What is the legal doctrine that grants states control over water resources within their borders?

The legal doctrine that grants states control over water resources within their borders is known as "prior appropriation." This doctrine operates on the principle of "first in time, first in right," meaning that the first user to divert water from a source for beneficial use has the right to continue using that water, even in times of shortage. This system is prevalent in many western states of the United States, where water is often scarce.

Prior appropriation allows states to allocate water resources effectively and manage them based on established legal rights. It emphasizes the importance of beneficial use, which must be maintained by the appropriator, ensuring that water is used for actual needs rather than being hoarded.

In contrast, other concepts like private mortgage, water rights, and public trust doctrine address different aspects of water management and ownership. Private mortgage relates to property financing, water rights can refer to general legal entitlements to water, and the public trust doctrine deals with the state's obligation to protect and ensure public access to certain natural resources. However, none establish the specific mechanism through which states exert control over water resources as prior appropriation does.