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

Abstract

Urban stream rehabilitation projects commonly include log placement to establish the types of habitat features associated with large woody debris (LWD) in undisturbed streams. Six urban in-stream rehabilitation projects were examined in the Puget Sound Lowland of western Washington. Each project used in-stream log placement as the primary strategy for achieving project goals; none included systematic watershed-scale rehabilitation measures. The effectiveness of LWD in these projects was evaluated by characterizing physical stream conditions using common metrics, including LWD frequency and pool spacing, and by sampling benthic macroinvertebrates. In all project reaches where pre-project data existed, pool spacing narrowed after LWD installation. All project sites exhibited fewer pools for a given LWD loading, however, than has been reported for forested streams. In project reaches where the objective was to control downstream sedimentation, only limited success was observed. At none of the sites was there any detectable improvement in biological conditions due to the addition of LWD. Our results indicate that, although LWD projects can modestly improve physical habitat in a stream reach over a time scale of 2–10 years, they apparently do not achieve commensurate improvement in biological conditions.