Many people around the world are familiar with the concept of a legal right to use water. That is, the right to divert water out of stream for consumptive use, or the right to use water instream to generate hydropower. This concept of a legal right is private in nature, a right held by the party that uses the water to grow crops, to provide water for domestic use, to operate a hydropower facility on a river. For example, in the American West, such private entitlements to water include riparian surface water rights (which derive from the early English common law) and prior appropriation surface water rights (which emerged after the California gold rush).

Alongside these accepted notions of private rights to water, however, there is an equally long tradition in many countries of a public right to keep water instream – for fisheries, for navigation, for flows to preserve water quality. In the Roman Empire, the Latin term was publicum ius aquae (public water right) and was closely related to the status of fish and instream flows as res communis (things held/owned in common by the public) under Roman law.