Sediment Budget of a Mixed-Use, Urbanizing Watershed

Abstract

The Issaquah Creek basin is an urbanizing watershed of 144 square km in western Washington, where sediment aggradation of the main channel and delivery of fine sediment into a large downstream lake have raised serious local and regional concerns. The basin has many water quality problems that may be associated with erosion occurring throughout the watershed. The water quality of Lake Sammamish, located at the outlet of the basin, is degrading with time, and fine sediment entering the lake from the watershed is a likely source of phosphorus during periods of lake anoxia. Additionally, there are flood-prone areas in the basin, particularly along the mainstem of Issaquah Creek in downtown Issaquah, that may be exacerbated by channel aggradation and subsequent reductions in flow conveyance. Another potential in-channel concern is the effect of fine sediment on spawning gravel for the salmon species that occupy Issaquah Creek. A sediment budget was constructed for this mixed-use, rapidly developing watershed to evaluate the relative effects of land-use practices, including urbanization, on watershed-scale sediment supply and delivery. It also can be used to identify the major sources of sediment, and thus guide the most effective remedial measures.