Integrated assessment of landslide effects triggered by the 2020-2021 rainfall in Uvira Territory, on the northwestern shore of Lake Tanganyika

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A combined analysis of field data and Google Earth satellite imagery identified 221 events related to landslides and erosion, including 125 landslides (56.56%), 82 cases of gully erosion (37.10%), and 14 instances of bank erosion (6.33%). Intense rainfall such as the 114 mm recorded over 9 hours in April 2020 has intensified these hazards, leading to sudden floods. Between 2020 and 2021, human activities like construction in vulnerable areas and slope deforestation further amplified the risks. At least 2,707 riverside dwellings are exposed to sediment deposits. Three severely degraded zones have been identified as priorities for reforestation: (1) from Kaala stream to the right slopes of the Mulongwe River, (2) from Kakungwe stream to Ruzozi stream, and (3) the area surrounding Kalundu port. Recommended interventions include mountain stabilization, reforestation of degraded areas, and relocation of buildings away from high-risk zones. Environmental Engineering landslides flood risk river sedimentation climate resilience Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Introduction Landslides, heavily influenced by human activities, pose a major challenge in many tropical regions(Mugaruka et al., 2024 ; Vincens, 1989 ). In Uvira territory, landslides in 2002 resulted in approximately fifty deaths and the destruction of several areas. These events have significant impacts on development. Key triggering factors include geology, topography, vegetation cover, water, human activities, and cyclical stressors such as wind, thermal shocks, and earthquakes(Amanejieu, 2018 ; Cherkaoui & Al Heib, 2014 ). Numerous studies conducted in the tropical regions of the East African Rift confirm its instability(Mugaruka et al., 2017 ; Sutton & Berg, 1958 ). In Uvira,Ndyanabo, Vandecasteele, Moeyesrsons, et al., ( 2010 ) identified 60 landslides linked to seismic activity, located along deeply incised river sections. These landslides are typically shallow, small-scale, and highly mobile. Their deposits can temporarily block riverbeds with rocks, protecting channels and limiting incision. However, such blockages may cause upstream flooding before dam failure and sudden downstream floods after rupture(Bravard et al., 2001 ; Michel, 1960 ). Additionally, increased rainfall reduces soil shear strength, saturates watersheds, and leads to higher river discharge and extreme flooding. Since the early 2000s, hydroclimatic risks have intensified in the Albertine Rift, often with catastrophic consequences. In the northern part of Lake Tanganyika and in Uvira, the topographic contrast steep, unvegetated slopes on Mount Mitumba west of the lake and in the central region exacerbates erosion risk. Debris flows from alluvial plains, river meandering, and gully formation destroy homes, pipelines, and roads in their path. Bank collapses and recurrent floods have destroyed houses and bridges in Kavimvira district, located in Uvira and northwest of Lake Tanganyika, during the rainy seasons of 2015, 2016, and 2017. Changes in vegetation cover, particularly deforestation and intensified agriculture, contribute to soil destabilization on slopes overlooking the city(El Jazouli, 2020 ; Keefer, 1984 ). The most unstable zones are found around gullies, major escarpments, rivers, and geological fractures. Landslides, often triggered during rainy periods, combined with reduced vegetation cover and rugged terrain, result in human casualties and negatively affect ecosystems, flora, and fauna(Dai et al., 2002 ; Matabaro et al., 2017 ). Kulimushi Matabaro et al. ( 2017 ) noted that, like Bukavu, Uvira is especially vulnerable to gravitational movements, with affected slopes among the most densely populated. Soil erosion and landslides are linked to natural causes such as steep relief, prolonged light rainfall, and intense downpours(Butara et al., 2015b ; Saidi et al., 2003 ). Uvira’s population growth (267,614 inhabitants between 2000 and 2019) is largely due to rural exodus driven by insecurity and poor living conditions. This influx has led to settlements in unsuitable and often prohibited areas for construction or agriculture, such as the steep slopes of Mount Mitumba, riverbeds, and lakeshores factors that exacerbate and sometimes cause flooding. This situation continuously exposes these populations to increasing flood risks in the study area. Moreover, the near-total lack of access to electricity intensifies demand for firewood, accelerating deforestation on Mount Mitumba’s slopes. These phenomena are directly or indirectly linked to the disasters of April 2020 and 2021, which this study focuses on by examining field-observed effects of persistent flood risks in Uvira, northwest of Lake Tanganyika. Landslides and soil erosion along rivers contribute to significant downstream sedimentation within the watershed(Becel, 2005 ; Sellier, 2021 ). Debris flows and soil erosion pose a threat by accumulating in riverbeds, leading to overflow and fluvial flooding. The risk is further amplified by population settlements in unsuitable zones. The key findings of this study are essential for guiding policymakers and local communities in combating flood hazards in Uvira and developing sustainable watershed management strategies for Lake Tanganyika. Materials and Methods Study site This study was conducted across the watersheds of Uvira’s main rivers Kavimvira, Mulongwe, Kalimabenge, Kaala, Nyarumanga, Kabindula, Karigo, Kamongola, Ruzozi, and Sangeza located in the northwestern region of Lake Tanganyika, within South Kivu Province, Democratic Republic of the Congo (Fig. 1 ). The area spans from 26.9° to 29.16° East longitude and from − 3.48° to -3.32° South latitude. It is bounded by the Kawizi River to the north, the Sangeza River to the south, the Mitumba Mountains to the east (maximum elevation: 3,200 m), and Lake Tanganyika to the west(Dubois, 1957 ; Kapepula et al., 2014 ). The region’s humid tropical climate, influenced by both the lake and surrounding topography, results in high hydrological variability. Annual rainfall averages between 800 and 1,000 mm, distributed over 130 to 150 rainy days(Akilimali et al., 2017 ; Ilunga, 2006 ). Two rainy seasons (January–May and mid-October–December) alternate with a dry season (June-August), marked by strong southern winds. These precipitation patterns intensify erosion and sediment transport into rivers and their outlets(Ilunga, 2007 ; Ongezo et al., 2014 ). On April 17, 2020, the city of Uvira experienced deadly floods, along with multiple landslides and erosion events. Uvira’s soils, predominantly sandy to silty, are composed of kaolinite and clay, which contribute to their permeability and vulnerability to erosion. Vegetation is mainly composed of grassy savannas and gallery forests along the waterways of the Mitumba Mountains. However, urban expansion, deforestation, and unsustainable agricultural practices have led to significant degradation of vegetative cover in the plains. Identification and selection criteria for high-risk sites The rivers selected for this study were chosen due to their particular susceptibility to flooding, resulting from interactions with landslides. The presence of landslides, observed erosion on upstream slopes, and debris deposits along riverbanks in downstream plains were key criteria for identifying high-risk study sites. These indicators formed the basis for selecting the most vulnerable locations. Intense rainfall and human activities on steep slopes within the Mitumba Mountain watershed in Uvira were consistently observed as triggering factors for landslides and subsequent flooding. Data Collection, design and analysis A multi-method approach was adopted for data collection. Historical information relevant to the research theme was gathered through an extensive literature review, including books, maps, technical reports, and urban planning documents related to Uvira’s watersheds, sourced both online and from local libraries. During field visits, direct observations were conducted to assess