Interstate compacts

The layer above the priority system

Prior appropriation decides who gets water inside Colorado. It does not decide how much water Colorado gets to keep. That is set by nine interstate compacts, two Supreme Court equitable apportionment decrees, and two treaties with Mexico — and they bind the state regardless of how senior anyone's decree is. When Colorado owes water at a state line, the State Engineer curtails within Colorado to deliver it, which is why compact compliance can shut off wells and ditches that are perfectly in priority under state law.

What this page is not. It does not compute compliance, state anyone's delivery obligation, or name official accounting points. Those belong to DWR and the compact commissions. What it does is show the compacts, the streams each one governs, and the 50 gages in our record that sit on those streams — several of them outside Colorado, because that is where the water is measured. Assignment of a gage to a compact is ours, made by basin, and is awaiting an expert read. The lesson explains how they work →

Rio Grande Compact 1938 · Colorado · New Mexico · Texas

Apportions the Rio Grande among Colorado, New Mexico and Texas. Colorado's obligation is measured as a delivery at the Colorado-New Mexico state line on the Rio Grande and the Conejos system, and varies with how much water the year brings rather than being a fixed amount.

Colorado's delivery is measured leaving the San Luis Valley -- the Rio Grande at the state line and the Conejos system. Otowi Bridge and Elephant Butte are downstream index points for the compact's later accounting, not Colorado's own delivery gages.

13 gage(s) on this compact's streams in our record, 11 reporting now.

Costilla Creek Compact 1944, revised 1963 · Colorado · New Mexico

Divides Costilla Creek, which crosses the Colorado-New Mexico line twice, between the two states. Revised in 1963.

6 gage(s) on this compact's streams in our record, 6 reporting now.

La Plata River Compact 1922 · Colorado · New Mexico

One of Colorado's oldest compacts. Governs the La Plata River across the Colorado-New Mexico line, with a seasonal regime that can require Colorado to rotate deliveries when the river runs low.

La Plata River at the Colorado-New Mexico state line is where Colorado's delivery would be read.

3 gage(s) on this compact's streams in our record, 3 reporting now.

Arkansas River Compact 1948 · Colorado · Kansas

Apportions the Arkansas River between Colorado and Kansas below John Martin Reservoir. The subject of long-running Supreme Court litigation (Kansas v. Colorado) over depletions to stateline flow.

3 gage(s) on this compact's streams in our record, 3 reporting now.

Republican River Compact 1942 · Colorado · Kansas · Nebraska

Allocates the Republican River basin's virgin water supply among three states. Groundwater pumping's effect on stream flow has been the central dispute, and compliance drove Colorado's Republican River pipeline and well retirements.

3 gage(s) on this compact's streams in our record, 3 reporting now.

South Platte River Compact 1923 · Colorado · Nebraska

Governs the South Platte at the Colorado-Nebraska line, with an irrigation-season flow obligation that binds Colorado's lower-river junior diverters during part of the year.

1 gage(s) on this compact's streams in our record, 1 reporting now.

Upper Colorado River Basin Compact 1948 · Colorado · New Mexico · Utah · Wyoming · Arizona

Divides the Upper Basin's share among the upper states -- Colorado receives much the largest percentage -- and apportions specific interstate tributaries, including the Little Snake between Colorado and Wyoming. It also created the Upper Colorado River Commission, which does the Upper Basin's accounting.

The Yampa, White and Dolores are Colorado's other Colorado-basin rivers; these are their lowest gages before the water leaves the state.

13 gage(s) on this compact's streams in our record, 11 reporting now.

Colorado River Compact 1922 · Colorado · Wyoming · Utah · New Mexico · Arizona · Nevada · California

The foundational division of the Colorado River, splitting it between an Upper Basin (Colorado, Wyoming, Utah, New Mexico) and a Lower Basin (Arizona, Nevada, California) at Lee Ferry, Arizona. Everything Colorado may consume in the Colorado River basin sits underneath it. It was negotiated on a run of unusually wet years, so the water it divided has turned out to be more than the river reliably carries -- which is why shortage on the Colorado is a structural argument about the compact itself, not just a run of dry years.

The apportionments in Article III(a) are of CONSUMPTIVE USE, not of flow, and the III(d) obligation is a non-depletion floor on a ten-year aggregate. Neither is something a streamgage reads directly, which is why the record below is presented as measured flow and nothing more.

2 gage(s) on this compact's streams in our record, 2 reporting now.

Animas-La Plata Project Compact 1969 · Colorado · New Mexico

A short compact clearing the way for the Animas-La Plata Project, which stores San Juan basin water for use in both states and for tribal settlement obligations.

No gage in our record sits on this compact's accounting reach.

Not compacts, and often confused with them

A Supreme Court equitable apportionment is a judgment, not an agreement between legislatures; a treaty binds the United States, not Colorado directly; and a transbasin project is engineering, not law. They are kept apart here because they are decided, argued and amended in different places.

Where this comes from

• DWR: the State Engineer is Colorado's commissioner on five compacts and assures compliance with all of them — https://dwr.colorado.gov/about-us ↗
• Colorado State University Water Knowledge: agreements, compacts and treaties — https://waterknowledge.colostate.edu/water-management-administration/agreements-compacts-treaties/ ↗
• Gage records and flow are DWR's own, cached and linked on each gage's page.

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