Water flowing from Helena Valley Tunnel at mile 2.8 discharges into the 300-cubic-foot-per-second Helena Valley Canal. The Canal is 31.7 miles long, with 10.2 miles unlined and 21.5 miles lined. at mile 11, the canal discharges into a 10,500-acre-foot-capacity regulating reservoir. Helena Valley Dam, which forms the reservoir, is an earthfill structure 91 feet high, with a crest length of 2,650 feet. A 600-foot-long dike extends from the left abutment of the dam.
The reservoir has 5,900 acre-feet of conservation storage for irrigation and municipal water. A outlet built into the dams supplies municipal water to the city of Helena. The 350-cubic-feet-per-second outlet to the Helena Valley Canal is located in the dike. Between the reservoir and mile 17, the canal provides facilities for future supplemental service to 10 irrigation ditches which have been diverting water from Prickly Pear Creek; between miles 17 and 22, facilities are provided for supplemental service to existing ditches diverting water from Prickly Pear and Tenmile Creeks. The canal terminates at Lake Helena, which occupies the lower part of the valley.
The reservoir area upstream from the dam has been earth blanketed to reduce seepage from the reservoir, and pressure relief wells have been installed near the downstream toe of the dam. These measure have not been totally effective and other measures are being considered to relieve the uplift pressures on the downstream toe of the dam.Foundation consists of an alternating sequence of tuff, tuffaceous sandstone, and conglomerate beds. These sediments are part of the Bozeman beds of Paleocene to the Miocene Age. Numerous faults and fractures cut these strata beneath and adjacent to the dam. One fault with a displacement of about 200 feet trends northward through the right abutment of the dam. Some fault and fracture zones have been healed by clastic material intruded during earthquake. Since initial filling of the reservoir in late 1959, excessive seepage and hydrostatic pressures under the dam have been encountered.