environmental degradation, particularly the impact of human activities on the slopes of Mount Mitumba within the river basins (Fig. 1 ). Measurements included recording geographic coordinates using a Garmin 76X GPS and determining the length and width of landslides using a 100 meter measuring tape and soil erosion zones in the river basins surrounding Uvira, during April 2020 and 2021. These measurements enabled the spatial distribution and sizing of landslides and erosion events. The approach combined fieldwork with documentary analysis. Landslides were classified into four categories based on their length (L): Very large: L > 250 m Large: 250 m ≥ L > 150 m Medium: 150 m ≥ L > 50 m Small: L ≤ 50 m In the laboratory, inaccessible areas particularly steep slopes were analyzed using © Google Earth satellite imagery. These images helped assess river impacts at their outlets and delineate zones degraded by agriculture and deforestation. To estimate the number of homes exposed to flood risk, flood boundaries from the 2020 and 2021 events were mapped in the field by marking points every 10 meters using the Garmin 76X GPS. These boundaries were overlaid with housing data downloaded from OpenStreetMap. In QGIS 3.3, the housing layer was intersected with the flood boundary layer, and the resulting data identified affected homes. The software automatically counted the number of impacted houses. Flood boundaries were mapped immediately following the 2020 and 2021 flood events. Rainfall data collected from the Uvira hydrobiology research station were analyzed to examine factors contributing to soil destabilization and saturation in areas with severely degraded vegetation. These precipitation events can trigger new landslides and erosion or reactivate existing ones. Rainfall data were processed in Excel to identify peak rainfall events responsible for initiating landslides and soil erosion in the study area. Finally, watershed boundaries were delineated using contour lines derived from SRTM satellite imagery processed in QGIS 3.2. River networks were mapped using OpenStreetMap data within the same software. The height of sediment deposits was measured using a measuring tap, based on visible marks on the outer walls of houses affected by the floods. This visual estimation method is simple and quick to use in the field and allowed use in the amount of sediment in the most affected areas. However, this method has some limitations: its accuracy depends on how clear the marks are, the stability of the buildings, and the uniformity of the deposits. To reduce possible errors, measurements were taken at several points and, when possible, compared with other direct observations or photos. Results Landslides and Soil Erosion The distribution of landslides and soil erosion across the various river basins in the city of Uvira revealed that 125 events were classified as landslides, accounting for approximately 56.56% of the total. Additionally, 82 cases were identified as gully erosion (37.10%), and 14 as bank erosion (6.33%). These results can be attributed to the lack of vegetation cover on steep slopes and the occurrence of intense rainfall. A total of 221 landslide and erosion events were identified and mapped along the slopes of Mount Mitumba, from the Kaala site to Ruzozi (Fig. 2 ). The landslides primarily consisted of debris flows that discharged materials into the rivers, with some forming temporary barriers that obstructed river flow. As a consequence, the collapse of these barriers led to large volumes of water overflowing downstream riverbeds, triggering floods and burying human infrastructure in Uvira under sediment deposits. A distribution map based on the size of landslides and erosion events was developed according to landslide dimensions (Fig. 3 ). Among these landslides and soil erosion events, several are in direct contact with rivers, transporting significant amounts of debris depending on their size. This process increases downstream sedimentation, particularly in Uvira’s neighborhoods and at the mouths of rivers flowing into Lake Tanganyika. As a result, river dredging is not a sustainable long-term solution. These landslides destroy homes, strip vegetation downstream, and negatively impact riverine biodiversity. The classification of landslides and erosion events by size enabled the identification of three major degraded zones requiring reforestation to mitigate flood risk in the study area. These zones are: From Kaala stream to the right slopes of the Mulongwe River From Kakungwe stream to Ruzozi stream Around the Kalundu port These areas are marked with black lines on the map (Fig. 3 ). Sedimentation and landscape degradation Landslide activity was observed on bare, steep slopes lacking vegetation (Fig. 5 ). In addition, field observations revealed a significant level of human activity and progressive landscape degradation in these regions (Figs. 4 and 5 ). Substantial sediment deposits approximately 4 meters in height caused by the Mulongwe River were recorded in the Mulongwe and Rombe II neighborhoods. These deposits pose a serious threat to downstream infrastructure and housing in the city of Uvira (Figs. 5 c and 5 d). Comparative Analysis of Landscape Destruction Using Google Earth Satellite Images A comparative analysis of Google Earth satellite images from 2001 and 2021 revealed sediment deposits along the rivers and at the mouths of the three main rivers crossing the city of Uvira (Fig. 6 ). The following observations were made within the watersheds of these rivers: Kavimvira and Mulongwe River Basins: In 2001, there were virtually no human settlements, buffer zones, or vegetation along these rivers or along the coastal area of Lake Tanganyika. The 2021 images show significant landscape degradation around the riverbanks, with a proliferation of housing and sediment deposits resulting from upstream landslides on Mount Mitumba. These sediments buried homes located in riverbeds and surrounding areas, as observed during the 2020 disaster events in the study area (Figs. 6 a & b, 6c & d). Kalimabenge River Basin (In 2001, no houses were present along the river or on the adjacent shoreline of Lake Tanganyika. By 2021, a concentration of housing and uncontrolled construction was observed within the buffer zones of the river and along the lake’s edge. However, sedimentation at the river mouth was minimal in this basin, likely due to local reforestation efforts on the steep, bare slopes. This was also evidenced by the presence of macrophytes at the mouth of the Kalimabenge River, highlighted by the green circle Figs. 6 e & f). Spatial analysis of hydrological hazards in Uvira revealed a high level of residential exposure to risks such as landslides and soil erosion, particularly during the rainy season. Flood events, which recur annually, have posed significant threats to both human life and infrastructure across the city’s seven major watersheds (Fig. 7 ). The cumulative assessment identified 2,707 houses exposed to landslide risk, with the Mulongwe watershed exhibiting the highest vulnerability (807 houses), followed by Kavimvira (638 houses) and Kabindula (523 houses) (Fig. 6 ). Hydrological risk was most severe in the Mulongwe basin, where flood events in April 2020 resulted in six fatalities, nine injuries, and the destruction of 1,048 houses, with an additional 32 houses partially damaged. Several public infrastructures were also inundated. In April 2021, two more deaths and further material losses were reported, reinforcing the pattern of seasonal vulnerability. Field observations and satellite imagery indicated that unregulated construction within riverbeds significantly exacerbated the impacts of flooding. In 2020, sediment deposition reached heights of over 4 meters in the Mulongwe River and 3 meters in the Kavimvira River, leading to the burial of numerous homes. These findings highlight the urgent need for relocation of residential structures situated along riverbanks and the implementation of land-use planning measures to reduce future exposure. Precipitation and anthropogenic activities Meteorological data from the CRH/Uvira station indicate significant variability in daily precipitation during the month