Supply and Demand Assessment (SDA) – Santa Maria River

The Santa Maria and Cuyama Rivers Watershed is a major coastal basin on California’s Central Coast, draining approximately 1,828 square miles. The watershed includes the Santa Maria River, a largely free-flowing system fed by 19 principal tributaries, including the Sisquoc River, Tebusquet Creek, Orcutt Creek, La Brea Creek, and Manzana Creek. From the confluence of the Sisquoc and Cuyama Rivers, the Santa Maria River flows west through the City of Santa Maria before reaching the Pacific Ocean. The elevation range of the watershed extends from sea level near the outlet to over 2,600 meters (8,500 feet) in the eastern highlands near the Los Padres National Forest. The region’s Mediterranean climate features wet winters and dry summers, with an average annual precipitation of about 14 inches. Land cover is dominated by shrub/scrub (55%) and grassland/herbaceous (22%) vegetation, with additional areas of cultivated crops (5%), evergreen forest (6%), mixed forest (5%), and developed lands (4%).

The watershed provides important habitat for both resident rainbow trout and anadromous steelhead. The Sisquoc River is a key spawning and rearing tributary for steelhead and joins the flow-regulated Cuyama River in the northern portion of the basin. Twitchell Dam, located on the Cuyama River, is operated primarily for groundwater recharge and flood control, which significantly limits surface flows through the Santa Maria River—flows that are critical for steelhead migration between the ocean and upstream habitats. Although the Santa Maria River is naturally intermittent, dam operations have further reduced the frequency and duration of fish-passable flows. In a recent U.S. Supreme Court decision, operations at Twitchell Dam were determined to be subject to the Endangered Species Act, requiring that dam management ensure protection of the endangered steelhead population.

In addition to hydrologic barriers, the Santa Maria River is listed as impaired by several pollutants, including metals, pesticides, nutrients, toxicity, and pathogens. These impairments are primarily attributed to agricultural and urban runoff, as well as stormwater discharges.