Union Reservoir is a 736 acre body of water providing many outdoor recreational activities. 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. In 1903, the Union Ditch Company began drilling a tunnel to release water into the St Vrain River. According to Colorado water law, that made Union a true reservoir. Today, visitors to Union Reservoir can enjoy fishing, swimming, wakeless boating, camping, windsurfing and picnicking. T