of April from 2020 to 2023 (Fig. 8 ). Among the four years analyzed, April 2020 recorded the highest rainfall levels. Notably, heavy rains began on April 12, 2020, with an extreme precipitation event occurring during the night of April 16–17, when 114 mm of rain fell within 9 hours. This was followed by an additional 57 mm between April 17 and April 23. The total rainfall for April 2020 reached 400 mm, representing 40% of the region’s average annual precipitation of approximately 1,000 mm. These intense rainfall events saturated the deforested soils, triggering landslides, increasing surface runoff, and elevating river discharge, which collectively led to widespread flooding during this period. The years 2020 and 2021were marked by a higher frequency of landslides, with major disaster events occurring in April of both years. The combination of extreme precipitation and anthropogenic pressures—particularly deforestation and unregulated land use contributed significantly to slope instability and hydrological hazards, underscoring the role of climatic and human factors in amplifying disaster risk in Uvira. Human activities and natural hazards Field observations and spatial analysis revealed that anthropogenic activities on the slopes of Mount Mitumbasignificantly contribute to land degradation and slope instability in the Uvira region (Fig. 9 ). These activities include unsuitable agricultural practices, tree cutting in landslide-prone areas, cultivation on steep slopes, vegetation clearing through burning, soil exposure, charcoal production (a key indicator of deforestation), extraction of construction materials, and grazing. Such practices are widespread in the mountainous zones surrounding Uvira and are closely linked to increased soil displacement and erosion. These human-induced pressures were compounded by fragile natural conditions, notably the presence of clay-sandy soilsand steep topography, which inherently predispose the region to landslides. The lack of vegetation cover observed in these areas is a direct consequence of sustained anthropogenic disturbance. In addition to the densely populated Uvira plain, several villages are located in the surrounding mountains, where local livelihoods further accelerate vegetation loss. Across both lowland and upland areas, human activities exert substantial pressure on the environment, intensifying the vulnerability of the landscape to natural hazards. Discussion Landslides represent a major environmental challenge in several areas of Uvira, resulting in loss of life, destruction of homes and infrastructure, and a significant barrier to local development(Azzouz et al., 2002 ; Haemouzi et al., 2018 ) [24, 25]. Between 2020 and 2021, a total of 221 ground movement events were recorded, including 125 landslides and 96 other forms of erosion. This marks a substantial increase compared to earlier data reported by(Ilunga, 2006 ), indicating a worsening trend over time. Soil movements observed along riverbanks have led to increased downstream sedimentation, disrupting the natural functioning of watersheds(Becel, 2005 ; Sellier, 2021 ). Debris deposits from landslides temporarily block riverbeds, causing upstream flooding and sudden downstream surges when natural dams collapse. These events result in extensive ecological and material damage, including the destruction of flora, fauna, homes, roads, bridges, and stormwater systems, as well as loss of life-particularly during the rainy season(Bravard et al., 2001 ; Bunduki et al., 2015 ; Cadier et al., 1996 ; Dai et al., 2002 ; Dionne, 2012 ; Faleh & Abdelhamid, 2002 ; Ilunga, 2006 ; Matabaro et al., 2017 ; Michel, 1960 ; Mwenyemali et al., 2020 ; Ndyanabo, Vandecasteele, Moeyersons, et al., 2010 ; Ndyanabo, Vandecasteele, Moeyesrsons, et al., 2010 ; Palhol et al., 2010 ; Quefféléan et al., 2021 ). In 2020 and 2021, several landslides triggered massive debris flows following floods, especially in riverine neighborhoods and at river mouths flowing into Lake Tanganyika. These processes severely altered the urban landscape and disrupted human activities. A notable case involved the Mulongwe River, where a landslide blocked the channel and caused a flash flood following the collapse of the natural dam. The situation is further exacerbated by unregulated urban expansion, particularly the construction of homes within minor riverbeds, making densely populated areas highly vulnerable to flooding. More than 2,700 homes in Uvira are currently exposed to this hazard. The causes of these landslides are multifactorial, involving both natural and anthropogenic drivers. Natural factors include steep terrain and intense tropical rainfall(Butara et al., 2015a ; Saidi et al., 2003 )[15, 34], while human-induced triggers include deforestation, unsustainable land use, cultivation on steep slopes, bushfires, charcoal production, overgrazing, and extraction of construction materials(Amanejieu, 2018 ; Avenard, 1965 ; Azzouz et al., 2002 ; Berg & Sutton, 1958 ; Cherkaoui & Al Heib, 2014 ; De Bremaecker, 1959 ; Keefer, 1984 ; Millies-Lacroix, 1968 ; Mugaruka et al., 2017 ; Ndyanabo, Vandecasteele, Moeyesrsons, et al., 2010 ; Zana, 1977 ). The mountainous topography of the Mitumba Range, combined with depleted vegetation cover especially west of Lake Tanganyika greatly increases erosion risk(Bravard et al., 2001 ; Nteranya, 2021 ). Heavy rainfall events in April 2020 and 2021 saturated deforested soils, contributing to increased landslide activity. Satellite imagery (Google Earth Pro) and field observations from those years confirm significant vegetation loss in the mountainous zones surrounding the city. Additionally, steep slopes and poorly cohesive clay-sandy soils make certain areas difficult to access and highly susceptible to mass movements. Three severely degraded zones were identified: (1) from Kaala stream to the right slopes of the Mulongwe River, (2) from Kakungwe stream to Ruzozi stream, and (3) the area surrounding Kalundu port. These zones, characterized by sparse vegetation and frequent landslides, require urgent reforestation to reduce runoff and stabilize soils. However, this study has certain limitations. The lack of precise and continuous climate data restricts the analysis of climate change impacts on the frequency and intensity of extreme events. Furthermore, although priority areas for reforestation have been identified, the absence of long-term monitoring makes it difficult to assess the effectiveness of restoration efforts. The findings of this study highlight the urgent need to implement slope stabilization strategies, sustainable environmental management, and targeted reforestation in the areas most affected by landslides and erosion in Uvira. Conclusion This study demonstrates the strong correlation between landslides and riverine flooding in Uvira, DRC. Using historical data, field observations, and satellite imagery, key zones vulnerable to ground movements between 2020 and 2021 were identified, along with primary triggers and impacts particularly flash floods. Watersheds of rivers such as Mulongwe, Kavimvira, Kabindula, and Kalimabenge are highly exposed due to sediment accumulation and landscape degradation. Intense rainfall events, combined with anthropogenic pressures (deforestation, slope farming, charcoal production, and material extraction), have amplified soil instability and flood risks. Precipitation analysis confirms that landslides and floods are concentrated during heavy rain periods, especially in the Mitumba Mountains upstream. This highlights the urgent need for slope stabilization in critical zones. The mapping approach proved effective for risk identification and mitigation planning. Priority actions include reforestation, slope stabilization, watershed management, and regulated urban development. Further studies are needed to assess climate change impacts and integrate them into long-term resilience strategies. Declarations Acknowledgments We thank the CRH-Uvira team (Kahindo Ndjungu, Mwenyemali Banamwezi, Muzumani Risasi, Maliyamungu Makubuli, Muhemura Francoise) for field access support. Appreciation also goes to Andrew Cohen, Pierre-Denis Plisnier, and Jean-Claude Maki Mateso for their valuable inputs