Water Allocation Law

Two fundamentally different legal systems govern the allocation of water in the U.S.  Under the riparian system, which applies in twenty-nine “wet” eastern states, ownership of land along a waterway determines the right to use that water.  In times of shortage all owners along a stream must reduce their use.  In contrast, western states typically rely on the prior appropriation system.  Under prior appropriation, a water right is obtained by diverting water and putting it to a beneficial use (e.g., irrigation, municipal use), with the date of first appropriation establishing the users priority (or seniority) on the stream system.  In times of water shortages, “senior” rights are fully satisfied first and completely before junior rights are met, sometimes resulting in “juniors” receiving no water at all.  Prior appropriation is state law; it does not typically apply across state lines, and is not immune from federal reserved water rights and environmental laws.  Additionally, many states that use prior appropriation to allocate surface waters use different allocation systems for groundwater. For more detailed information on state or federal law and interstate arrangements and compacts click on any of the links below the picture or to the left.