Delta Lake, in located 4 miles north of Monte Alto, in the Nueces-Rio Grande basin. Previously known as the Monte Alto Reservoir, it has also been called the Mestenas Reservoir and the Willacy Reservoir. As a man-made reservoir, Delta Lake is an off-channel storage reservoir to the Rio Grande River.
Constructed in 1939, it is owned and operated by the Hidalgo-Willacy Counties Water Control and Improvement District No. 1. 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 the Mestenas Canal, for distribution to land during the irrigation season. It has a capacity of 25,000 acre-feet and a surface area of 2,371 acres.
As a project of Hidalgo County, it is used for recreational purposes by area residents. Closed to the public in the late 1970s after it was found to be contaminated by an excess of microorganisms, it was reopened in 1998, although swimming is still prohibited.