Water law in the United States refers to the Water resources law laws regulating water as a resource in the United States. Beyond issues common to all jurisdictions attempting to regulate water's uses, water law in the United States must contend with: Public regulation of waters, including flood control, environmental regulation—state and federal, public health regulation and regulation of fisheries The interplay of public and private rights in water, which draws on aspects of eminent domain law and the federal commerce clause powers; Water project law: the highly developed law regarding the formation, operation, and finance of public and quasi-public entities which operate local public works of flood control, navigation control, irrigation, and avoidance of environmental degradation; and Treaty rights of Native Americans.
The law governing these topics derives from all layers of US law. Some derives from common law principles which have developed over centuries, and which evolve as the nature of disputes presented to courts change. For example, the judicial approach to landowner rights to divert surface waters has changed significantly in the last century as public attitudes about land and water have evolved. Some derives from state statutory law. Some derives from the original public grants of land to the states and from the documents of their origination. Some derives from state, federal, and local regulation of waters through zoning, public health, and other regulation. (Federally recognized tribes may have water rights, but non-federally recognized Indian tribes generally do not.)
Common law sources of water law
The United States inherited the British common law system which develops legal principles through judicial decisions made in the context of disputes between parties. Statutory and constitutional law forms the framework within which these disputes are resolved, to some extent, but decisional law developed through the resolution of specific disputes is the great engine of water law.
At common law any rights to water must be claimed based on a claim against the land over which water flows or rests. A downstream landowner can bring an action against an upstream owner for excessively diminishing the quantity and quality of water arriving at a downstream location.
Water Disputes arise in a number of contexts. When the state, local, or federal government takes private property that has water rights associated with private ownership, the value of that property is significantly affected by its water rights. And, properties located along public waters are quite common, because of the importance of public waters to commerce, the environment, and recreation. These taking cases represent a major source of the law defining the limits of private rights in water and public rights.
A second context for the development of water law arises from disputes among private parties over the extent of their respective water rights; e.g., a landowner upstream seeks to cut off the flow of surface water downstream and appropriate these surface waters for its exclusive use. The downstream owner claims that the upstream landowner has appropriated water that belongs to its property. A downstream owner seeks to stop the flow of excess water that will otherwise flood its land thereby increasing flood damage on the upstream owner. Each party claims that the other's conduct interferes with the rights associated with their respective ownership of the property.
The third context for water law arises from disputes regarding flooding or other invasions of private property by water. In these cases, the private party claims that private or public actions have damaged its private property, and the court must decide the nature of the respective rights of public and private parties arising from the alteration of the hydrology of a watershed.