Geomorphic impacts of large wood restoration in an urban Colorado stream

Abstract

The effects of urbanization on river systems lead to degradation and simplification, reducing beneficial ecosystem functions. Prior to urbanization, many rivers in the Mountain West were geomorphologically complex, hydrologically interconnected, interspersed with large wood, and connected to their floodplains, providing numerous ecosystem services that led to functional resilience. Process-based restoration techniques are being implemented to reintroduce and support these lost functions in both rural and urban areas as an alternative to form-based restoration. Minimal research around process-based techniques has been done in urban systems and understanding how this approach impacts geomorphic response in varied biomes provides practitioners a basis to evaluate future projects. The objective of this study is to analyze the effects of a large wood process-based restoration in an urban Colorado corridor (Cache la Poudre River, Fort Collins, CO) by monitoring changes in 1) site erosion and deposition, 2) geomorphic unit heterogeneity, 3) large wood volume and porosity, 4) evaluate whether monitoring geomorphic units is an adequate metric of project success, and 5) to compare restoration techniques throughout the site by analyzing sediment changes. Additionally, I review and recommend monitoring methodologies and discuss how water policy affects restoration in Colorado. The site restoration included reconnecting the floodplain, reconstructing site bathymetry, and adding large wood to both the floodplain and active channel. Following restoration construction and one runoff season, digital elevation models were analyzed to delineate geomorphic units and compare restoration approaches. iPad LiDAR was collected at six constructed large wood structures to determine volumetric and porosity changes. Sediment analysis shows net aggradation of sediment around structures, supporting the project goal of mitigating an impending head cut. Large wood analysis results varied based on structure location within the channel and with respect to other large wood structures. The accumulation and dispersion of wood throughout the site was captured by newly formed islands induced by restoration. Both the largest patch index and patch density heterogeneity metric either stayed consistent or increased after the runoff season. This restoration project demonstrates to policymakers the geomorphological, ecosystem, and social benefits while showcasing the low-risk nature of carefully designed process-based restoration using large wood. Greater utilization of large wood in urban restoration more broadly supports the ecosystem services that benefit the communities that live in and around the river.