Union Reservoir is a 736-acre body of water providing recreational opportunities all-year round for visitors and residents alike. Originally called Calkins Lake, the reservoir was carved out during the last glacial age and is one of only a few natural lakes in Colorado. You can swim, sunbathe, fish, walk, sail, kayak or stand-up paddle board at Union. The Reservoir is located approximately 7 miles west of I-25 and only 3 miles from Downtown Longmont, making it a central hub for outdoor activities on your next trip to Longmont! There is a daily admission fee to get into Union Reservoir.