News Release

Potomac River Algae Blooms Beginning to Dissipate

Aug 15, 2011

Washington, D.C. –The algae blooms on the Potomac River that were reported on August 5 are beginning to dissipate according to regional experts monitoring the river; however, dense patches may still exist in localized areas.  The earlier advisories issued by the Maryland Department of Health and Mental Hygiene and the Stafford County, VA Department of Health recommend avoiding contact with the water and remain in effect until a more comprehensive assessment of the Potomac is completed the week of August 15th. 

The Metropolitan Washington Council of Governments (COG) is working with state and local governments, universities, and scientists from around the region to monitor and provide up-to-date information on the algae blooms that have developed in the river near Mattawoman Creek in Maryland and Aquia Creek in Virginia – more than 30 miles downstream of the region’s drinking water intakes. 

The algae, called Microcystis, appear like bright green paint on the surface of the water and can produce toxins which may cause skin irritation and nausea for people and pets that swallow or have prolonged contact with it.  The algae can also cause environmental problems for the river. In some circumstances, algae blooms can block sunlight and use up all the oxygen in the water, killing other plants and animals; however, the current bloom is not dense enough to cause these issues.  More information on Microcystis and other potentially harmful algae can be found on the Maryland Department of Natural Resources web sites at .

According to experts, this year’s weather has provided ideal conditions for the algae blooms—a combination of sunny, summer days, high temperatures, light winds, and low streamflows.  In addition, above normal rainfall and runoff during early spring may have provided nutrients needed to feed a bloom. 

“COG and our regional partners are regularly sharing information and assessing water quality conditions throughout the Potomac to gain a better understanding of the blooms and determine trends between river conditions and bloom growth,” said Stuart Freudberg, COG’s Director of Environmental Programs.  “We are also using monitoring data and environmental and computer models, which we hope will help understand the causes of this summer's bloom and forecast the likelihood of future blooms.” 

Microcystis blooms are fairly common in the summer months in areas with fresh water or little salt.  On the Potomac, the blooms stretch as far south as Maryland Point, depending on the amount of freshwater flow.  Typically, the blooms are localized and break up quickly.  In the summer of 1983, the tidal freshwater portion of the Potomac turned bright green in color due to a Microcystis bloom and covered 20 miles between Piscataway Creek, MD and Quantico, VA.  Improvements in wastewater treatment since 1983 have lead to much better Potomac water quality and fewer significant blooms.

COG is coordinating with officials from the State of Maryland, Commonwealth of Virginia, and District of Columbia as well as academics from George Mason University, the University of Maryland, and Virginia Tech. For the latest information and advisories concerning the algae bloom please visit 

You can also visit the Maryland Department of Mental Health and Hygiene website at  and the Stafford County Department of Health website at:

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