DELTA LAKE. Delta Lake, in the Nueces-Rio Grande basin four miles north of Monte Alto and twenty-four miles northeast of McAllen in Hidalgo County (at 26°25' N, 97°56' W), was previously called Monte Alto Reservoir and has also been called Mesteñas Reservoir and Willacy Reservoir. It is an off-channel storage reservoir to the Rio Grande. The project, constructed in 1939, is owned and operated by the Hidalgo-Willacy Counties Water Control and Improvement District No. 1. It has a capacity of 25,000 acre-feet and a surface area of 2,371 acres at the elevation of 56½ feet above mean sea level and consists of two units. Water is diverted from the Rio Grande in Cameron County by a gravity canal system. From this main canal water is diverted to another canal, known as Mesteñas Canal, for distribution to land during the irrigation season or to a pumping plant that lifts surplus water to the reservoir for storage. When needed for irrigation, the water is released and flows back to the Mesteñas Canal for distribution. Until the late 1970s the lakes formed by the reservoir were used as recreational facilities by area residents. After the water was discovered to be contaminated by an excess of microorganisms, it was closed to swimmers, and the park no longer attracted visitors.