The Maryland Department of Natural Resources says algae mats have been detected in the Potomac River from the Williamsport area to Riley Lock and they could be harmful to animals when ingested. Pet owners should make sure their animals stay out of the water in the affected areas until further notice. The DNR says the algae looks like mats of fine, green cotton and they are found on the shoreline, on rocks and plants floating downstream. The agency will monitor the algae and let the public know when conditions improve.