Onion Valley, located in Montrose, Colorado, is a privately owned dam with a primary purpose of irrigation. Completed in 1954, this earth dam stands at a height of 85 feet and has a hydraulic height of 105 feet. With a storage capacity of 10,380 acre-feet, the dam serves to regulate water from Iron Creek for agricultural use in the region.

The dam has a surface area of 340 acres and a drainage area of 22 square miles, with a maximum discharge capacity of 502 cubic feet per second. Despite its fair condition assessment, Onion Valley is classified as having a high hazard potential. The dam's spillway type is listed as 'None', with outlet gates consisting of a slide (sluice gate) and uncontrolled mechanisms.

Although the dam is not under the jurisdiction of the US Army Corps of Engineers, it is subject to state regulation and enforcement. With its strategic location and critical role in providing water for irrigation, Onion Valley is a significant structure that contributes to the water resource management in the region.