Amistad Dam is a large dam across the Rio Grande on the United States-Mexico border. It serves to store water for irrigation and generate hydroelectricity. The dam was first proposed in a 1944 treaty between the United States and Mexico. Construction began in 1963 and was finished in 1969, with the hydroelectric plants constructed in the 1980s. Of the dam's roughly US$72 million cost, Mexico shouldered about 4/7 of the funds and the United States about 3/7. At roughly 5 miles (8.0 km) long, the dam lies mostly on the Mexican side of the border and forms Amistad Reservoir.