Publication Summary

One of the Department of Ecology′s highest priorities is to help ensure there is enough water to meet the present and future needs of people and the natural environment. And protecting stream flows - that is, the naturally varying amount of water flowing in a stream or river - is one of our most important water management tools. Specific stream flow amounts protected in a regulation are called instream flows.

This Ecology Report defines instream flows, explains why instream flows are important, describes instream flow methodology, explains the history of Ecology's instream flow program and the future direction of the program with watershed planning.

The legal and administrative meaning of "instream flow" is different from either base flow or stream flow, being more abstract. The volume of water required for an instream flow is developed by considering existing data, the hydrology of a stream and its natural variations in stream flow and base flow over the course of the year, studying the need for fish habitat, as well as many other factors.

Ecology is required by law to protect instream flows by adopting regulations and to manage water uses that affect stream flows. Once adopted, an instream flow rule acquires a priority date similar to that associated with a water right. Water rights existing at the time an instream flow is adopted are unaffected by the rule and those issued after the rule adoption are subject to the requirements of the instream flow rule. A "junior" water right would contain provisions requiring the diversion of water authorized by the water right to cease when the stream flow drops to the levels protected by the rule.