EFFECTS OF URBANIZATION ON A LOWLAND STREAM IN WESTERN WASHINGTON

The effects of nonpoint source pollution due to urbanization and stormwater runoff on the structure and function of an urban stream in western Washington, Kelsey Creek, were investigated in comparison to a nearby, control stream, Bear Creek. The results of the study indicated that changes in stream hydrology and geomorphology were the critical agents in causing alterations in the ecological structure of the stream.;Changes in the chemical environment appeared to be less important. The annual export of particulates was approximately 3 times greater in Kelsey than in Bear. More than half of the annual transport of suspended solids occurred in less than 10 percent of the time. The association of lead with the suspended load suggested that it (and similar toxicants) may be unavailable to the biota much of the time.;A variable pattern of food resource availability in both space and time appeared due to bottom scouring and rapid replacement by primary producers. A decrease in the efficiency of organic carbon utilization (as measured in a mass continuity budget) was due more to downstream transport of available carbon, the lack of in-stream storage sites and lower faunal diversity, rather than decreased quality or quantity of food resources.;Relatively undisturbed sections of the urban stream system appeared to provide refugia for organisms unable to survive in the more disturbed areas, while the disturbed sections supported dense populations of a few tolerant species that had adapted to the variable conditions of habitat and food supply.;Important changes in the hydrologic regime included increased peak flows, decreased minimum flows and an acceleration in hydrograph rise and fall. Constriction of the channel resulted in a substantial reduction in stream surface area, the space available for biotic activity. The number and size of debris dams, which provide storage sites and protected habitat, were greatly diminished. Higher stream power resulted in increased fluvial transport, channel erosion and bed instability.;Future management strategies for urban streams should be based on a clear designation of public use functions since some functions (e.g. maintenance of a natural diverse fauna and surface drainage) appear to be incompatible.