Under Oregon law, all water belongs to the public. With some exceptions, cities, irrigators, businesses, and other water users must obtain a permit or license from the Water Resources Department to use water from any source - whether it is underground, or from lakes or streams. Generally speaking, landowners with water flowing past, through, or under their property do not automatically have the right to use that water without authorization from the Department.
Oregon’s water laws are based on the principle of prior appropriation. This means the first person to obtain a water right on a stream is the last to be shut off in times of low streamflows. In water-short times, the water right holder with the oldest date of priority can demand the water specified in his or her water right without regard for the needs of junior users. If there is a surplus beyond what is necessary to fulfill the senior right, the water right holder with the next oldest priority date can take what is available to satisfy needs under his or her right. This continues down the line until there is no surplus or until all rights are satisfied. The date of application for a permit to use water usually becomes the priority date of the right.
For more information please view the Water Rights In Oregon Publication.