The dam is 3,979 feet long and 94 feet high at its highest point, with four generating units inside that can produce 124 megawatts of electricity when all are operating. Potential floodwaters surging downstream on the Tennessee River collect in Lake Guntersville, then are gradually sent through the dam by carefully controlled releases. As water moves through the dam, it also generates hydroelectric power. Inside is a tunnel called a penstock, where water surges down and strikes the propeller blades of four turbines. The force spins shafts extending from the turbines into two generators, which transform this natural energy into hydroelectric power. High-voltage power lines then deliver the electricity to utility companies for distribution to businesses and homes in their service areas. Construction of Guntersville Dam and Reservoir began in 1935, and operation began in 1939. At full capacity, its four units generate 124 megawatts, enough to power 72,540 homes. The reservoir has 67,900 acres of water surface and 949 miles of shoreline. Main and auxiliary locks allow commercial and recreational vessels to pass from one reservoir to another, cranes help TVA move heavy equipment for maintenance, spillway gates move water through the dam for flood control, navigation, and cooling other power plants downstream, and the powerhouse encloses the turbines and generators that produce electricity.