Holter Dam is a hydroelectric straight gravity dam on the Missouri River about 45 miles (72 km) northeast of Helena, Montana, in the United States. The dam, which was built between 1908 and 1918, is 1,364 feet (416 m) long and 124 feet (38 m) high. The reservoir formed by the dam, Holter Lake (also known as Holter Reservoir) is 25 miles (40 km) long and has a storage capacity of 243,000 acre-feet (300,000,000 m3) of water when full. The dam is a "run-of-the-river" dam because it can generate electricity without needing to store additional water supplies behind the dam.