What is the Equitable Apportionment?

Colorado water flows to eighteen downstream states and the Republic of Mexico. Colorado is a party to nine interstate compacts and two equitable apportionment decrees of the United States Supreme Court. The equitable apportionment decrees are for water of the North Platte and Laramie Rivers. The Republic of Mexico is entitled to water of the Rio Grande and Colorado Rivers, under the 1944 Treaty between the United States and Mexico. But what exactly is an equitable apportionment decree? The Colorado Foundation for Water Education Compacts Guide lists it as follows:
“A state may file a lawsuit in the U.S. Supreme Court asking for an equitable apportionment of interstate stream waters among the states, where the court decides how to fairly divide the waters. Congress can also make an equitable apportionment, as it did by the Boulder Canyon Project Act for Arizona, California, and Nevada’s shares of Colorado River Water. By entering into a compact, states can fashion specific enforceable provisions to share the water of an interstate stream. A compact avoids the risk of repeated Supreme Court lawsuits to determine each state’s fair share of the water. The state has a duty to regulate water rights within its own state to avoid breaching the rights of another state.” (p. 7).