Long‐term storage and transport length scale of fine sediment: Analysis of a mercury release into a river

Excessive suspended sediment concentrations create important water quality problems, but scientists disagree on how to predict its movement through watersheds. Most models assume that fine-grained sediment moves rapidly far downstream, without recognizing the importance of episodic, long-term storage. Here a historic industrial release of mercury is interpreted as a decadal sediment tracer experiment, releasing sediment particles “tagged” with mercury that are deposited on floodplains. As expected, floodplain mercury inventories decrease exponentially downstream, with a characteristic decay length of 10 km (95% confidence interval: 5–25 km) that defines the distance suspended particles typically move downstream before entering storage. Floodplain mercury inventories are not significantly different above and below three colonial age mill dams (present at the time of mercury release but now breached), suggesting that these results reflect ongoing processes. Suspended sediment routing models that neglect long-term storage, and the watershed management plans based on them, may need revision.