The Naches River drains some of the highest and wettest terrain in the Yakima Basin. The Naches and Tieton Rivers are considerably steeper and naturally more confined than the most of the Upper Yakima River. Nevertheless, the predevelopment floodplain reaches of the mainstem Naches River and its tributaries provided a labyrinth of channels surrounded by extensive riparian forests that maintained cool summer temperatures and habitat complexity for all life stages of salmon and steelhead (Kinnison and Sceva 1963; Snyder and Stanford 2001; Stanford et al. 2002). In an unregulated condition, the flow of the Naches River would be characteristic of snowmelt-dominated systems where discharge peaks between May and June concurrent with melting snow, and reaches base flow in August and September. Late autumn rainfall and minor snowmelt would augment summer base flow, with occasional winter high water events under the influence of Chinook winds. Above the confluence with the highly regulated Tieton River, the Naches River and its tributaries have some of the most natural flow regimes in the Yakima Basin. Below the Tieton confluence, the Naches River floodplain widens, although a highway and other structures have isolated the river from part of the active floodplain. Rimrock Reservoir blocks passage to upstream habitat and has a major effect on flows in the Tieton River and lower Naches mainstem.