Idaho Code § 42-607
It shall be the duty of said watermaster to distribute the waters of the public stream, streams, or water supply comprising a water district among the water users taking water therefrom according to the prior rights of each respectively, in whole or in part, and to shut and fasten, or cause to be shut or fastened, under the direction of the department of water resources, the headgates or controlling works for the diversion of water from such stream, streams, or water supply, during times of water scarcity, in order to supply the prior rights of others from such stream or water supply; provided, that any person or corporation claiming the right to the use of the waters of the stream or water supply comprising a water district, but not owning or having the use of an adjudicated or decreed right therein, or right therein evidenced by permit or license issued by the department of water resources, shall, for the purposes of distribution during times of water scarcity, be held to have a right subsequent to any adjudicated, decreed, permit, or licensed right from such stream or water supply, and the watermaster shall close all headgates or controlling works of diversions having no adjudicated, decreed, permit or licensed right if necessary to supply adjudicated, decreed, permit or licensed right in such stream or water supply. As long as a duly elected watermaster is charged with the administration of the waters within a water district, no water user within such district can adversely possess the right of any other water user.
Idaho Code § 42-607