Lake Buchanan was formed by the construction of Buchanan Dam by the Lower Colorado River Authority to provide a water supply for the region and to provide hydroelectric power. Buchanan Dam, a structure over 2 mi (3.2 km) in length, was completed in 1939. Lake Buchanan was the first of the Texas Highland Lakes to be formed, and with 22,333 acres (34.9 sq mi; 90.4 km²) of surface water, it is also the largest. The surface of the lake includes area in both Burnet and Llano Counties.