Stream Quality Assessment Program

A stream is a combination of all of its physical, chemical and biological characteristics. Human activities shape and alter many of these characteristics. The health of our rivers and streams is closely linked to their surrounding watersheds. Ecologists measure the extent to which human activities affect watershed health by gathering and recording information on physical conditions, water quality and living communities in streams and in the surrounding environment. The long-term stream monitoring program meets the requirements of state and federal regulations and supports the Board of Supervisors' environmental improvement program by providing an ongoing evaluation of the streams. The program maintains a substantial database, which over time will be used to determine the overall rate of change or trends in the conditions of our waterways.

Stream Conditions

Based on biological data collected at 40 randomly selected locations throughout Fairfax County in 2015, approximately 80 percent of Fairfax County's streams are in fair to very poor condition. This indicates that many of our streams are significantly impaired and lack biological diversity.

The Stream Quality Index (SQI) is based on annual data collected on resident populations of benthic macroinvertebrates (animals without a backbone that live on the bottom of a stream, river, lake and are visible to the naked eye). As benthic macroinvertebrates are good indicators of water quality, the SQI is used to evaluate long-term trends in the overall health of streams. Each of the 40 randomly selected locations is placed in one of five rating categories (excellent, good, fair, poor or very poor) based on the diversity of benthic macroinvertebrates found in that stream segment. An index value ranging from one to five, with a higher number indicating better stream quality, is calculated for the year based on the percent of sampling locations that fall into each rating category. Results from 2004 through 2015 can be found in the table below. With ten years of reference data available, the SQI from 2013 and forward are calculated using a new cumulative reference data. This process will reduce the variability created by yearly disturbances to the reference sites (i.e. drought). Previous years' SQIs have been back calculated with this new reference set and could show different results than seen in previous publications.

More information about the 2015 biological monitoring results can be found in the 2016 Fairfax County Stormwater Status Report. Over the past ten years of sampling, a very small increase in the overall index has emerged. As more data are reported annually, trends can be identified with greater certainty.

Sampling Year Percent Very Poor Percent Poor Percent Fair Percent Good Percent Excellent Stream Quality Index
2004 40.0 30.0 16.7 13.3 0 2.0
2005 15.0 32.5 35 7.5 10 2.7
2006 38.6 36.3 11.4 11.4 2.3 2.0
2007 17.5 35 12.5 20 15 2.8
2008 27.5 30 20 15 7.5 2.5
2009 37.5 37.5 12.5 7.5 5 2.1
2010 17.5 37.5 22.5 15 7.5 2.6
2011 12.5 35 20 20 12.5 2.9
2012 33.3 30.8 20.5 5.1 10.3 2.3
2013 22.5 27.5 17.5 15 17.5 2.8
2014 27.5 25 15 27.5 5 2.6
2015 10 30 40 17.5 2.5 2.7

Monitoring Methods

Stream and watershed health are evaluated using a variety of indicators such as water quality parameters, bacteria levels, resident invertebrate and fish communities and habitat conditions. The results of the yearly monitoring events are compiled into annual reports and are used to support other environmental initiatives. Additional information on monitoring methods can be found by selecting one of the pictures below.

For more information on the Stream Quality Assessment Program, please email the Stormwater Planning Division or call 703-324-5500, TTY 711.

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