The endangered, semi-aquatic plant Ptilimnium nodosum had historically been known in the Potomac River drainage only from one site on Sleepy Creek, West Virginia and from the Potomac River, Maryland, where it was thought to have been extirpated. After extensive surveys in the Potomac drainage, the species has been found to occur along short to extensive stretches of the Cacapon River and Sleepy Creek in West Virginia and along Fifteen Mile Creek and Sideling Hill Creek, Maryland. It is also extant, in very small numbers, along the mainstem Potomac River.