Gross Reservoir, Reservoir in Boulder County, Colorado

Gross Reservoir is a 440-acre reservoir in Boulder County, Colorado, held by a 340-foot concrete dam on South Boulder Creek. It also receives water from the western side of the Continental Divide through the Moffat Tunnel. The dam's grey face has vertical seams and panel lines, with a railing and a red metal gantry along the top.

Denver Water built this reservoir in 1954, naming it after Dwight D. Gross, a former chief engineer of the utility. The facility was created to provide reliable water storage for the region's growing communities.

Gross Reservoir is a Denver Water facility where fishing, hiking, canoeing and kayaking are allowed, but swimming and wading are not, and only non-motorized boats may be launched. The reservoir's expansion has drawn lawsuits from environmental groups over the Clean Water Act, Endangered Species Act and National Environmental Policy Act, and Boulder County approved the project in 2021 with a $12.5 million mitigation deal.

The reservoir is open for boating and paddling activities from late May through late September, with parking areas at North Shore. Visitors should be prepared for the high elevation and changing mountain weather conditions.

Gross Reservoir's dam, 340 feet high, will rise another 131 feet under the expansion approved in 2021, making it the tallest dam in Colorado. The reservoir holds 440 acres of water at a spillway elevation of 7,225 feet. The coldest temperature recorded here was −24 °F on February 2, 2011.