Rio Grande Basin

The Arkansas River Basin covers a huge territory, more than 28,000 square miles, encompassing more than a fifth of Colorado’s total land area. The Arkansas River flows from its headwaters near Leadville through the Front Range cities of Cañon City and Pueblo before unspooling onto the Eastern Plains and crossing into Kansas near the town of Holly. Eventually, the Arkansas River flows into the Mississippi River on its way to the Gulf of Mexico.

Max inches of annual precipitation in Colorado’s arid Rio Grande Basin

More than 600,000 acres of irrigated land in the San Luis Valley is used for agricultural purposes. Producers in the valley are the second-largest provider of fresh potatoes in the United States (behind the well-known Idaho potatoes). Barley and alfalfa are other major crops. Non-irrigated areas in the valley are mostly classified as shrubland (24%) and grassland (31%).

Because the Rio Grande Basin receives such little precipitation, groundwater pumped from the confined and unconfined aquifers beneath the San Luis Valley is critical for agriculture. However, long-term drought and over-use have led to the decline of these aquifers and state rules now require the replacement of well-pumping depletions. Farmers have responded with an innovative program to replenish the aquifer, where subdistricts of the Rio Grande Water Conservation District assess fees on groundwater withdrawals and the proceeds are used to pay farmers to fallow some of their land.

More than half of the basin is public land, including the Rio Grande National Forest and the Great Sand Dunes National Park and Preserve. Those lands support a dynamic tourism industry that features activities like angling, boating, birdwatching and camping. Alamosa is the largest city in the basin with a population of approximately 10,000 people.

The northern third of the basin is considered a “closed basin” because it is separated, through a hydraulic divide, from the Rio Grande and therefore does not naturally contribute surface flows to the river. The Closed Basin Project delivers water from the closed basin to the Rio Grande for a few reasons: to help meet the state’s compact obligation to send water downstream to New Mexico and Texas, to maintain the Alamosa National Wildlife Refuge, and to deliver water to San Luis Lake.

Water users and managers convene and cooperate through the Rio Grande Basin Roundtable where they discuss and support projects to help manage, protect and sustain water use into the future.

Water use is limited by several interstate water sharing agreements. The 1938 Rio Grande Compact between Colorado, New Mexico and Texas requires that water be delivered to New Mexico at varying rates depending on the native flow of the Rio Grande each year. The 1944 Costilla Creek Compact, amended in 1963, divides the waters of a major tributary to the Rio Grande, giving roughly one-third of Costilla Creek’s water to Colorado and about two-thirds to New Mexico.