Aruncus dioicus; Rosaceae (rose family)
According to one expert from the Maryland Native Plant Society, goatsbeard grows in the coastal plain and is rare in the piedmont, so I’m a little tickled that I found it near Cabin John Creek. It can grow up to six feet tall, and likes wet soils. The plants I found were growing right out of a steep hillside next to the water.
Goatsbeard is one of the few native perennials showy enough to be a common garden plant. You can find cultivars for sale at any decent plant nursery.