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

Urban stream rehabilitation projects commonly include log placement to esta blish 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 use d in-stream log placement as the primary strategy for achieving project goa ls; none included systematic watershed-scale rehabilitation measures. The e ffectiveness of LWD in these projects was evaluated by characterizing physi cal stream conditions using common metrics, including LWD frequency and poo l spacing, and by sampling benthic macroinvertebrates. In all project reach es where pre-project data existed, pool spacing narrowed after LWD installa tion. All project sites exhibited fewer pools for a given LWD loading, howe ver, 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 tha t, 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 comm ensurate improvement in biological conditions. (C) 2001 Elsevier Science B. V. All rights reserved.