Holter Dam is a four-unit hydroelectric plant on the Missouri River, about 43 miles northeast of Helena, Mont. The units have a total generating capacity of 48 megawatts. (One megawatt can satisfy the average energy needs of 750 households.) Holter Dam, which began operation in 1918, is 1,364 feet long and 124 feet high. Its reservoir is 27 miles long and has a storage capacity of 240,000 acre-feet. The dam is classified as a “run-of-river” project because it can generate electricity using the water that flows down the river, without the need to store additional water supplies. Click here for more photos.