It is 92 miles (148 km) long,[3] flowing from east to west through the Santa Ynez Valley, reaching the Pacific Ocean at Surf, near Vandenberg Space Force Base and the city of Lompoc.

Instead, the river and mountains took the name of Mission Santa Inés (also spelled "Ynés" or "Ynéz" in New Spain), which was established in 1804 (dedicated to Saint Agnes of Rome, virgin and martyr).

West of Red Rock Canyon the river leaves Los Padres National Forest and its valley widens considerably.

Water from the lake is diverted into the Tecolote Tunnel, which passes south under the mountains to the Santa Barbara area.

Just west of Salsipuedes Creek the Santa Ynez River flows past the largest city in the valley, Lompoc.

A few miles west of Lompoc the river reaches the Pacific Ocean at a location known as Surf, where there is a beach and an Amtrak station.

While there is public access to Surf and the mouth of the Santa Ynez River, most of the land between Lompoc and the ocean is part of Vandenberg Space Force Base.

Water from Lake Cachuma is diverted into the Tecolote Tunnel, which passes south under the Santa Ynez Mountains.

These process changes have led to alteration of sedimentation and erosion patterns and converted wetland habitats to upland.

"[14] In the 1940s the Santa Ynez River was thought to have the largest run of steelhead (Oncorhyncus mykiss irideus) south of San Francisco Bay.

[15] Prior to the completion of Cachuma Dam in 1953, the steelhead run on the Santa Ynez River was estimated to be as high as 25,000 adults.

[16] Three decades earlier, in 1920, the Gibraltar Dam was built and blocked access to spawning in the upper watershed, so early twentieth century steelhead runs were likely much higher still.

[18] Genetic analysis of the steelhead in the Santa Ynez River watershed has shown them to be of native and not hatchery stocks.

The beavers may play a critical role for steelhead populations as their ponds replenish aquifers, allowing groundwater to recharge streams in dry summers, and provide perennial pools for oversummering trout smolts.

[28] Father Pedro Font, on the second de Anza Expedition in 1776, described the coastal Chumash women as wearing beaver capes.

[citation needed] The Santa Ynez River beaver were likely trapped out until re-introduction in the 1940s by the California Department of Fish and Game.