When rivers were our highways, the Roanoke River system was tantalizingly close to being a 400-mile-long superhighway. It had the potential to connect a bountiful Piedmont with seaports and far-flung markets. But at the fall line, where the river tumbled down to the coastal plain, roiling, bolder-strewn rapids brought heavily laden boats to a standstill. Maybe a canal could circumvent those rough waters...