Large Woody Debris in Urban Streams of the Pacific Northwest

Large woody debris (LWD) performs key functions in undisturbed streams that drain lowland forested watersheds, including dissipation of flow energy, stabilization of bedforms and channel banks, entrapment of sediment, and formation of pools. These functions vary between individual channels, however, depending on the size and morphology of the stream, which in turn depend on climate, watershed size, valley slope, geologic substrate, and relative inputs of water and sediment. Loss of LWD will alter channel form and processes, yielding greater sediment fluxes, more rapid bank erosion and incision, and loss of heterogeneity in bed morphology. Just as LWD is ubiquitous in undisturbed lowland streams of the Pacific Northwest, it is significantly depleted in urbanized systems where it is lost through washout, downcutting, and direct removal. Given the dramatic changes in runoff processes and sediment delivery that typify urban watersheds, we doubt that simple reintroduction of LWD will fully restore the lost functions of urban streams. Instead, projects that replace LWD may be best suited to recover a more limited set of rehabilitation goals; they are also necessary components of more comprehensive restoration efforts in once-forested lowland landscapes. Project designs range from the visually pleasing to the hydraulically engineered, but most approaches nonetheless fail rapidly in a dynamic stream environment. Stable configurations of LWD are best recognized through the careful observation of form and riverine context in natural systems, where years of varying water and sediment discharges have obliterated all but the most stable arrangements. 1 Director, Center for Urban Water Resources Management, Department of Civil Engineering, University of Washington, Box 352700, Seattle, Washington 98195 Phone (206) 543-7923; Fax (206) 685-3836; Email [email protected] 2 Department of Geological Sciences, University of Washington, Seattle, WA 98195 3 King County Surface Water Management Division, Seattle, WA 98104