EXTRACTING SUSPENDED SEDIMENT TRAVEL TIMES FROM FLUVIAL SEDIMENT BUDGETS – GEOMORPHOLOGY CAN SUPPORT WATERSHED MANAGEMENT DECISIONS

Reducing the loading of suspended sediment and/or particle-borne contaminants (heavy metals, nutrients) is often a goal of watershed Best Management Practices (BMPs), but BMPs are usually evaluated using naïve watershed models that neglect geomorphic processes of erosion and deposition. Since the 1960s, sediment budgets have been used by geomorphologists to document patterns of sediment movement through watersheds, but even though sediment budgets quantify the fluxes between different storage reservoirs, they are not sediment routing tools that can track a specific population of sediment. Using principles of conservation of mass, spatially averaged sediment routing models can be developed that directly utilize sediment budget data, thereby providing geomorphically realistic predictions of the length and time scales of downstream transport through watersheds. The necessary reach averaging, however, introduces a new unknown variable: the length of time particles remain in storage, referred to as the particle’s transit time. While rates of erosion and deposition are well-studied in fluvial systems, sediment transit time distributions are virtually unknown, and simplifying assumptions (treating floodplains as “well-mixed”, for example) are untenable. Fortunately, transit time distributions can be estimated from highly resolved realizations of fluvial paleotopography, but only if detailed stratigraphic and geochronologic data are available, and most importantly, if the bias resulting from incomplete preservation can be accounted for. In the Chesapeake Bay watershed, the mean transit time for floodplain deposits is currently ~ 500 years, while average downstream transport velocities (including the time particles spend in storage) are 4-6 orders of magnitude slower than water velocities in the channel, suggesting that BMPs located in the headwaters will not appreciably benefit the Bay for 100-1000 years. These results suggest that geomorphically informed models of sediment routing can significantly improve the effectiveness of watershed restoration planning.