MONITORING THE CHANNEL PROCESS OF A STREAM RESTORATION PROJECT IN AN URBANIZING WATERSHED: A CASE STUDY OF KELLEY CREEK, OREGON, USA

摘要

Pacific Northwest (PNW) streams in the United States were impacted by the 20th century development, when removal of instream structure and channelization degraded an aquatic habitat. The lower Kelley Creek in southeast Portland, USA was channelized during the 1930's Works Progress Administration (WPA) projects. Stream restoration reintroduced pool-riffle sequences and heterogeneous substrates to protect salmonids while mitigating impacts from flooding. We investigated whether the restored pool-riffle morphology changed substantially following effective discharge events. We examined channel forms for four reaches representing three time periods-pre-development (two reference reaches), development and restoration. We conducted thalweg profiles, cross-sections and pebble counts along the reaches to examine how channel geometry, residual pool dimensions and particle size distribution changed following effective discharge events. The effective discharge flows altered the restoration reach more substantially than the reference reaches. The restoration reach decreased in median particle size, and its cross-sectional geometry aggraded near its margins. However, the residual pool morphology remained in equilibrium. Richardson Creek's reference reach degraded at the substrate level, while Kelley Creek's reference reach remained in equilibrium. The restoration reach's aggradation may have resulted from sedimentation along the nearby Johnson Creek. In contrast, Richardson Creek's degradation occurred as upstream land use may have augmented flows. Stream channels with low gradient pool-riffle morphologies are ideal for salmonid spawning and rearing and should be protected and restored within urban corridors. The findings of our study suggest that the connectivity of streams and the dynamic fluvial geomorphology of stream channels should be considered for stream restoration projects in humid temperate climates.