Southern Steelhead trout ("Oncorhynchus mykiss") are thought to represent about 1% of the historic population and most of the Santa Maria River run is thought to have continued up the Sisquoc River to spawn. Major dams on the Santa Ynez, Ventura, and Santa Clara Rivers prevent steelhead from accessing hundreds of miles of habitat and have drastically reduced the steelhead runs. The construction of Twitchell Dam on the Cuyama River in the late 1950s blocked at least 264 miles of stream and more than 60% of the Santa Maria River watershed and tributary habitat found upstream. This makes the Sisquoc River, which remains undammed, an important stream for surviving southern steelhead populations.