ON A DREARY STRETCH of Interstate 95, shadowed by the corporate office towers of Wilmington, Delaware, an overpass spans a narrow valley. Passing motorists, zooming through on their way toward Baltimore or Philadelphia, might cross a hundred times and barely notice the gorge’s existence: hardly more than a wrinkle in the landscape, a glimpse of treetops vanishing in the rearview mirror. They would hardly guess at the little world enfolded within it, much less that it conceals what is arguably one of America’s great waterways.
The Brandywine River is not, to be sure, the Mississippi, the Colorado, or the Hudson. It has inspired no epic novels, gouged no mighty canyons, carried on its bosom no famous ships. Indeed, in most places the Brandywine is navigable only by canoe or kayak,…