City role: indirect
| Percentage of stream sampling locations with Water Quality Index greater than 40; and, percentage of twelve significant streams with BIBI score of 35 or greater.
The Washington State Department of Ecology developed a Water Quality Index (WQI) tool that synthesizes a variety of water quality field data into one score between zero and 100. WQI scores of 80 and above indicate a stream can support fish and wildlife, and is safe for human contact. Streams with scores between 40 and 79 are considered “marginal.” WQI scores of less than 40 indicate streams are of "greatest concern."
To determine a stream's WQI score, eight water quality parameters are measured:
- temperature (high temperature is bad)
- dissolved oxygen (low concentration is bad)
- pH (acidity too high or too low is bad)
- fecal coliform bacteria (pathogens, high is bad)
- nitrogen (high is typically bad)
- phosphorous (high is typically bad)
- total suspended solids (high is typically bad)
- turbidity (high is typically bad)
In 2009 the City implemented an updated WQI sampling protocol. Results from past years have been restated to conform to the new protocol. Water quality samples are now collected on a monthly basis over the water year (October to September). Samples are not taken during, or soon after, rain storms. This change produces more accurate index values as the WQI was designed to measure non-storm-related, low-flow stream conditions. Water quality typically declines during periods of stormwater runoff.
Percentage of Sites with WQI Greater than 40
The Benthic Index of Biotic Integrity (BIBI), also known as the “bug index” is a measure of the ecological health of Redmond streams and whether or not they can support native habitat. A score of 35 or higher is necessary to support native habitat. Scores range from 10 to 50. In 2013 the average score was 19, down from 21 in 2012.
Source: Natural Resources Division
Updated March 2014
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