and discussions on the early version of this MS. Author Contributions TKP: study design, data collection/analysis, manuscript drafting and final validation; PMM: Study supervision, methodological support, result interpretation, initial draft; SFB: Objective formulation, data analysis, technical review, manuscript refinement; BM: Data interpretation and writing support; SRR: Field data collection, methodology and analysis sections; BP: Initial data analysis and manuscript drafting; DMC: Scientific guidance, study design, data collection/analysis, manuscript drafting, content revision. Funding and/or Conflicts of Interest / Competing Interests Funding The Center for Hydrobiology Research provided logistical support by supplying drinking water during the fieldwork and helping us access the study sites. No direct financial funding was received for this research. Conflicts of Interest / Competing Interests The authors declare that they have no financial or personal conflicts of interest related to this study. References Akilimali, J. I., Wasso, D. S., Baenyi, P., & Bajope, J. B. (2017). Caractérisation des systèmes de production porcine de petits exploitants dans trois zones agro-écologiques du Sud-Kivu en République Démocratique du Congo. Journal of Applied Biosciences , 120 , 12086–12097. Amanejieu, A. (2018). Analyse temporelle de la représentation du risque d’inondation de 1980 à 2018 à douala-Cameroun . Avenard, J. M. (1965). La cartographie de l’érosion actuelle dans le bassin du Sebou (Maroc). Service de La Cartographie, Section Érosion , 1–25. 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Additional Declarations The authors declare no competing interests. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8658281","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":578021146,"identity":"dafbc413-ca7c-4a38-a35b-0d961e32269f","order_by":0,"name":"Patient 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07:08:09","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":659522,"visible":true,"origin":"","legend":"\u003cp\u003eDistribution of landslide and erosion events across the three main river basins: Kavimvira(a), Mulongwe(b), Kalimabenge(c), and surrounding areas.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8658281/v1/4dbed4eaf778b0cf14c63254.png"},{"id":101397619,"identity":"97faab1c-0616-4bb9-8db7-462f7f0b2115","added_by":"auto","created_at":"2026-01-29 09:32:49","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":529281,"visible":true,"origin":"","legend":"\u003cp\u003eDistribution of landslides and soil erosion by size in the study area.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8658281/v1/83db44b6da1136aed5968e7d.png"},{"id":100949512,"identity":"7c758ce2-eaf8-4780-954d-9504b98bdb37","added_by":"auto","created_at":"2026-01-23 07:03:30","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":795213,"visible":true,"origin":"","legend":"\u003cp\u003eExtent of land without vegetation cover, with reforestation zones highlighted in green\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8658281/v1/03eb57627e8a14d867a3dc2a.png"},{"id":100866768,"identity":"d879fb36-6b09-474e-97b6-e2f0ab78bff4","added_by":"auto","created_at":"2026-01-22 08:35:36","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":596061,"visible":true,"origin":"","legend":"\u003cp\u003eLandslides and sedimentation in Uvira: \u0026nbsp;Landslide on bare land in Kavimvira(a); Landslide along Mulongwe River banks(b); Unregulated construction in Mulongwe’s minor bed (sediment height \u0026gt;4 m) (c); Sedimentation and material extraction at the river mouth (April 2020) (d).\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8658281/v1/4d8af5d87dc5c0ecc4ac313c.png"},{"id":100866780,"identity":"48abd5ee-95aa-4527-a846-efe36da43cde","added_by":"auto","created_at":"2026-01-22 08:35:37","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":819487,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in land use and sedimentation at river mouths: \u0026nbsp;Minimal land use and sedimentation at Kavimvira (2001) (a); Sediment deposits \u0026gt;4 m at Kavimvira (2021) (b); Low land use and sedimentation at Mulongwe (2001) (c); Heavy sedimentation at Mulongwe (2021) (d); Kalimabenge River impact (2001) (e); Kalimabenge mouth with macrophytes(f).\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8658281/v1/03bfbbe3bb950b390e5a08ce.png"},{"id":100866764,"identity":"83eb38af-f087-4c0f-a277-30f96f175cf9","added_by":"auto","created_at":"2026-01-22 08:35:36","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":59762,"visible":true,"origin":"","legend":"\u003cp\u003eNumber of houses exposed to risks (landslides and soil erosion) in the various watersheds of Uvira city.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-8658281/v1/0ddf1830c3a223c2f1bbf949.png"},{"id":100866775,"identity":"2181a924-37eb-445c-8e11-21df16662658","added_by":"auto","created_at":"2026-01-22 08:35:37","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":191151,"visible":true,"origin":"","legend":"\u003cp\u003eDaily precipitation for the month of April from 2020 to 2023 (CRH-Uvira meteorological station).\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-8658281/v1/f22d99c8206e552ce6fcb454.png"},{"id":100949847,"identity":"5d13e963-63c5-46c0-a884-960c92dbb468","added_by":"auto","created_at":"2026-01-23 07:06:01","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":1088784,"visible":true,"origin":"","legend":"\u003cp\u003eAnthropogenic activities on the slopes of Mount Mitumba: \u0026nbsp;Unsuitable agricultural practices (a); Tree cutting in landslide-prone areas (b); Cultivation on steep slopes(c); Slash-and-burn practices and bare soils (d); Charcoal production (indicator of deforestation) (e); Deforestation (f).\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8658281/v1/0a80769a5e21860ecf01aa07.png"},{"id":101398812,"identity":"2ba3e2cf-5280-498c-8524-644c599ef8ab","added_by":"auto","created_at":"2026-01-29 09:48:09","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5983827,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8658281/v1/74d95191-3798-473f-adbb-285236810d12.pdf"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eIntegrated assessment of landslide effects triggered by the 2020-2021 rainfall in Uvira Territory, on the northwestern shore of Lake Tanganyika\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eLandslides, heavily influenced by human activities, pose a major challenge in many tropical regions(Mugaruka et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Vincens, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). In Uvira territory, landslides in 2002 resulted in approximately fifty deaths and the destruction of several areas. These events have significant impacts on development. Key triggering factors include geology, topography, vegetation cover, water, human activities, and cyclical stressors such as wind, thermal shocks, and earthquakes(Amanejieu, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Cherkaoui \u0026amp; Al Heib, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eNumerous studies conducted in the tropical regions of the East African Rift confirm its instability(Mugaruka et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Sutton \u0026amp; Berg, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1958\u003c/span\u003e). In Uvira,Ndyanabo, Vandecasteele, Moeyesrsons, et al., (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) identified 60 landslides linked to seismic activity, located along deeply incised river sections. These landslides are typically shallow, small-scale, and highly mobile. Their deposits can temporarily block riverbeds with rocks, protecting channels and limiting incision. However, such blockages may cause upstream flooding before dam failure and sudden downstream floods after rupture(Bravard et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Michel, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1960\u003c/span\u003e). Additionally, increased rainfall reduces soil shear strength, saturates watersheds, and leads to higher river discharge and extreme flooding. Since the early 2000s, hydroclimatic risks have intensified in the Albertine Rift, often with catastrophic consequences. In the northern part of Lake Tanganyika and in Uvira, the topographic contrast steep, unvegetated slopes on Mount Mitumba west of the lake and in the central region exacerbates erosion risk. Debris flows from alluvial plains, river meandering, and gully formation destroy homes, pipelines, and roads in their path.