The summer low flow indicator measures long-term trends in river low flows that occur during summer months when there is less rain and temperatures are warmer. The indicator for summer low flow is designed to evaluate the Partnership’s 2020 ecosystem recovery targets identified for each of 12 major rivers in Puget Sound.
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The percentage change per year (1975-2017) in summer low flows, calculated by dividing the gain or loss of flow (cfs) per year by the average 30-day summer low flow over the 1975-2017 period, for unregulated and regulated Puget Sound rivers. Values shown indicate the trend in summer low flow (cfs) per year. Note: the Dungeness and Skagit rivers show increasing percentages per year, but with no significant change detected.

Key Indicator Results

Trends in summer low flow for the period 1975 to 2017 at gauging stations on the 12 large rivers described in the 2020 recovery target has varied from river to river. The majority of rivers without dams have been on a declining trend since the 1970s. Regulated rivers, expectedly, have had relatively stable or even increasing flows. Because the direction of the trends vary across rivers, we conclude that progress is “mixed”, with many river flows not yet meeting their expected 2020 target trends. 

Patterns in the trends in summer low flows generally did not change much over the last few years and most are on the same trajectory. Most rivers with stable summer low flows a few years ago continue to have stable flows, rivers with decreasing trends continue to show decreasing trends, and rivers with increasing trends, continue to show increasing trends.

  • Five of the seven unregulated rivers have declining trends in summer low flow.
  • Only two of the seven non-regulated rivers, the Dungeness and Puyallup, meet the 2020 target value (stable flows). Both of these rivers have glaciers in their headwaters.
  • Of the seven rivers already meeting their 2020 target values (stable or increasing flows), five are regulated rivers with large dams, an expected result given the license-based requirements to release agreed upon in-stream flows.

Contributing Partners

Recovery Target

Stable flows in unregulated rivers that were stable in 2010: Puyallup (near Orting WA), Dungeness, and Nooksack rivers.

Stable Snohomish River summer flows.

Weak decreasing trend in summer low flows in the Deschutes River, North Fork Stillaguamish River, and Issaquah Creek.

Stable or increasing flows in highly regulated rivers: Nisqually, Cedar, Skokomish, Green, and Skagit.

Monitor low flow in the Elwha River after dam removal.

Data Source

U.S. Geological Survey Stream Gauging network, compiled by the Streamflow Monitoring Program at the Washington Department of Ecology

Methods and More Results
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