Almaden Reservoir, reservoir in Santa Clara County, California

Almaden Reservoir is a man-made lake in the hills south of San Jose created by a dam built across Alamitos Creek. The earthen dam is about 110 feet (33 meters) high, and the water eventually flows through Guadalupe River toward San Francisco Bay.

Almaden Dam was built in 1936 across Alamitos Creek near the community of New Almaden, forming the reservoir. The earthen structure stands 110 feet (34 m) high and 500 feet (150 m) long, holding 250,000 cubic yards (190,000 m3) of material, with its crest at 615 feet (187 m) above sea level. Water released from the reservoir reaches San Francisco Bay via the Guadalupe River.

The reservoir takes its name from New Almaden, the mercury-mining community beside Alamitos Creek; the mines were worked from the 1840s and the area is now within Almaden Quicksilver County Park. Visitors today picnic, hike and ride horses on the park's trails above the water, and fishing is catch-and-release only.

Trails in the adjoining 4,163-acre Almaden Quicksilver County Park give access to the reservoir's shoreline; the reservoir itself is closed to swimming and boating. Fishing is catch-and-release only, and the state health office advises against eating any fish caught here. The dam lies near the former community of New Almaden, south of San Jose.

The California Office of Environmental Health Hazard Assessment has issued a "Do Not Eat" advisory for any fish caught in Almaden Reservoir because of elevated mercury levels. The contamination stems from historical mercury mining near New Almaden, and the advisory applies to all fish, not just certain species.