[2][3] The entire river defines the border between northern Santa Barbara County and southern San Luis Obispo County, up to the Sisquoc River, with a major bridge on Highway 101 passing over it.

Twitchell Dam was built by the United States Bureau of Reclamation and provides flood control and groundwater recharge of the aquifer.

[3] During much of the year, the Santa Maria River has very little water, but it can swell greatly during winter storms.

[3] The section adjacent to the city of Santa Maria has experienced encampments of unhoused people.

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