Medina Lake Medina Lake was constructed between 1911-1912 as an irrigation reservoir. An extensive canal system delivers water to 34,000 acres of blackland prairie farmlands below the Balcones escarpment around Castroville. At the time it was constructed, it was the biggest irrigation project west of the Mississippi. At spillway capacity, Medina Lake covers about 5,575 acres, has a length of 18 miles, a maximum width of three miles, and 110 miles of shoreline. In addition to the main dam, there is a smaller dam about four miles downstream that creates Diversion Lake, from which discharges are made to the canal system.