Water Quality Index for Streams and Rivers

Summary of King County Water Quality Index Ratings

Sixty-three sites in the Lake Washington and Green-Duwamish drainage basins were sampled monthly as part of King County’s Routine Stream Monitoring Program. Numerous water quality conventional parameters are measured, including those used to determine the Water Quality Index (WQI) – temperature, pH, fecal coliform bacteria, total suspended solids, dissolved oxygen, and nutrients. (See the Water Quality Index Background tab for information about this index.) On occasion, extreme weather conditions occur during routine sampling events (which is different from a pre-determined wet weather sampling event). Data collected during these events are not included in the WQI calculation to allow for the year-to-year comparison of routine events. Figure 1 shows the percentages of King County streams within each of the three WQI categories both with and without the wet weather events that occurred during the 2006-07 and 2000-01 water years to illustrate the impact that these events can have on the rating.

Based on the 2008 WQI calculations, 19 percent of the sixty-three sampling sites were rated “low concern”, 57 percent were rated “moderate concern”, and 24 percent were rated “high concern”. All sites rated of high concern were impacted in part by elevated concentrations of nitrogen and/or phosphorus. In addition, low dissolved oxygen concentrations, high fecal coliform bacteria, elevated temperatures, and elevated suspended solids/turbidity affected the WQI ratings of most of the high concern sites. View a table of WQI scores.

Of the 23 sites in WRIA 9 (seven sites are located on Vashon Island), eight were rated of low concern, 13 were of moderate concern, and 2 were of high concern (see WRIA 9 chart). Of the 40 sites in the WRIA 8, four were rated of low concern, 23 were of moderate concern, and 13 were of high concern (see WRIA 8 chart).

Environmental Factors Influencing the Water Quality Index

Overall stream water quality in King County is impacted by increased development in our region — primarily through stormwater runoff. In 2007-08, cumulative rainfall was below average compared to historical values, even with an exceptionally wet December. This is reflected in the somewhat higher WQI scores for the year. (Check out the County Hydrologic web site for more information about rainfall patterns in the last few years.)

Stormwater, combined sewer overflows (CSO's), waterfowl and pet wastes are the most likely sources of bacteria in urban streams. Poor livestock manure management and failing septic systems can be a potential source of bacteria in agricultural and suburban areas. In wetlands, wildlife excrement and stagnant water conditions can lead to elevated bacteria counts. High phosphorus concentrations are found in fecal material and elevated concentrations are often linked to similar sources as bacteria. In addition, elevated phosphorus concentrations are linked to areas undergoing development since phosphorus occurs naturally in soils, as well as organic material.

Low dissolved oxygen concentrations can be associated with low flows, wetlands, high temperatures (colder water holds more oxygen), and high levels of organic matter (bacteria use up oxygen in the process of decomposing).

The WQI indicator highlights the need for comprehensive and coordinated approaches to resolving problems related to instream flow management since lower flows and excessive storm water runoff exacerbates every water quality measurement of the WQI.