Evaluating the performance of instream structures for a stream restoration project in Colorado

Metals pollution and channel disturbance associated with historical mining, land use, and water development degraded aquatic and riparian habitat along the upper Arkansas River near Leadville, Colorado. Stream restoration was conducted for an 8 km reach to improve aquatic habitat and increase trout populations. Instream structures were prescribed to stabilize streambanks, create diverse stream morphology, and provide overhead cover, refuge, spawning, and overwinter habitat for trout. At least 90% of all structures were expected to be stable and functional three years after implementation. The objectives of this study were to investigate structure performance by (1) evaluating the integrity and function of instream structures and (2) evaluating the change in residual pool depths (RPD). Annual surveys utilized a rapid assessment procedure to qualitatively rank integrity, erosion, and deposition at each structure (n = 137). Rankings were investigated with ordinal regression to determine if performance varied by structure type and year. Longitudinal profile surveys were conducted annually and used to estimate RPD for 86 pools. The change in RPD was investigated with repeated measures ANOVA to determine if RPD varied between structure types and changed over time. Results suggest that some structures were more prone to failure, with higher rankings observed for boulder toe, log vanes, log toe, and boulder vanes. Pool depths increased during construction, decreased following the first runoff, and then remained relatively stable in subsequent years. Understanding the performance of instream structures from this case study will help inform the design, evaluation, and expectations for future stream restoration projects.