Supply and Demand Assessment (SDA) – Putah Creek

The Putah Creek watershed lies within California’s Coast Ranges, draining roughly 600 square miles across Lake and Napa counties before reaching Lake Berryessa—one of the largest reservoirs in the state. Originating at springs on the eastern side of Cobb Mountain, the creek descends steeply through rugged terrain formed by complex geology, including volcanic rock and serpentine soils. This upstream portion of the watershed is characterized by oak woodlands, chaparral, and steep canyons, and plays a critical role in capturing runoff that feeds Lake Berryessa, which in turn supports the Solano Project and regional water supply. Downstream of Monticello Dam, Putah Creek continues through Yolo and Solano counties, transitioning from a regulated flow corridor into an ecologically significant riparian system that supports wildlife habitat, agriculture, and recreation before ultimately reaching the Yolo Bypass and the Sacramento River. Seasonal flows are heavily influenced by both natural precipitation and regulated releases from Monticello Dam, and land use across the watershed includes rangeland, recreation, and scattered rural development.

Hydrologic conditions in Putah Creek have been significantly altered by the Solano Project, especially the construction of Monticello Dam, which has changed the timing and volume of flows downstream. Although the dam provides important water supply and flood control benefits, it has also contributed to ecological challenges, including the disconnection of historic floodplains, reduction in instream gravel for spawning, and suppression of natural flow variability. Despite these impacts, the watershed continues to support cold freshwater habitat and remnant populations of native fish species, such as rainbow trout. Regional partners, including the Upper Putah Creek Stewardship and other local groups, are actively engaged in habitat restoration, invasive species management, and community-based monitoring efforts to improve conditions in the watershed and enhance long-term resilience.