Algal Mat Alerts Lifted for the North Fork of the Shenandoah River 

Algal Mat Alerts Lifted for the North Fork of the Shenandoah River near Town of Strasburg, VA – 2022 Harmful Algal Response Season Complete

November 4, 2022
Media Contact: Lorrie Andrew-Spear,

(Strasburg, Va.) – An Algal Mat Alert which was issued for the North Fork Shenandoah River for the Town of Strasburg in July and August has been lifted due to the discontinuation of HAB Response for the season (May – October). The HAB Task Force suspends response sampling at the end of October each year, when the recreational (swimming) season concludes and as temperatures begin cooling in natural waters. This enables conservation of response resources for peak swimming use. Algae mats which may still be sporadically present in October/November in areas of a waterbody may persist into late fall and possibly winter months. While cyanobacteria concentrations fluctuate during seasonal transitions, resources are not available to continue the response efforts in off-season months.

The original mat alert area began above the town at approximately Route 644 (Deep Hollow Lane), to include the Deer Rapids Road Bridge, to below the town at approximately Route 611 (Long Meadow Road), for a total of approximately 11.5 miles. No additional sampling in this vicinity could be conducted due to limited resources however recent reported observations suggest the mats have largely dissipated. The public should use caution while recreating in this vicinity and avoid algal mats if they are observed present along the bottom of the river or along shorelines.

The public, in general, is advised to avoid areas in natural waterways that have algal mats or discolored, scummy water. People should not allow pets to swim in areas where mat material is observed. Contact with these mats may cause skin rash and gastrointestinal illnesses, such as upset stomach, nausea, vomiting and diarrhea. If mats are producing toxins, consumption of mats could be fatal to dogs and other animals when ingested. Humans should never consume water or material from a natural waterbody because this water is not treated water and is not suitable for consumption.

VDH has observed no evidence of impacts to drinking water. The Office of Drinking Water is working with drinking water utilities to protect drinking water sources.

Algae mats typically occur when warm water and nutrients combine to make conditions favorable for algae growth. While late fall and winter blooms are uncommon, they may occur. Most algae species are harmless; however, some species may produce irritating compounds or toxins. Avoid discolored water, scums or mat material that are green or bluish-green because they are more likely to contain toxins.

To prevent illness, people should:

  • Avoid contact with mats which may be present in North Fork Shenandoah River above and below the Town of Strasburg.
  • If mats are unavoidable, do not attempt to recreate in the waterbody.
  • WHEN IN DOUBT, KEEP PEOPLE AND PETS OUT! Use your best judgment before recreating in natural water bodies.
  • Do not allow children or pets to drink from natural bodies of water or consume material in the water or along the shoreline.
  • Keep small children, pets, and livestock out of the areas experiencing an Algal Mat Alert. They do not understand the risks associated with mats and may drink river water or consume mats which could cause illness.
  • If you or your animals experience symptoms after swimming in or near the area under an Algal Mat Alert, seek medical/veterinarian care. You may also contact your local poison control center.
  • Additional resources for pet owners and veterinarians are available from the CDC at
  • To ensure fish filets are safe to eat, properly clean fish by removing skin, discarding all internal organs, and cooking fish to the proper temperature.
  • To view the Algal Mat Alert area, view the HAB Map online or the Algal Mat Status Report for the North Fork Shenandoah River 8.5.22.
  • If you suspect you or your animal experienced health-related effects following exposure to a bloom, contact the Virginia Harmful Algal Bloom Hotline at 1-888-238-6154.

For more information about harmful algae blooms, Algal Mat Alerts and Recreational Water Advisories, visit