Prior appropriation water rights, sometimes known as the "Colorado Doctrine", is a system of allocating water from a water source that is markedly different from Riparian water rights. It is commonly used in the western United States, and was developed due to the scarcity of water in that area.

The legal details vary from state to state, however the general principal is that water rights are unconnected to land ownership, and these rights can be sold or mortgaged like other property. The first person to use a quantity of water from a water source for a beneficial purpose has the right to continue to use that quantity of water for that purpose. Subsequent users can use the remaining water for their own beneficial purposes provided that they do not impinge on the rights of previous users.

Beneficial purposes are commonly defined as agricultural, industrial or household use. Ecological purposes, such as maintaining a natural body of water and the wildlife that depends on it are not generally held to be beneficial uses.

Each water right has a yearly quantity and an appropriation date. Each year, the user with the earliest appropriation date (known as the "senior appropriator") may use up to their full allocation (provided the water source can supply it). Then the user with the next earliest appropriation date may use their full allocation and so on. In times of drought, users with junior appropriation dates might not receive their full allocation or even any water at all.

When a water right is sold, it maintains its original appropriation date. If a water right is not used for a beneficial purpose for a certain period of time it expires.

For water sources with many users, a government or quasi-government agency is usually charged with overseeing allocations. Allocations involving water sources that cross state borders or international borders can be quite contentious, and are generally governed by federal court rulings, interstate agreements and international treaties.