The Arkansas River Compact

What it is

The Arkansas River Compact was signed by commissioners for Colorado and Kansas and a federal representative on December 14, 1948, ratified by both legislatures and approved by Congress in 1949. Colorado's copy is in state law at C.R.S. 37-69-101. Its stated purposes are to settle existing disputes and remove causes of future controversy between the two states over the Arkansas River, and to apportion the waters and the benefits of John Martin Reservoir.

John Martin Reservoir

John Martin Reservoir, on the Arkansas below Las Animas, is operated by the U.S. Army Corps of Engineers for flood control and water conservation. The compact divides the reservoir's conservation storage and releases between Colorado ditches and Kansas. In practice, much of the compact's day-to-day administration is about when water is stored in John Martin, who can call for releases, and how they are accounted for. The Arkansas River Compact Administration, with representatives from both states, oversees it and meets each year.

The central promise

Article IV-D of the compact says that future development in Colorado may not materially deplete the usable flows of the Arkansas available to Kansas and Colorado water users. The test is not a fixed delivery number; it is whether new development after the compact has depleted the water that would otherwise have been usable at the state line.

Kansas v. Colorado

In 1985 Kansas sued Colorado in the U.S. Supreme Court. Kansas argued that pumping from wells drilled in Colorado after the compact had depleted the river's usable flows. In 1995 the Court agreed that post-compact well pumping in Colorado had violated Article IV-D (Kansas v. Colorado, 514 U.S. 673). A damages phase followed, and in 2005 Colorado paid Kansas about $34.6 million for depletions of usable state-line flows from 1950 through 1996, according to the Kansas Department of Agriculture. The case continued on other issues for several more years.

What changed in Colorado

The case reshaped how Colorado administers groundwater in the Arkansas Valley. Colorado adopted rules requiring wells in the basin to replace their depletions to the river, typically through augmentation plans or replacement plans operated by well associations and water districts. Later rules also address surface-water efficiency improvements, such as lining ditches or converting to sprinklers, because those can reduce return flows that downstream users and Kansas rely on. Farmers in the valley who pump wells or modernize irrigation encounter these rules directly, and the Division 2 office in Pueblo administers them.

Calls and the compact in Division 2

Inside Colorado, the Arkansas is administered by prior appropriation like every other river, and Division 2 often has many active calls on the main stem and its tributaries — Fountain Creek, the Purgatoire, the Huerfano and others. The compact sits on top of that system: it governs what Colorado as a whole owes Kansas and how John Martin storage is shared, not which Colorado ditch is senior to which.

The Arkansas basin also receives transmountain water from the Colorado River system, notably through Reclamation's Fryingpan–Arkansas Project, which delivers to reservoirs such as Turquoise Lake, Twin Lakes and Pueblo Reservoir. That imported water is accounted for separately from the native flow of the river.

Since 1980, the Compact Administration's Operating Plan for John Martin Reservoir has divided conservation storage into accounts — one for Kansas and others for Colorado ditches, most of them in Water District 67 below the dam. Each account's owner decides when to call for releases, so in dry summers those ditches depend on the plan as much as on the river.

Following it in BasinMind

The live box above shows current storage in the Division 2 reservoirs BasinMind tracks, including John Martin. The Reservoirs page charts storage over time, and Today lists the division's active calls. Compact accounting itself is published by the Arkansas River Compact Administration and DWR; BasinMind does not calculate compact compliance.