Report of Investigations in Instream Flow: High Flows for Fish and Wildlife in Washington

The Washington Department of Fish and Wildlife (WDFW) develops instream flow recommendations to protect and restore fish and wildlife habitat in streams. Recent advances in instream flow science document the importance of protecting and restoring high flows that create habitat. High flows provide desirable conditions for fish migration and spawning, flush organic matter from the channel, maintain channel geometry by transporting sediment, and form new channels by stream meandering, side-channel activation, and renewal of riparian and floodplain vegetation. Recommendations for protecting and restoring high flows include high flow pulses and flushing flows, channel maintenance flows, and channel forming flows. High flows are quantified by their magnitude, duration, frequency, timing (rate of change), and seasonality. Each of these characteristics may vary among different hydrologic regimes in the state. High flow pulses and flushing flows on unregulated streams are on the order of mean annual discharges. Channel maintenance flows on unregulated streams are on the order of 1.5 to 2-year recurrence interval peak flows. Channel forming flows on unregulated streams are on the order of 10 to 25-year recurrence interval peak flows. High flows on regulated streams may vary according to flow influences due to storage and diversion for agricultural irrigation, municipal water use, hydropower generation, flood control, or other purposes. This report presents proposed changes to high flow recommendations in the WDFW Instream Flow Guidelines.