\u003c/p\u003e \u003cp\u003eBank collapses and recurrent floods have destroyed houses and bridges in Kavimvira district, located in Uvira and northwest of Lake Tanganyika, during the rainy seasons of 2015, 2016, and 2017. Changes in vegetation cover, particularly deforestation and intensified agriculture, contribute to soil destabilization on slopes overlooking the city(El Jazouli, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Keefer, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1984\u003c/span\u003e). The most unstable zones are found around gullies, major escarpments, rivers, and geological fractures.\u003c/p\u003e \u003cp\u003eLandslides, often triggered during rainy periods, combined with reduced vegetation cover and rugged terrain, result in human casualties and negatively affect ecosystems, flora, and fauna(Dai et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Matabaro et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Kulimushi Matabaro et al. (\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) noted that, like Bukavu, Uvira is especially vulnerable to gravitational movements, with affected slopes among the most densely populated. Soil erosion and landslides are linked to natural causes such as steep relief, prolonged light rainfall, and intense downpours(Butara et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2015b\u003c/span\u003e; Saidi et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2003\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eUvira\u0026rsquo;s population growth (267,614 inhabitants between 2000 and 2019) is largely due to rural exodus driven by insecurity and poor living conditions. This influx has led to settlements in unsuitable and often prohibited areas for construction or agriculture, such as the steep slopes of Mount Mitumba, riverbeds, and lakeshores factors that exacerbate and sometimes cause flooding. This situation continuously exposes these populations to increasing flood risks in the study area. Moreover, the near-total lack of access to electricity intensifies demand for firewood, accelerating deforestation on Mount Mitumba\u0026rsquo;s slopes. These phenomena are directly or indirectly linked to the disasters of April 2020 and 2021, which this study focuses on by examining field-observed effects of persistent flood risks in Uvira, northwest of Lake Tanganyika. Landslides and soil erosion along rivers contribute to significant downstream sedimentation within the watershed(Becel, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Sellier, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDebris flows and soil erosion pose a threat by accumulating in riverbeds, leading to overflow and fluvial flooding. The risk is further amplified by population settlements in unsuitable zones. The key findings of this study are essential for guiding policymakers and local communities in combating flood hazards in Uvira and developing sustainable watershed management strategies for Lake Tanganyika.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eStudy site\u003c/h2\u003e \u003cp\u003eThis study was conducted across the watersheds of Uvira\u0026rsquo;s main rivers Kavimvira, Mulongwe, Kalimabenge, Kaala, Nyarumanga, Kabindula, Karigo, Kamongola, Ruzozi, and Sangeza located in the northwestern region of Lake Tanganyika, within South Kivu Province, Democratic Republic of the Congo (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The area spans from 26.9\u0026deg; to 29.16\u0026deg; East longitude and from \u0026minus;\u0026thinsp;3.48\u0026deg; to -3.32\u0026deg; South latitude. It is bounded by the Kawizi River to the north, the Sangeza River to the south, the Mitumba Mountains to the east (maximum elevation: 3,200 m), and Lake Tanganyika to the west(Dubois, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e1957\u003c/span\u003e; Kapepula et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe region\u0026rsquo;s humid tropical climate, influenced by both the lake and surrounding topography, results in high hydrological variability. Annual rainfall averages between 800 and 1,000 mm, distributed over 130 to 150 rainy days(Akilimali et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Ilunga, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Two rainy seasons (January\u0026ndash;May and mid-October\u0026ndash;December) alternate with a dry season (June-August), marked by strong southern winds. These precipitation patterns intensify erosion and sediment transport into rivers and their outlets(Ilunga, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Ongezo et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). On April 17, 2020, the city of Uvira experienced deadly floods, along with multiple landslides and erosion events.\u003c/p\u003e \u003cp\u003eUvira\u0026rsquo;s soils, predominantly sandy to silty, are composed of kaolinite and clay, which contribute to their permeability and vulnerability to erosion. Vegetation is mainly composed of grassy savannas and gallery forests along the waterways of the Mitumba Mountains. However, urban expansion, deforestation, and unsustainable agricultural practices have led to significant degradation of vegetative cover in the plains.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eIdentification and selection criteria for high-risk sites\u003c/h3\u003e\n\u003cp\u003eThe rivers selected for this study were chosen due to their particular susceptibility to flooding, resulting from interactions with landslides. The presence of landslides, observed erosion on upstream slopes, and debris deposits along riverbanks in downstream plains were key criteria for identifying high-risk study sites. These indicators formed the basis for selecting the most vulnerable locations. Intense rainfall and human activities on steep slopes within the Mitumba Mountain watershed in Uvira were consistently observed as triggering factors for landslides and subsequent flooding.\u003c/p\u003e\n\u003ch3\u003eData Collection, design and analysis\u003c/h3\u003e\n\u003cp\u003eA multi-method approach was adopted for data collection. Historical information relevant to the research theme was gathered through an extensive literature review, including books, maps, technical reports, and urban planning documents related to Uvira\u0026rsquo;s watersheds, sourced both online and from local libraries.\u003c/p\u003e \u003cp\u003eDuring field visits, direct observations were conducted to assess environmental degradation, particularly the impact of human activities on the slopes of Mount Mitumba within the river basins (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Measurements included recording geographic coordinates using a Garmin 76X GPS and determining the length and width of landslides using a 100 meter measuring tape and soil erosion zones in the river basins surrounding Uvira, during April 2020 and 2021. These measurements enabled the spatial distribution and sizing of landslides and erosion events. The approach combined fieldwork with documentary analysis.\u003c/p\u003e \u003cp\u003eLandslides were classified into four categories based on their length (L):\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eVery large: L\u0026thinsp;\u0026gt;\u0026thinsp;250 m\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eLarge: 250 m\u0026thinsp;\u0026ge;\u0026thinsp;L\u0026thinsp;\u0026gt;\u0026thinsp;150 m\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eMedium: 150 m\u0026thinsp;\u0026ge;\u0026thinsp;L\u0026thinsp;\u0026gt;\u0026thinsp;50 m\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eSmall: L\u0026thinsp;\u0026le;\u0026thinsp;50 m\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eIn the laboratory, inaccessible areas particularly steep slopes were analyzed using \u0026copy; Google Earth satellite imagery. These images helped assess river impacts at their outlets and delineate zones degraded by agriculture and deforestation.