Madison Dam is a four-unit hydroelectric plant on the Madison River at the head of Bear Trap Canyon, about 10 miles north of Ennis, Mont. The units have a total generating capacity of 8 megawatts. Madison Dam, which began operation in 1906, is 257 feet long and 35 feet high. Its reservoir is 2.6 miles long and has a storage capacity of 27,200 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 information.