Series: Colorado Water Law Basics #15 - Colorado's Interstate Compacts

In last week’s blog, LWS discussed Colorado as The Headwaters State since it is the source of significant surface water supplies for 14 states. As the state of origin for the river basins listed below, there is the potential for Colorado to control the flow and use of water from each of these river basins, whose waters flow through 13 other states that rely on the waters of these rivers. To protect the availability of water to the downstream states (where these rivers flow), Congress has enacted interstate Compacts that determine the terms and conditions in which water is apportioned to Colorado and the other downstream states.

There are 11 interstate Compacts and decrees that regulate use of water in multiple basins in Colorado, including:

Colorado River Basin

• Colorado River Compact

• Upper Colorado River Compact

• La Plata River Compact

• Animas-La Plata River Compact

Arkansas River

• Arkansas River Compact

South (North) Platte River Basin

• South Platte River Compact

• Republican River Compact

• North Platte River Decree

• Laramie River Decree

Rio Grande

• Rio Grande Compact

• Costilla Creek Compact

This week we provide a few details on the Colorado River and Arkansas River Compacts, and will discuss the Compacts in the South Platte River and Rio Grande in a blog next week.

The Colorado River Compact includes several river basins in Colorado including the Animas, Dolores, Gunnison, La Plata, San Juan, White, and Yampa rivers. The Compacts regulating the flow and use of Colorado River water have been discussed in the May 4, 2021 LWS blog, along with our blog about the importance of Lee Ferry, relative to the apportionment of water. The recent lack of water that has been apportioned, but is not physically available, has been discussed in the LWS blog on the Colorado River Drought Contingency Plan.

However, in addition to the overall Colorado River Compacts that apportion water to the seven basin states, the La Plata River Compact, signed in 1922, allocates water of the river to Colorado and New Mexico during most of the year (February 16-November 30) but does not have limitations on flow during the winter months (December 1-February 15). The apportionment of flow is based on a gaging station on the La Plata River at Hesperus and, during the period that flow is regulated, if the flow is below 100 cubic feet per second at the gage, Colorado has to pass half the flow to New Mexico. If the flow is greater than 100 cfs then flow can be used unrestricted by both states.

The 1968 Animas-La Plata Compact relates to a storage structure identified in the Colorado River Storage Project Act. The resulting dam and reservoir, i.e., Lake Nighthorse south of Durango, Colorado, provides for water availability to New Mexico under its apportionment in the Colorado River Compact (850,000 KAF) and also provides water to the Ute Tribe as part of a settlement agreement on Native American water rights. Lake Nighthorse has a storage capacity of approximately 123.5 KAF.

The 1948 Arkansas River Compact apportions water use between Colorado and Kansas. John Martin Reservoir, located near Las Animas, Colorado was constructed to assist with the allocation of water to Kansas. To regulate the apportionment of water, there are many storage and use limitations in the Compact. However, in general inflows into John Martin Reservoir between November 1 and March 31 are stored but Colorado can request releases of the inflow, up to a maximum of 100 cfs. Similarly, summer storage is from April 1 through October 31, but Colorado can request releases of the inflow, up to 500 cfs while Kansas can request releases of the inflow that is between 500 and 750 cfs. Maximum deliveries to Colorado cannot exceed 750 cfs, while releases to Kansas cannot exceed 500 cfs. More limited releases occur when storage in John Martin is below 20 KAF.

While the Arkansas River Compact sought to “settle existing disputes and remove causes of future controversy between the states of Colorado and Kansas,” it hasn’t quite worked out that way. In 1985 Kansas alleged that Colorado had violated the terms of the 1948 Compact by overusing its allocation under the Compact and significant litigation ensued for the next 20 years. Kansas ultimately prevailed in their allocation and Colorado agreed, among other things, to use the hydrologic-institutional model (“H-I Model”) as a means to better understand water use and account for deliveries to Kansas. The H-I Model is used to determine depletions and accretions to flow at the Colorado-Kansas stateline caused by groundwater pumping and replacement by simulating the hydrologically-connected Arkansas River surface flows and associated alluvial groundwater flows. The H-I Model was used, among other purposes, to develop rules that govern the diversion of surface water under the priority system in Colorado and the operation of John Martin Reservoir by predicting diversions, stream flows and storage of water in, and releases of water from, John Martin Reservoir and other reservoirs.

Look for our blog next week on the Compacts that regulate use in the South Platte River Basin and the Rio Grande Basin.

If you have any water resources issues, LWS can help. For more information on this subject or any Western water rights issue, please contact us for help at 303-350-4090 or by email.

Bruce Lytle, P.E. bruce@lytlewater.com

Chris Fehn, P.E., P.G. chris@lytlewater.com

Anna Elgqvist, EIT anna@lytlewater.com

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