Expanding catchment-scale hydrologic restoration in suburban watersheds via stream mitigation crediting—A Northern Kentucky (USA) case study

Abstract The need for hydrologic restoration is well established in stream ecosystems across the world; however, available funding for catchment-scale restoration typically falls far short of what is required to produce in-stream results. Hydrologic restoration can be particularly important for improving stream integrity in urban watersheds, but implementation can be challenging due to high property values and limited space for retroactive stormwater control measures. This Northern Kentucky (USA) case study summarizes how stormwater mitigation interventions could be, and have already been, used to credit stream mitigation projects via conventional US Army Corps of Engineers crediting protocols. Hydrologic restoration can generate stream mitigation credits by directly improving the flow class and/or by indirectly improving the habitat quality. For example, a stormwater intervention could create a shift from an ephemeral to intermittent flow class, while at the same time facilitating greater substrate stability, lower embeddedness, and other geomorphic improvements, that subsequently improve the categorical habitat rating. The ecological lift of such hydrologic interventions could be further expanded via concurrent in-stream mitigation measures such as re-establishing a jurisdictional stream in place of a drainage ditch or installing habitat structures such as toe wood and log steps, among other activities. Such process-based hydrologic restoration is consistent with the goals of the Clean Water Act and has the potential to be more beneficial to greater portions of stream networks and greater numbers of stakeholders than conventional habitat restoration alone.