Somerville Lake is located in the south central part of Burleson County and the north central part of Washington County, with the upper reaches of the lake extending into Lee County. The dam site is located on Yegua Creek, 20.0 river miles upstream from its confluence with the Brazos River. The normal lake pool elevation is 238.0 feet (above mean sea level) which encompasses 11,460 acres. There is approximately 85 miles of shoreline. Total land and water cover approximately 30,000 acres.
The Yegua Creek watershed is about 62 miles long and 32 miles wide, having a drainage of 1,320 square miles. About 76 percent of the area is controlled by Somerville Dam, an earthfill embankment 20,210 feet long. The total length of the dam, including dike and spillway, is 26,175 feet. The earthfill dike, located on the northeast bank, is about 4,715 feet long, with an access road to the overlook area along the top. The lake provides flood protection for about 9,000 acres of land along the Yegua Creek and assists in the flood protection and irrigation of 887,000 acres of agricultural land along the Brazos River below the mouth of Yegua Creek.
The water surface level is maintained at elevation 238.0 as much of the time as possible; however, this level is subject to fluctuation upward when flood waters are being stored, or downward, depending upon the withdrawals for water supply from the conservation storage, release rates and daily evaporation.
The project has a storage capacity of 337,700 acre-feet (one acre-foot is one acre of water one foot deep, or about 326,000 gallons), in the portion of the project allocated to flood control. The flood control storage space is that portion of the lake between the top of the conservation pool, elevation 238.0 and the spillway crest 258.0.
The water surface level is maintained through the use of a conduit and gates. The outlet works consist of a 10 foot diameter conduit controlled by two 5 by 10 foot tractor-type gates. The conduit is located in the main embankment about 1300 feet to the right or south of the original river channel. The conduit, including the gate passages, is about 400 feet long.
The spillway is an uncontrolled concrete ogee weir about 1,250 feet long.