\u003c/p\u003e \u003cp\u003eTo estimate the number of homes exposed to flood risk, flood boundaries from the 2020 and 2021 events were mapped in the field by marking points every 10 meters using the Garmin 76X GPS. These boundaries were overlaid with housing data downloaded from OpenStreetMap. In QGIS 3.3, the housing layer was intersected with the flood boundary layer, and the resulting data identified affected homes. The software automatically counted the number of impacted houses. Flood boundaries were mapped immediately following the 2020 and 2021 flood events.\u003c/p\u003e \u003cp\u003eRainfall data collected from the Uvira hydrobiology research station were analyzed to examine factors contributing to soil destabilization and saturation in areas with severely degraded vegetation. These precipitation events can trigger new landslides and erosion or reactivate existing ones. Rainfall data were processed in Excel to identify peak rainfall events responsible for initiating landslides and soil erosion in the study area.\u003c/p\u003e \u003cp\u003eFinally, watershed boundaries were delineated using contour lines derived from SRTM satellite imagery processed in QGIS 3.2. River networks were mapped using OpenStreetMap data within the same software.\u003c/p\u003e \u003cp\u003eThe height of sediment deposits was measured using a measuring tap, based on visible marks on the outer walls of houses affected by the floods. This visual estimation method is simple and quick to use in the field and allowed use in the amount of sediment in the most affected areas.\u003c/p\u003e \u003cp\u003eHowever, this method has some limitations: its accuracy depends on how clear the marks are, the stability of the buildings, and the uniformity of the deposits. To reduce possible errors, measurements were taken at several points and, when possible, compared with other direct observations or photos.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eLandslides and Soil Erosion\u003c/h2\u003e \u003cp\u003eThe distribution of landslides and soil erosion across the various river basins in the city of Uvira revealed that 125 events were classified as landslides, accounting for approximately 56.56% of the total. Additionally, 82 cases were identified as gully erosion (37.10%), and 14 as bank erosion (6.33%). These results can be attributed to the lack of vegetation cover on steep slopes and the occurrence of intense rainfall.\u003c/p\u003e \u003cp\u003eA total of 221 landslide and erosion events were identified and mapped along the slopes of Mount Mitumba, from the Kaala site to Ruzozi (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The landslides primarily consisted of debris flows that discharged materials into the rivers, with some forming temporary barriers that obstructed river flow. As a consequence, the collapse of these barriers led to large volumes of water overflowing downstream riverbeds, triggering floods and burying human infrastructure in Uvira under sediment deposits.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA distribution map based on the size of landslides and erosion events was developed according to landslide dimensions (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Among these landslides and soil erosion events, several are in direct contact with rivers, transporting significant amounts of debris depending on their size. This process increases downstream sedimentation, particularly in Uvira\u0026rsquo;s neighborhoods and at the mouths of rivers flowing into Lake Tanganyika. As a result, river dredging is not a sustainable long-term solution. These landslides destroy homes, strip vegetation downstream, and negatively impact riverine biodiversity.\u003c/p\u003e \u003cp\u003eThe classification of landslides and erosion events by size enabled the identification of three major degraded zones requiring reforestation to mitigate flood risk in the study area. These zones are:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eFrom Kaala stream to the right slopes of the Mulongwe River\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eFrom Kakungwe stream to Ruzozi stream\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eAround the Kalundu port\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eThese areas are marked with black lines on the map (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eSedimentation and landscape degradation\u003c/h2\u003e \u003cp\u003eLandslide activity was observed on bare, steep slopes lacking vegetation (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). In addition, field observations revealed a significant level of human activity and progressive landscape degradation in these regions (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Substantial sediment deposits approximately 4 meters in height caused by the Mulongwe River were recorded in the Mulongwe and Rombe II neighborhoods. These deposits pose a serious threat to downstream infrastructure and housing in the city of Uvira (Figs.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec and \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eComparative Analysis of Landscape Destruction Using Google Earth Satellite Images\u003c/h3\u003e\n\u003cp\u003eA comparative analysis of Google Earth satellite images from 2001 and 2021 revealed sediment deposits along the rivers and at the mouths of the three main rivers crossing the city of Uvira (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). The following observations were made within the watersheds of these rivers:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eKavimvira and Mulongwe River Basins: In 2001, there were virtually no human settlements, buffer zones, or vegetation along these rivers or along the coastal area of Lake Tanganyika. The 2021 images show significant landscape degradation around the riverbanks, with a proliferation of housing and sediment deposits resulting from upstream landslides on Mount Mitumba. These sediments buried homes located in riverbeds and surrounding areas, as observed during the 2020 disaster events in the study area (Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea \u0026amp; b, 6c \u0026amp; d).\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eKalimabenge River Basin (In 2001, no houses were present along the river or on the adjacent shoreline of Lake Tanganyika. By 2021, a concentration of housing and uncontrolled construction was observed within the buffer zones of the river and along the lake\u0026rsquo;s edge. However, sedimentation at the river mouth was minimal in this basin, likely due to local reforestation efforts on the steep, bare slopes. This was also evidenced by the presence of macrophytes at the mouth of the Kalimabenge River, highlighted by the green circle Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee \u0026amp; f).\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSpatial analysis of hydrological hazards in Uvira revealed a high level of residential exposure to risks such as landslides and soil erosion, particularly during the rainy season. Flood events, which recur annually, have posed significant threats to both human life and infrastructure across the city\u0026rsquo;s seven major watersheds (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). The cumulative assessment identified 2,707 houses exposed to landslide risk, with the Mulongwe watershed exhibiting the highest vulnerability (807 houses), followed by Kavimvira (638 houses) and Kabindula (523 houses) (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eHydrological risk was most severe in the Mulongwe basin, where flood events in April 2020 resulted in six fatalities, nine injuries, and the destruction of 1,048 houses, with an additional 32 houses partially damaged. Several public infrastructures were also inundated. In April 2021, two more deaths and further material losses were reported, reinforcing the pattern of seasonal vulnerability.