Stream Quality Assessment Program

Stream Conditions

Based on biological data collected at 40 randomly selected locations throughout Fairfax County in 2017, approximately 75 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 starting 2004 through the most recent monitoring year 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 2016 biological monitoring results can be found in the 2017 Fairfax County Stormwater Status Report. As more data are reported annually, trends can be identified with greater certainty.

Monitoring Methods

Countywide monitoring is conducted annually using a probabilistic design approach. Using this approach, statistically valid inferences may be made about the condition of the county's streams. Each year, all potential sampling sites are stratified by stream order and 40 sampling locations are selected randomly. These 40 locations are then evaluated for bacteria levels, benthic macroinvertebrates, fish community and physical habitat during that year. Water quality parameters (such as water temperature, dissolved oxygen, specific conductance, pH and levels of nitrogen and phosphorus) are also collected. The results of the yearly monitoring events are compiled into annual reports  and are used to support the county's Environmental Quality Advisory Council's Annual Report on the Environment.The protocols for macroinvertebrate and fish monitoring and habitat assessment are based on the EPA's Rapid Bioassessment Protocol for Use in Wadeable Streams and Rivers. The modified protocols can be found in the Standard Operating Procedures Manual for the Fairfax County Biological Stream Monitoring Program.