Lake Conroe is a 21,000-acre (85 km2) lake in Montgomery County, Texas, United States. Even though it is named Lake Conroe, only a small portion of the lake is in Conroe, Texas. Most of the lake is in unincorporated Montgomery County. The lake lies on the West Fork of the San Jacinto River just west of Interstate 45 off of State Highway 105 in Montgomery and Walker counties. The lake runs through classic East Texas Piney Woods forests. Water quality generally is good, fairly clear, with an average depth of 20.5 feet (6.2 m), and a maximum depth of 75 feet (23 m). The controlling authority of the lake is the San Jacinto River Authority. The City of Houston owns a two-thirds interest in the lake; the SJRA owns one-third. The lake extends about 21 miles (34 km) in length and covers 20,118 acres (81.41 km2) with 5,000 acres (20 km2) in Sam Houston National Forest and capacity of 430,260 acre feet (530,720,000 m3). Lake Conroe is a reservoir built as an alternate water supply for the city of Houston by the City of Houston, the San Jacinto River Authority, and the Texas Water Development Board in January 1970, the lake was completed January 1973 and it was filled on October 31, 1973.