Longmont Reservoir, located in Lyons, Colorado, is a concrete gravity dam built in 1900 primarily for water supply purposes. With a height of 30 feet and a storage capacity of 75 acre-feet, this reservoir serves as a key water resource for the surrounding area. Situated along the North St. Vrain Creek, the reservoir covers an area of 5 acres and has a drainage area of 108 square miles, with a maximum discharge capacity of 3000 cubic feet per second.

Despite its age, Longmont Reservoir has been well-maintained and is in satisfactory condition according to its last inspection in 2015. Classified as a low hazard potential structure, the reservoir has a regular inspection frequency of every 6 years. While it lacks a spillway, the dam's risk assessment is considered high, indicating the importance of ongoing monitoring and risk management measures. With its strategic location and vital role in water supply, Longmont Reservoir is a significant reservoir for water resource and climate enthusiasts to study and appreciate in the Boulder County region.