Fairfax County and USGS Partner for Watershed Monitoring

Automated Stream Gage at Flatlick Branch

Under a joint funding agreement between Fairfax County and the United States Geological Survey (USGS), the Stormwater Planning Division began a long-term monitoring effort in 2007 to identify countywide conditions and trends in stream water quality and quantity. The information collected will be used to evaluate the benefits of future watershed improvement projects.

Initially, there were 14 sites in the monitoring network. After five years of successful data collection, the decision was made to increase the spatial coverage of the network. In 2012, the study expanded to 20 sites to provide a more comprehensive overview of Fairfax County. Fifteen of these sites are monitored manually on a monthly basis; the remaining five sites are equipped with automated stream gages (pictured to the right) which are monitored continuously. Instruments at the five automated gages measure six indicators, including:

  • stream height (how much does the elevation of water change between normal conditions and storm events)
  • water temperature
  • dissolved oxygen (how much oxygen is in the water)
  • pH (how acidic or basic is the water)
  • specific conductance (a measurement of the dissolved solids in the water)
  • turbidity (how clear is in the water)

The automated stations measure these six indicators every 15 minutes, creating approximately 44,000 data points per year. Data are transmitted via satellite and posted hourly to a USGS stream gage website. These automated stations also collect stream water samples during storm events. These samples are analyzed for sediment and nutrient (nitrogen and phosphorus) concentrations. Monthly samples are collected at all 20 sites under various weather conditions and analyzed.  Approximately 300 water samples are collected and analyzed each year. Results from the 2012 sampling year are published in the 2012 Stormwater Status Report.

Interpretation of water quality conditions and trends requires multiple years of data; therefore, trend analyses are not yet available for this study. 

For additional information, please email the Stormwater Planning Division or call 703-324-5500, TTY 711.

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