Oregon's water laws are based on the principle of prior appropriation often referred to as “first in time, first in right." This means the first person to obtain a water right on a stream is the last to be shut off in times of low streamflow. The priority date of a right is typically the date the Department received the application for a permit to use water.
When there is insufficient water to satisfy all water rights, water users with senior priority dates can make a “call" to receive water regardless of the needs of junior users. If there is a surplus beyond the needs of the senior right holder, the water right holder with the next oldest priority date can take as much as necessary to satisfy needs under their right and so on down the line until there is no surplus.
Prior to distributing water from junior users to senior users, the Department's Watermaster staff make field measurements to validate the call and determine whether there is adequate water at the senior right's point of diversion to satisfy the senior user's water right demand. Once the calls are validated, the Department must begin distributing water from junior (newer) water right diversions to benefit the senior users. When the junior water right holder is notified, they are required to shut off their diversions, pumps, or wells. Watermaster staff follows up with field checks to individual points of diversion to ensure that water use is in compliance with regulation orders.
Each summer as streamflows drop, Watermasters regulate junior users to provide water to the more senior users. By the end of summer, there is typically only enough water to supply users who established their rights in the late 1800s in many areas of the state.