Briones Reservoir, water storage reservoir in Contra Costa County, California, USA

Briones Reservoir is an open-cut terminal storage reservoir in the Briones Hills of Contra Costa County, California, owned and operated by the East Bay Municipal Utility District. It is impounded by Briones Dam, an earthen dam completed in 1964. The reservoir is the largest of EBMUD's five East Bay terminal reservoirs, with a capacity of 60,510 acre-feet, and its irregular branching shape fills a valley partly submerged from the former Bear Creek drainage.

The reservoir was built in the 1960s by the East Bay Municipal Utility District, which was established in 1923 to supply water to growing communities around the San Francisco Bay. It was one of several projects the district undertook to secure a reliable water supply for the region over the long term.

The reservoir's name comes from the Briones Hills and Briones Regional Park, which border it on the east. The 14-mile Bear Creek and Oursan trail loop is open to hikers and horseback riders, but mountain biking is not permitted. Dogs are allowed only on the Oursan Trail segment and must be leashed.

Hikers need a Trail Use Permit from the East Bay Municipal Utility District, obtainable online before setting out. The 14-mile Bear Creek and Oursan trail loop starts at the Bear Creek Staging Area inside Briones Regional Park. Mountain bikes are barred; dogs are allowed only on the Oursan segment and must be leashed.

Swimming is not allowed in the reservoir even though it is a large body of open water, because it remains an active drinking water source for the region. The dam crossing on the loop trail gives one of the few spots where the full shape of the water basin becomes visible in a single view.