An estimated 132,000 miles of river and stream channels, representing the state’s 17 major river systems, are carved into Alabama’s landscape, with about 61 percent flowing permanently throughout the year and 39 percent flowing only intermittently during wetter times. At 1,438 miles, Alabama leads the nation in miles of navigable channels, with 16 lock-and-dam structures on six river systems. More than 20 hydroelectric generating facilities and 20 or more impoundments on smaller streams for public water supply have been built throughout the state. The total surface area of lakes, ponds, and reservoirs is 563,000 acres, and about 33.5 trillion gallons of water flow through Alabama’s river and stream channels every year. Also, Alabama’s underground water supplies are estimated at about 553 trillion gallons, or more than 16 times the amount of surface water.