Rend Lake is a 13-mile long, 3-mile wide reservoir located in Southern Illinois in Franklin and Jefferson Counties near the town of Benton. It contains 18,900 acres of water, stores 185,000 acre-feet of water, and supplies over 15 million gallons of water per day to 300,000 persons in over 60 communities. The reservoir is up to 35 feet deep, but its aver…Rend Lake is a 13-mile long, 3-mile wide reservoir located in Southern Illinois in Franklin and Jefferson Counties near the town of Benton. It contains 18,900 acres of water, stores 185,000 acre-feet of water, and supplies over 15 million gallons of water per day to 300,000 persons in over 60 communities. The reservoir is up to 35 feet deep, but its average depth is closer to 9.7 feet. Its elevation is 405 feet above sea level.