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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 broad 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 development, forestry practices, 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 evidence that substantiates some of the reports that flooding has gotten worse in the Snoqualmie Basin:

  • Small and moderate flood events appear to be occurring more often at several gages in the basin.
  • There are increasing trends in fall and spring high flows at several gages. This corroborates residents’ observations that spring and fall flooding has become more severe.

These trends are consistent with observed changes in precipitation patterns at several nearby weather stations, as well as climate change projections for the Pacific Northwest. The investigation also found a statistically significant trend toward increased peak flood levels at the Carnation gage, which is likely due to a combination of higher peak flows and localized sedimentation. Forestry, land development, and sedimentation were found to have a relatively minor effect on large flood events in the Snoqualmie Valley.

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. Analysis of residents’ highwater mark data revealed no clear change in flood behavior in recent decades.

The analysis also revealed the complexity and variability of flood behavior in the Snoqualmie, and concluded that much of the change in flooding reported by residents may be attributed to the unique characteristics of each flood event.  Differences in how precipitation falls across the basin during a storm and how the river channel naturally evolves over time can cause noticeable fluctuations in flood levels at a given location in the Valley. 

The study also concluded 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.

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. Ed McCarthy, PhD, 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 scope was expanded a second time to include an analysis of flood stage at the Carnation gage and highwater marks from past floods, based on public feedback on the draft report.

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 hydrology and evaluated possible causes of change, such as land development, forestry practices, sedimentation, and climate change.
  • Investigated three recent flood events to explain unusual flooding behavior and to illustrate the complexity of flooding in the Snoqualmie basin.
  • Analyzed highwater mark data from past flood events collected by Valley residents for evidence of changes in flood behavior.

Project Schedule

  •  September 2015 - Phase 2 Study initiated
  • March 2017 - Draft study posted to project webpage
  • April 18, 2017 – Presentation & Community Forum
  • May 3, 2017 – Deadline for public comments on draft report.
  • July 2018 - Phase 2 Study finalized

Next Steps

King County is planning to explore opportunities to add river gages in the lower Snoqualmie Valley to provide residents with more accurate and localized information about flood conditions. The County is working to support citizen-led efforts to improve flood monitoring capabilities throughout the lower Valley.

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

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