Rio Grande Compact
The Rio Grande Compact is an interstate compact signed in 1938 in the United States between the states of Colorado, New Mexico, and Texas, and approved by the United States Congress, to equitably apportion the waters of the Rio Grande Basin.
The treaty was ratified by Colorado's legislature on February 21, 1939, then by Texas and New Mexico's legislatures on March 1. It was finally adopted on December 19, 1939 after passing through Congress (as Public Act No. 96, 76) and being signed into law on May 31, 1939.
The act was amended at the thirteenth Annual Meeting, on February 25, 1952.
Colorado committed to deliver a certain amount of water to the New Mexico state line, based on water runoff as measured at four index stations located in the area of the Rio Grande's headwaters. The compact provides for an indexed schedule of required water deliveries from Colorado, based on gauged stream flows, and, under certain circumstances, a system of debits and credits in water deliveries that also permits water storage with the combined capacity of Elephant Butte and Caballo Reservoirs.
The compact also sets a minimum quality standard, as in the event that Colorado was to construct water projects later for the purpose of delivering water into the Rio Grande from the Closed Basin, the state would not be credited with the water delivered unless the sodium ions in the water do not exceed 45% of the total positive ions when the total dissolved solids in the water exceeds 350ppm.
Between 1939-1966, Colorado virtually ignored the compact. The result was that by 1966, due to Colorado's non-compliance with the Compact, Colorado owed New Mexico one-million acre feet (1.2 km³) of water, and New Mexico owed Texas 500,000 acre feet (620,000,000 m3). New Mexico and Texas collectively sued Colorado to try to force compliance, and between 1966–1967, the case progressed and was eventually heard by the Supreme Court of the United States.
In 1985, after several affluent years of water flow from the Rio Grande, the Elephant Butte and Caballo reservoirs reached capacity, having received more water than they could hold. The terms of the Compact state that once the Elephant Butte reservoir is filled, New Mexico and Colorado are immediately released from any water debt they have accrued. Colorado's debt to New Mexico at the time, 500,000 acre feet (620,000,000 m3) of water, was immediately cleared, settling the balance between the two states, but leaving New Mexico in an unfortunate position in its debt to Texas.
...
Wikipedia