Effectiveness of large woody debris in stream rehabilitation projects in urban basins

Abstract

In-stream rehabilitation projects are commonly built in response to problems that result from both local sources and diffuse watershed degradation. Local problems, such as an improperly sized culvert, are relatively easily identified and corrected. Reversing the consequences of watershed degradation, such as channel widening and incision, is much more difficult if conditions that led to stream degradation remain unchecked. Despite this challenge, large amounts of money are being spent on in-stream projects in urban or urbanizing basins, because of numerous recognized problems on these streams, the interest of local communities in restoring the amenities these streams provide (Riley 1998, MacDonald 1995), and the relative ease and economy of site-specific in-stream work. This study investigates the effectiveness of one common technique, placement of in-stream large woody debris (LWD), to reverse local effects of watershed degradation in the absence of any systematic watershed-scale rehabilitation measures. To accomplish this, six stream rehabilitation projects in western Washington state that employ LWD were examined with the objective of answering the following questions: · Does in-stream placement of LWD produce physical channel characteristics typical of streams in less-disturbed watersheds? · Does biological integrity improve after LWD is added? · How can LWD project designs be improved? · Does watershed-scale disturbance, generally unaffected by LWD projects, extent an equivalent or greater effect on the physical and biological recovery of the channel than the local in-channel conditions that are addressed by the LWD?