Water Right in water law refers to the right of a user to use water from a water source, e.g., a river, stream, pond or source of groundwater. In areas with plentiful water and few users, such systems are generally not complicated or contentious. In other areas, especially arid areas where irrigation is practiced, such systems are often the source of conflict, both legal and physical. Some systems treat surface water and ground water in the same manner, while others use different principles for each.
Under the prior appropriation doctrine, water rights are "first in time, first in right." That is, the older, or senior, water right may operate to the exclusion of junior water rights. The concept of "priority date" is significant.
Water rights are generally established pursuant to State law, but there are exceptions, most notably, the concept of federal reserved water rights. Reserved water rights are rights that are established when the federal government reserves land for a specific federal purpose. Courts have held that there is an implied water right to satisfy the primary purposes of the reservation Examples of reservations include Indian reservations, national wildlife refuges, federal forests and military bases.
Proceedings to determine the relative priority of claims to water rights are known as adjudications. Through Congress's passage of the McCarren amendment, the federal government has consented to having its claims adjudicated in state courts.
All states offer mechanisms for changing how a water right is exercised, e.g., amending the point of diversion or withdrawal, the place of use and the purpose of use. In reviewing such requests, the state must guard against the impairment of other water rights, the enlargement of the water right and injury to the public interest.
In the United States, Navigable waters are subject to the commerce clause of the U.S. Constitution. The commerce clause provides the federal government the ability to restrict state issued water rights via, for example, the enforcement of water quality standards via the Federal Water Pollution Control Act (Clean Water Act), the Federal Power Act and the protection of endangered species via the Endangered Species Act.