LITITZ, Pa. — Dorothy J. Merritts, a geology professor at Franklin & Marshall College in Lancaster, Pa., was not looking to turn hydrology on its ear when she started scouting possible research sites for her students a few years ago. But when she examined photographs of the steep, silty banks of the West Branch of Little Conestoga Creek, something did not look right. The silt was laminated, deposited in layers. She asked a colleague, Robert C. Walter, an expert on sediment, for his opinion. “Those are not stream sediments,” he told her. “Those are pond sediments.” In short, the streamscape...