\u003c/p\u003e \u003cp\u003eField observations and satellite imagery indicated that unregulated construction within riverbeds significantly exacerbated the impacts of flooding. In 2020, sediment deposition reached heights of over 4 meters in the Mulongwe River and 3 meters in the Kavimvira River, leading to the burial of numerous homes. These findings highlight the urgent need for relocation of residential structures situated along riverbanks and the implementation of land-use planning measures to reduce future exposure.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003ePrecipitation and anthropogenic activities\u003c/h3\u003e\n\u003cp\u003eMeteorological data from the CRH/Uvira station indicate significant variability in daily precipitation during the month of April from 2020 to 2023 (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). Among the four years analyzed, April 2020 recorded the highest rainfall levels. Notably, heavy rains began on April 12, 2020, with an extreme precipitation event occurring during the night of April 16\u0026ndash;17, when 114 mm of rain fell within 9 hours. This was followed by an additional 57 mm between April 17 and April 23. The total rainfall for April 2020 reached 400 mm, representing 40% of the region\u0026rsquo;s average annual precipitation of approximately 1,000 mm.\u003c/p\u003e \u003cp\u003eThese intense rainfall events saturated the deforested soils, triggering landslides, increasing surface runoff, and elevating river discharge, which collectively led to widespread flooding during this period. The years 2020 and 2021were marked by a higher frequency of landslides, with major disaster events occurring in April of both years.\u003c/p\u003e \u003cp\u003eThe combination of extreme precipitation and anthropogenic pressures\u0026mdash;particularly deforestation and unregulated land use contributed significantly to slope instability and hydrological hazards, underscoring the role of climatic and human factors in amplifying disaster risk in Uvira.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eHuman activities and natural hazards\u003c/h2\u003e \u003cp\u003eField observations and spatial analysis revealed that anthropogenic activities on the slopes of Mount Mitumbasignificantly contribute to land degradation and slope instability in the Uvira region (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). These activities include unsuitable agricultural practices, tree cutting in landslide-prone areas, cultivation on steep slopes, vegetation clearing through burning, soil exposure, charcoal production (a key indicator of deforestation), extraction of construction materials, and grazing. Such practices are widespread in the mountainous zones surrounding Uvira and are closely linked to increased soil displacement and erosion.\u003c/p\u003e \u003cp\u003eThese human-induced pressures were compounded by fragile natural conditions, notably the presence of clay-sandy soilsand steep topography, which inherently predispose the region to landslides. The lack of vegetation cover observed in these areas is a direct consequence of sustained anthropogenic disturbance.\u003c/p\u003e \u003cp\u003eIn addition to the densely populated Uvira plain, several villages are located in the surrounding mountains, where local livelihoods further accelerate vegetation loss. Across both lowland and upland areas, human activities exert substantial pressure on the environment, intensifying the vulnerability of the landscape to natural hazards.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eLandslides represent a major environmental challenge in several areas of Uvira, resulting in loss of life, destruction of homes and infrastructure, and a significant barrier to local development(Azzouz et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Haemouzi et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) [24, 25]. Between 2020 and 2021, a total of 221 ground movement events were recorded, including 125 landslides and 96 other forms of erosion. This marks a substantial increase compared to earlier data reported by(Ilunga, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), indicating a worsening trend over time.\u003c/p\u003e \u003cp\u003eSoil movements observed along riverbanks have led to increased downstream sedimentation, disrupting the natural functioning of watersheds(Becel, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2005\u003c/span\u003e; Sellier, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Debris deposits from landslides temporarily block riverbeds, causing upstream flooding and sudden downstream surges when natural dams collapse. These events result in extensive ecological and material damage, including the destruction of flora, fauna, homes, roads, bridges, and stormwater systems, as well as loss of life-particularly during the rainy season(Bravard et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Bunduki et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Cadier et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Dai et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Dionne, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Faleh \u0026amp; Abdelhamid, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Ilunga, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Matabaro et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Michel, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1960\u003c/span\u003e; Mwenyemali et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Ndyanabo, Vandecasteele, Moeyersons, et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Ndyanabo, Vandecasteele, Moeyesrsons, et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Palhol et al., \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Queff\u0026eacute;l\u0026eacute;an et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). In 2020 and 2021, several landslides triggered massive debris flows following floods, especially in riverine neighborhoods and at river mouths flowing into Lake Tanganyika. These processes severely altered the urban landscape and disrupted human activities. A notable case involved the Mulongwe River, where a landslide blocked the channel and caused a flash flood following the collapse of the natural dam.\u003c/p\u003e \u003cp\u003eThe situation is further exacerbated by unregulated urban expansion, particularly the construction of homes within minor riverbeds, making densely populated areas highly vulnerable to flooding. More than 2,700 homes in Uvira are currently exposed to this hazard. The causes of these landslides are multifactorial, involving both natural and anthropogenic drivers. Natural factors include steep terrain and intense tropical rainfall(Butara et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2015a\u003c/span\u003e; Saidi et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2003\u003c/span\u003e)[15, 34], while human-induced triggers include deforestation, unsustainable land use, cultivation on steep slopes, bushfires, charcoal production, overgrazing, and extraction of construction materials(Amanejieu, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Avenard, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e1965\u003c/span\u003e; Azzouz et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Berg \u0026amp; Sutton, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1958\u003c/span\u003e; Cherkaoui \u0026amp; Al Heib, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; De Bremaecker, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e1959\u003c/span\u003e; Keefer, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1984\u003c/span\u003e; Millies-Lacroix, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e1968\u003c/span\u003e; Mugaruka et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Ndyanabo, Vandecasteele, Moeyesrsons, et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Zana, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e1977\u003c/span\u003e). The mountainous topography of the Mitumba Range, combined with depleted vegetation cover especially west of Lake Tanganyika greatly increases erosion risk(Bravard et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Nteranya, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eHeavy rainfall events in April 2020 and 2021 saturated deforested soils, contributing to increased landslide activity. Satellite imagery (Google Earth Pro) and field observations from those years confirm significant vegetation loss in the mountainous zones surrounding the city. Additionally, steep slopes and poorly cohesive clay-sandy soils make certain areas difficult to access and highly susceptible to mass movements. Three severely degraded zones were identified: (1) from Kaala stream to the right slopes of the Mulongwe River, (2) from Kakungwe stream to Ruzozi stream, and (3) the area surrounding Kalundu port. These zones, characterized by sparse vegetation and frequent landslides, require urgent reforestation to reduce runoff and stabilize soils.