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Indicator: Stream and River Flows

Median Flashiness Index by Year

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Upstream Indicators
  •   Urbanization and Impervious Surface Changes --- The number of acres of urban land, clear-cut, and regenerating forest have increased in King County since 1986. Acres of grass and deciduous, mixed and coniferous forest have decreased. Acres of rural forests and privately owned working forests have declined in recent years  (view data graph)
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Downstream Indicators
  •   Drinking Water Quality --- Seattle Public Utilities (SPU) regularly tests its water for 179 compounds, including metals, chemical contaminants and disease-causing organisms. From 2001 through 2008, SPU-supplied water easily met all state and federal drinking water standards. No compounds were detected above allowable levels.  (view data graph)
  •   Fecal Coliform at Swimming Beaches --- Status: High bacterial counts at several beaches monitored in Lake Washington (Juanita, Magnusson Off Leash Park, Gene Coulon, and Meydenbauer) during the 2007 summer season resulted in swimming beach closures. Bacteria levels were low in Green Lake for the fifth year in a row while Lake Sammamish remained fairly consistent, with slight variability from year to year. The 2008 target and long-term outcome for swimming beaches on large lakes is that none of the testing sites violate both parts of the Washington Department of Health fecal coliform bacteria target which is a geometric mean of 200 colonies per 100 ml with no single sample exceeding 1000 colonies per100 ml.   (view data graph)
  •   Wild Salmon --- Many of Puget Sound's salmon stocks have experienced substantial declines over the past 50-100 years due to overfishing, dams, and habitat degradation. Although the number of mature, adult chinook salmon returning to their stream of origin were almost twice in 2006 as high as 2005 levels, they comprised only 25 percent of the long-term chinook population target.  (view data graph)
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Related Goals

Sustainability Snapshot:

Urbanization in King County is resulting in faster surface runoff and peak stream flow rise and fall than has previously occurred. An increase in the "flashiness" index score represents the loss of water storage capability of soils and vegetation due to urbanization. Increased peak flows and "flashiness" leads to the most obvious effects from a human perspective — flash flooding and channel erosion. From a biological perspective, streams with greater "flashiness" are disturbed more often. Organisms that survive in these conditions are those that have adapted to more frequent and severe disturbances.

Sustainability Trend:

The median of the "flashiness" index scores across all streams measured in King County has increased between 1945 and 2007.

Data Discussion

The Indicator Defined

This indicator uses a stream "flashiness" index — based on the reciprocal of the fraction of days during the year that the flow rises above the annual mean daily flow.

Data Interpretation/Evaluation

Because peak stream flow rises and falls more quickly in urban areas than forested areas, urban streams tend to have a smaller fraction of days during the year when the flow is above the annual mean daily flow, and a higher "flashiness" index score. This increase in the "flashiness" index score represents the loss of water storage capability of soils and vegetation due to urbanization. To assess conditions throughout the county, the median stream "flashiness" is calculated each year across all streams where flow is measured. The median stream "flashiness" score represents the degree of water storage ability where half of the streams are flashier and half are less flashy.

Flows from 16 stream sites in King County were measured and their "flashiness" calculated during the 2007 water year (October 2006 — September 2007). Flows for four of these streams were measured by the United States Geological Survey. The median of the "flashiness" index scores across all streams measured in King County has increased between 1945 and 2007 and was lower in 2007 than in 2006.

 

These data suggest that increased urbanization in King County has resulted in faster surface runoff and peak stream flow rise and fall than previously occurred for some of the streams.

Data Source and Limitations

 Flows from 16 stream sites in King County were measured and their "flashiness" calculated during the 2007 water year (October 2006 — September 2007). Flows for four of these streams were measured by the United States Geological Survey.

 

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