Allen Lake, located in Boulder, Colorado, is a privately-owned reservoir with a primary purpose of irrigation. Built in 1928, the earth dam stands at 24 feet high and spans 1525 feet in length, providing a maximum storage capacity of 865 acre-feet. The reservoir covers a surface area of 36 acres and is fed by Left Hand Creek-OS, with a significant hazard potential due to its uncontrolled spillway and satisfactory condition assessment.

The dam at Allen Lake is regulated by the Colorado Division of Water Resources, with state permitting, inspection, and enforcement in place to ensure its safety and compliance. The dam is equipped with a slide (sluice gate) outlet gate and has a moderate risk assessment rating. The surrounding area is under the jurisdiction of Longmont city and falls within the congressional district of Joe Neguse (D). Water resource and climate enthusiasts can appreciate the historical significance and functional importance of Allen Lake in sustaining agricultural activities and providing water for various purposes in the region.