\u003c/p\u003e \u003cp\u003eHowever, this study has certain limitations. The lack of precise and continuous climate data restricts the analysis of climate change impacts on the frequency and intensity of extreme events. Furthermore, although priority areas for reforestation have been identified, the absence of long-term monitoring makes it difficult to assess the effectiveness of restoration efforts.\u003c/p\u003e \u003cp\u003eThe findings of this study highlight the urgent need to implement slope stabilization strategies, sustainable environmental management, and targeted reforestation in the areas most affected by landslides and erosion in Uvira.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study demonstrates the strong correlation between landslides and riverine flooding in Uvira, DRC. Using historical data, field observations, and satellite imagery, key zones vulnerable to ground movements between 2020 and 2021 were identified, along with primary triggers and impacts particularly flash floods.\u003c/p\u003e \u003cp\u003eWatersheds of rivers such as Mulongwe, Kavimvira, Kabindula, and Kalimabenge are highly exposed due to sediment accumulation and landscape degradation. Intense rainfall events, combined with anthropogenic pressures (deforestation, slope farming, charcoal production, and material extraction), have amplified soil instability and flood risks.\u003c/p\u003e \u003cp\u003ePrecipitation analysis confirms that landslides and floods are concentrated during heavy rain periods, especially in the Mitumba Mountains upstream. This highlights the urgent need for slope stabilization in critical zones.\u003c/p\u003e \u003cp\u003eThe mapping approach proved effective for risk identification and mitigation planning. Priority actions include reforestation, slope stabilization, watershed management, and regulated urban development. Further studies are needed to assess climate change impacts and integrate them into long-term resilience strategies.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank the CRH-Uvira team (Kahindo Ndjungu, Mwenyemali Banamwezi, Muzumani Risasi, Maliyamungu Makubuli, Muhemura Francoise) for field access support. Appreciation also goes to Andrew Cohen, Pierre-Denis Plisnier, and Jean-Claude Maki Mateso for their valuable inputs and discussions on the early version of this MS.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cul\u003e\n \u003cli\u003eTKP: study design, data collection/analysis, manuscript drafting and final validation;\u003c/li\u003e\n \u003cli\u003ePMM: Study supervision, methodological support, result interpretation, initial draft;\u003c/li\u003e\n \u003cli\u003eSFB: Objective formulation, data analysis, technical review, manuscript refinement;\u003c/li\u003e\n \u003cli\u003eBM: Data interpretation and writing support;\u003c/li\u003e\n \u003cli\u003eSRR: Field data collection, methodology and analysis sections;\u003c/li\u003e\n \u003cli\u003eBP: Initial data analysis and manuscript drafting;\u003c/li\u003e\n \u003cli\u003eDMC: Scientific guidance, study design, data collection/analysis, manuscript drafting, content revision.\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp\u003e\u003cstrong\u003eFunding and/or Conflicts of Interest / Competing Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003cbr\u003e\u0026nbsp;The Center for Hydrobiology Research provided logistical support by supplying drinking water during the fieldwork and helping us access the study sites. No direct financial funding was received for this research.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of Interest / Competing Interests\u003c/strong\u003e\u003cbr\u003e\u0026nbsp;The authors declare that they have no financial or personal conflicts of interest related to this study.\u003c/p\u003e"},{"header":"References","content":"\u003cp\u003eAkilimali, J. I., Wasso, D. S., Baenyi, P., \u0026amp; Bajope, J. B. (2017). Caract\u0026eacute;risation des syst\u0026egrave;mes de production porcine de petits exploitants dans trois zones agro-\u0026eacute;cologiques du Sud-Kivu en R\u0026eacute;publique D\u0026eacute;mocratique du Congo. \u003cem\u003eJournal of Applied Biosciences\u003c/em\u003e, \u003cem\u003e120\u003c/em\u003e, 12086\u0026ndash;12097.\u003c/p\u003e\n\u003cp\u003eAmanejieu, A. 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(2021). \u003cem\u003eD\u0026eacute;veloppement de m\u0026eacute;thodes de tra\u0026ccedil;age s\u0026eacute;dimentaire pour quantifier l\u0026rsquo;impact des mines de nickel sur l\u0026rsquo;hyper-s\u0026eacute;dimentation des rivi\u0026egrave;res et l\u0026rsquo;envasement des lagons de Nouvelle-Cal\u0026eacute;donie\u003c/em\u003e.\u003c/p\u003e\n\u003cp\u003eSutton, G., \u0026amp; Berg, E. (1958). Seismological Studies of the Western Rift Valley of Africa. \u003cem\u003eTransaction, American Geophycal Union\u003c/em\u003e, \u003cem\u003e39\u003c/em\u003e, 474\u0026ndash;481.\u003c/p\u003e\n\u003cp\u003eVincens, A. (1989). Paleoenvironnements Du Bassin Nord-Tanganyika ( Zaire , Burundi , Tanzanie ) Au Cours Des 13 Derniers. \u003cem\u003eReview of Paleobotany and Palynology\u003c/em\u003e, \u003cem\u003e61\u003c/em\u003e, 69\u0026ndash;88.\u003c/p\u003e\n\u003cp\u003eZana, N. (1977). \u003cem\u003eThe sismicity of the Western Rift Valley of Africa and related problems\u003c/em\u003e.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"landslides, flood risk, river, sedimentation, climate resilience","lastPublishedDoi":"10.21203/rs.3.rs-8658281/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8658281/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe city of Uvira faces increasing risks of landslides and flooding, driven by both human activities (deforestation, unsuitable and intensive agriculture practices) and natural factors (steep slopes, heavy rainfall). A combined analysis of field data and Google Earth satellite imagery identified 221 events related to landslides and erosion, including 125 landslides (56.56%), 82 cases of gully erosion (37.10%), and 14 instances of bank erosion (6.33%). Intense rainfall such as the 114 mm recorded over 9 hours in April 2020 has intensified these hazards, leading to sudden floods. Between 2020 and 2021, human activities like construction in vulnerable areas and slope deforestation further amplified the risks. At least 2,707 riverside dwellings are exposed to sediment deposits. Three severely degraded zones have been identified as priorities for reforestation: (1) from Kaala stream to the right slopes of the Mulongwe River, (2) from Kakungwe stream to Ruzozi stream, and (3) the area surrounding Kalundu port. Recommended interventions include mountain stabilization, reforestation of degraded areas, and relocation of buildings away from high-risk zones.\u003c/p\u003e","manuscriptTitle":"Integrated assessment of landslide effects triggered by the 2020-2021 rainfall in Uvira Territory, on the northwestern shore of Lake Tanganyika","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-22 08:35:25","doi":"10.21203/rs.3.rs-8658281/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"7a2728bb-081b-4726-aaa5-97d50459b7af","owner":[],"postedDate":"January 22nd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":61560448,"name":"Environmental Engineering"}],"tags":[],"updatedAt":"2026-01-26T10:47:15+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-22 08:35:25","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8658281","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8658281","identity":"rs-8658281","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}