How a river transports sediment

Fine-grained sediment is both light enough to be carried by a river’s current and heavy enough to settle on its floodplain. How far do grains travel before settling? To answer that question, Jim Pizzuto of the University of Delaware turned an episode of industrial pollution into an experiment. From 1929 to 1950, DuPont’s rayon mill in Waynesboro, Virginia, used mercury as a catalyst. Mercury subsequently leaked into the nearby South River, where it remains a health hazard. Because mercury readily attaches to suspended grains, its presence in a river’s floodplain can be used to trace sediment transport both in space (by extracting cores at different locations) and in time (by dating a core’s layers). Pizzuto analyzed the spatial distribution of mercury taken from cores along a 37-km-long stretch of the South River downstream of Waynesboro. A previous study had established that the concentration of mercury averaged along the same stretch peaked sharply between 1940 and 1960. Mercury-tagged sediment evidently made its way downstream in a single, decades-wide pulse. Indeed, Pizzuto found that the mercury concentration falls off exponentially as a function of downstream distance, consistent with a moving, gradually depleting pulse. The derived decay length of just 10 km suggests that fine sediment moves down the South River in a sequence of short hops that occur only when flood conditions strengthen the river’s current. (J. E. Pizzuto, Geophys. Res. Lett., in press.)