In the Fall Zone

Rare Geology Creates Rare Species—and Lots of Them—in the Potomac Gorge

Eastern rivers tumble quickly from the hard bedrock of the Piedmont to the sandy Atlantic coastal plain: the Susquehanna in northeastern Maryland, the Rappahannock in Fredricksburg, Virginia, the James in Richmond. In many such fall zones, species from interior lands mingle with those of the coastal plain, creating a rich stew of biodiversity. But even among fall zones, the Potomac Gorge is special.

Here, the lazy Potomac River turns rambunctious, frothing wildly as the contents from 11,500-square-miles of its watershed funnels through a narrow bedrock gorge. Add rain from a hurricane or snowmelt from a blizzard, and the swollen Potomac can overtop the gorge’s 50-foot cliffs, periodically stripping river terraces of plants and soil, sometimes down to bare stone.

The gorge’s unusual geology, combined with its wildness and fall-zone location, has helped create “one of the greatest concentrations of rare species and natural communities in the eastern United States,” says Stephanie Flack, The Nature Conservancy’s Potomac Gorge Project director. (The Conservancy and the National Park Service co-own Bear Island, a 96-acre island in the gorge’s heart.) The gorge’s 30-plus types of natural vegetation communities include several that thrive on such natural disturbances. Among them is the globally rare riverside prairie, where Midwestern prairie plants like Indian grass and wild blue indigo grow on rocky riverside terraces. In forest farther from the river, groundwater from seeps and springs nurtures globally rare invertebrates called amphipods, some no bigger than a grain of rice. All told, more than 240 state and globally rare species have been recorded in the gorge. To preserve these treasures, the Conservancy and the Park Service in 2001 developed the area’s first comprehensive conservation plan. In the process, they took stock of what they knew about the gorge’s species, learning a lot about birds, mammals and trees, but very little about less charismatic species. This year’s bioblitz filled in those gaps.

Good news was plentiful: Volunteer scientists and naturalists found 1,034 species, including 169 species of beetles, 114 kinds of moths, and 47 types of dragonflies and damselflies. They found two plants that hadn’t been seen in Great Falls Park since the 1880s—a black birch tree and wavy hair grass. They found isolated seeps with a globally rare amphipod and an equally rare snail. They found a fly never before seen east of Iowa and a bee new to Virginia. But there was bad news, too: an aggressive, non-native crayfish that drives out native ones or tears them apart, as well as a slug from Europe and gypsy moths.

The bioblitz yielded a snapshot of biodiversity that lays the groundwork for more intensive surveys. Says Flack: “You have to know what’s out there to know what you need to conserve.” —D.F.