Changes in stream ecosystem structure as a function of urbanization: Potential recovery through stream restoration

Abstract

I documented reach scale changes in the physical structure of 12 stream channels in the summer months of 2006, comparing four small streams draining forested catchments with eight streams from developed watersheds of similar catchment size. Study sites in four of the urban streams are within recently implemented natural channel design restoration projects. To assess whether restoration projects increase stream habitat and flow heterogeneity and increase water exchange with floodplain and hyporheic sediments I compared reach-scale geomorphic (e.g. slope, cross section, degree of incision, variation in water depth) and hydrologic (e.g. transient storage volume (TS), surface-water groundwater exchange, fine scale variation in velocity) features of each stream. I used ArcGIS to compile watershed maps and to produce detailed maps of reach habitat for each stream, and the hydrologic model OTIS-P to estimate transient storage from field rhodamine releases. Minimally impacted reaches were found to have shallower average depths with a greater variation in depth than urban or restored stream reaches. Streams restored to provide habitat had the lowest flow habitat heterogeneity of the three stream classes. Channel incision was the only physical channel feature for which the urban restored streams were more similar to the forested streams than the urban degraded condition. Surprisingly, I was unable to detect significant differences in transient storage volume or hyporheic exchange between our three stream classes. My results suggest that restoration designs are placing inadequate attention on recreating the physical template seen in less degraded streams.