The Rio Grande Compact

What it is

The Rio Grande Compact is an agreement among Colorado, New Mexico and Texas, signed at Santa Fe on March 18, 1938, and approved by Congress in 1939. Colorado's copy is in state law at C.R.S. 37-66-101. It apportions the waters of the Rio Grande above Fort Quitman, Texas, and is administered by the Rio Grande Compact Commission, with one commissioner from each state and a federal representative. Colorado's State Engineer is, by the compact's terms, Colorado's commissioner.

How Colorado's obligation is set

The compact does not give Colorado a fixed number of acre-feet. Instead, Article III ties what Colorado must deliver at the New Mexico state line to how much water the river produced that year, measured at index gauges:

• Rio Grande near Del Norte — above the main diversions into the San Luis Valley, corrected for reservoirs built after 1937.
• Conejos Index — the Conejos River near Mogote, plus the Los Pinos and San Antonio rivers near Ortiz from April through October.

Two tables in the compact relate each index flow to a delivery. Colorado's obligation, measured at or near Lobatos on the state line, is 10,000 acre-feet less than the sum of the quantities from those two tables. In general, the more water the river produces, the larger the share Colorado must pass downstream; in dry years the share is smaller. Deliveries are tallied by calendar year, and the compact has provisions for debits and credits carried between years.

Curtailment: how it is met

To meet the obligation, Colorado's Division 3 office projects the year's index flows and curtails diversions on the Rio Grande and the Conejos by a percentage, so that enough water passes the state line. The percentage can change through the season as forecasts change, and it is generally different on the two rivers. This compact administration sits on top of ordinary priority administration: a senior ditch can be in priority and still be curtailed by the compact percentage.

For an irrigator in the valley, that means the water you can take depends on the year's runoff forecast as much as on your priority date. Spring snowpack in the San Juan Mountains feeds most of the index flow, so NRCS snowpack readings and forecasts are watched closely in Alamosa.

Storage is tied to the ledger too. When Colorado is in debit, Article VI requires it to hold water in reservoirs built after 1937 above Lobatos, within their capacity, to the extent of the debit. Article VII bars Colorado and New Mexico from increasing storage in reservoirs built after 1929 while usable water in the downstream Rio Grande Project storage is below 400,000 acre-feet.

Groundwater

The San Luis Valley also relies on groundwater. Well pumping that depletes the river can reduce what reaches the state line, so Colorado has adopted groundwater rules for Division 3 that require wells to replace their injurious depletions, often through groundwater management subdistricts. Those rules are separate from the compact text but serve the same goal of keeping the river whole for senior surface rights and for compact deliveries.

Why it matters beyond the valley

The Rio Grande supports farms and cities in New Mexico and Texas, and interstate disputes over it have reached the U.S. Supreme Court. Colorado's compliance record, the state engineer's administration and the treatment of groundwater are all part of that picture. For the current status of any litigation or Commission decisions, check DWR and the Rio Grande Compact Commission's annual reports rather than a summary.

Following it in BasinMind

The live box above shows a current Division 3 flow. Many Division 3 flows come from DWR's telemetry stations (such as Rio Grande near Del Norte), which do not come with a USGS median, so BasinMind shows the flow without a percent-of-median rather than guess. Today's calls in the division are on Today; acequias on the Culebra and other creeks are explained in Acequias in Colorado. The Division Engineer in Alamosa is the authority on curtailment.