Effects of Urbanization on the Biological Integrity of Puget Sound Lowland Streams: Restoration with a Biological Focus

Abstract

Rapid urbanization threatens the condition of streams and rivers across the Pacific Northwest. Efforts to protect and control urban streams have traditionally focused on physical channel conditions and chemical water quality. This study is designed to use biological attributes of these streamsmeasured with the multimetric index of biological integrity (B-IBI) based on benthic macroinvertebratesto assess stream condition. Between 1997 and 1999, invertebrate samples were collected from 45 sites in second and third order streams of the Puget Sound lowlands of Western Washington. The locations of 14 sites were chosen to evaluate the placement of large woody debris (LWD) as a restoration technique on five streams. Urbanization was characterized by a 1998 satellite land cover classification and was measured across several spatial scales. The relationships among metrics of the B-IBI and stream substrate and hydrologic features were also evaluated at a sub-set of sampling sites. B-IBI declined as urban land cover increased both across the entire basin and within riparian zones. The effectiveness of localized patches of riparian corridor in maintaining biological integrity varied as a function of the percentage of urban land cover in the sub-basin. Channel roughness and hydrologic flashiness were also correlated with B-IBI. Below the five restoration projects, there was no overall detectable improvement in B-IBI. The aquatic biota is sensitive to a variety of urban impacts controlled over both large and small spatial scales. Restoration efforts that deal with only one local impact type without addressing larger scale issues are unlikely to successfully restore the biota of degraded streams.