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The Snoqualmie River Hydrologic Study: Evaluation of Flooding Trends and Current Conditions is the second phase of the Snoqualmie River H&H Study. The Study is a broader investigation into a variety of issues related to river gages, historical trends, basin hydrology, and recent flood events. The study looked for annual and seasonal trends in basin flood hydrology and evaluated possible causes of change, such as land use, forestry practices, increased sedimentation, and climate change. The Phase 2 Study also included a review of the USGS gaging program in the Snoqualmie basin and recommendations to improve the system for flood warning. The Phase 2 Study found little evidence that flooding has gotten significantly worse in the Snoqualmie basin but found some evidence of increasing frequency of flood events and upward trends in fall and spring precipitation and high flows. The analysis concludes that much of the change in flooding reported by residents is likely due to variations in how precipitation falls across the basin during a storm, and how this shapes the floodwave as it moves through the river system.

Public and stakeholder involvement

              Attn: Chris Ewing

              King County Department of Natural Resources and Parks

              201 South Jackson Street, Suite 600

              Seattle, WA 98104

lower-snoqualmie-valley-near-duvall-dec-9-2015-flood Lower Snoqualmie Valley near Duvall, December 9, 2015 flood

About the study

The Snoqualmie River Hydrologic Study: Evaluation of Flooding Trends and Current Conditions is an independent technical analysis of a variety of issues related to potential flooding changes in the Snoqualmie River basin. The study was conducted by a consulting firm, Watershed Science & Engineering, on behalf of King County. Dr. Ed McCarthy conducted an independent technical review on behalf of Snoqualmie Valley residents and stakeholders to verify the integrity of the methods and to ensure the study addressed community concerns.

The scope of the second phase was based on public feedback that flooding has worsened in recent years, particularly in the lower Snoqualmie Valley. Many community members have also said that the USGS river gage measurements are inconsistent and often do not correlate with flooding impacts. The study’s scope was expanded based on input from the Snoqualmie Valley Preservation Alliance and comments received at the March 22, 2016, public meeting for the Phase 1 Study to include a greater investigation into sediment accumulation and how this may affect flooding changes.

The Phase 2 Study:

  • Reviewed the USGS gaging program in the Snoqualmie basin and provided recommendations to improve the system for flood warning.
  • Examined annual and seasonal trends in basin flood hydrology and evaluated possible causes of change, such as land use, forestry practices, increased sedimentation, and climate change.
  • Investigated three recent flood events to explain unusual flooding behavior and to illustrate the complexity of flooding.

The study found little evidence that flooding has significantly worsened in the Snoqualmie basin. No statistically significant trend was found in annual peak flows, flood travel time, or rate of rise, suggesting major floods are not getting larger or arriving faster. However, the frequency of flood events in the lower Snoqualmie Valley appears to be increasing based on a weak upward trend at the Carnation gage, though this trend was not seen at any other gages. Increasing trends in high flow and precipitation were also identified at several gages during some spring and fall months, corroborating both the agricultural community’s observations and climate change projections. Though not statistically significant, slight increases were also found in much of the available flow and precipitation data, reflecting reports voiced by valley residents. Despite these instances of slight flooding changes, the analysis concludes that much of the change in flooding residents observe is likely due to how a flood responds to natural variability in rainfall distribution and intensity. The study’s investigation into timber harvest, urban development, and sediment deposition as causes of change suggests their effects are small and/or local. 

The study also concludes that the USGS gages generally provide high-quality real-time and historical flow information but notes some quality issues, specifically at the Carnation gage. The Carnation gage is known to be inaccurate at high flows due to river flow that bypasses the gage as well as complex flow dynamics at this location. King County is coordinating with the USGS to improve estimates of high flows.

snoqualmie-basin-aerialAerial Photo of Lower Snoqualmie Valley near Fall City

For more information about the Snoqualmie River Phase 2 Hydrologic Study, please contact Chris Ewing, Engineer, King County River and Floodplain Management Section.