Lake Washington, the largest lake in Washington State west of the Cascades, has a surface area of 34.6 mi2 (89.6 km2), with a length of 18.6 mi (30 km) (north-south) and an average width of 1.5 mi (2.4 km). The mean and maximum lake depths are 108 ft and 220 ft (33 and 67 m), respectively. Lake Washington receives inflow from the Cedar and Sammamish rivers, as well as numerous creeks such as Kelsey, Thornton, Juanita, McAleer, Lyon and May. The lake drains through the Ship Canal to Puget Sound. The lake has over 80 miles of lake shoreline and almost all of the area surrounding the lake is developed for residential and commercial uses and, as such, the majority of the lake shoreline (>82%) is armored (Fresh and Lucchetti 2000; Weitkamp et al. 2000). The shoreline also contains numerous overwater structures (>2,700; Kerwin 2001). Lake Washington is used by all three populations in WRIA 8 as a migratory and rearing area.

Shoreline habitat conditions are important for juvenile Chinook using Lake Washington, particularly those from the Cedar River population. Degraded shoreline conditions resulted originally from lowering the lake water surface levels when the Locks were constructed. Further adverse impacts are a result of urbanization and the majority of the lake shoreline is now used for urban residential uses. Landscaped yards and bank armoring (bulkheads and riprap) have reduced the amount of riparian vegetation and woody debris contributed to the lake. Armoring has also modified substrates in shallow areas due to prevention of bank erosion and altering sediment dynamics at the water-land interface. Overwater structures have increased shading and segmented the lake shoreline and nearshore areas, affecting aquatic organisms such as benthic invertebrates, a prey item of juvenile Chinook (Warner and Fresh 1998; Kahler et al. 2000; Koehler 2002). Docks and piers also affect the migration movements of juvenile Chinook. These alterations have reduced the amount and quality of shallow water habitat, an important habitat for rearing juveniles (Tabor and Piaskowsi 2002; Tabor et al. 2003).