The Cedar River is the largest tributary to Lake Washington and drains an elongated basin of 188 square miles that extends from the crest of the Cascade Mountains to the southern shore of Lake Washington in the City of Renton. The Cedar River was rerouted from the Black River to Lake Washington in 1916. The upper two-thirds of the subarea is owned and managed by the City of Seattle and supplies drinking water to two-thirds of Seattle and its regional customers. The Cedar River Municipal Watershed is almost entirely coniferous forest, and its management is governed by the Cedar River Watershed Habitat Conservation Plan. The lower third of the Cedar River subarea below the Landsburg Diversion Dam includes 21 miles of mainstem river and 15 tributaries, and drains a 66-square-mile area. The lower Cedar River mainstem and four main fish-bearing tributaries provide the majority of the current spawning habitat for chinook and sockeye salmon and steelhead trout in the WRIA 8 system as well as significant spawning and rearing habitat for coho salmon and cutthroat trout. The four main tributaries for Chinook are: Lower Rock Creek, Walsh Lake Diversion, Peterson Creek, and Taylor Creek. Most of the lower Cedar River subarea is rural and forested, except for the cities of Renton and Maple Valley, where the subarea is urbanized.

The Cedar River/South Lake Washington population (Cedar) spawns in the Cedar River and in some of its tributaries (Taylor, Peterson, and lower Rock creeks and the Walsh Lake Diversion Ditch) between September and November. Juveniles, after emerging from the gravel, migrate into the south end of Lake Washington either as fry or fingerlings between February and June. While in the lake, the juveniles rear and migrate north along the shoreline in shallow habitats with gentle gradient and small substrates (Tabor and Piaskowski 2002). They also utilize small creek mouths (Tabor et al. 2003). Once they become larger (May or June), most of the juveniles move offshore and prepare to exit WRIA 8 through the Ship Canal and Hiram M. Chittenden Locks (Locks). Chinook smolts typically enter saltwater between May and July (DeVries 2001; DeVries 2002). They then spend time rearing in the marine nearshore environment of WRIA 8 and other areas of Puget Sound before migrating to the larger ocean.