Summer Stream Flows

Summer flows in streams and rivers occur at the time of year characterized by warm temperatures, little rainfall, and depleted snow packs, and when water demands are greatest yet supply is lowest. Some seasonal variation in summer stream flows is normal, but exceptionally low flows exacerbated by development that draws water away from streams in the summer can cause problems for salmon and people. This Vital Sign helps us track the effects of climate change and variability on stream flows and whether or not management actions intended to reduce withdrawals and increase recharge in developed areas are working to restore summer flows.


Key Vital Sign Messages

  • Patterns in the trends in summer low flows generally did not change much over the last few years. 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 out of the seven rivers evaluated without dams have been on a declining trend since the 1970s. Regulated rivers have had relatively stable or even increasing flows, an expected result given the license-based requirements to release agreed upon instream flows.
  • Summer low flows in the Puget Sound basin respond to a variety of drivers including rainfall, snowfall, temperature, evapotranspiration, land-use conversion, forest practices, and human water use. This indicator aims to describe the change pattern in summer low flow based on the net effects of all factors combined; it does not evaluate the potential impact of individual factors on a trend.
  • Climate change is expected to impact aspects of the water cycle, including streamflow and snowpack (UW Climate Impacts Group 2015). Summer streamflow is projected to decrease along with declines in summer precipitation and winter snow accumulation.
  • The current Regional Priority for summer stream flows is to develop an Implementation Strategy in close collaboration with LIOs and tribal partners. Preparatory work on this Implementation Strategy has begun.

Strategies, Actions, and Effectiveness

Contributing Partners

PSEMP Freshwater Work Group (chair is Rick Dinicola, USGS)
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