Event-based sediment budgets were developed in a small agricultural sub-watershed using radionuclide tracers in conjunction with traditional monitoring techniques. The result of these budgets quantified the flux of material from each sediment source in the sub-watershed. The first step in the study was to quantify the net flux of material through the watershed outlet for runoff events. Results indicated that a pronounced clockwise hysteresis effect occurred during all studied events. The cause of the hysteresis effect was attributed to exhaustion of the upland (i.e., hillslopes and floodplains) sediment source. The hysteresis effect was dampened during an extreme flash flood event that caused overbank flow. This dampening was attributed to an increase in upland sediment mobilization resulting from overbank flow. Results of the event-based monitoring were compared against a previously developed sediment rating curve that assumed a power-law relationship between suspended sediment transport and water flow rates. The results indicate that the power-law relationship grossly under predicted the sediment flux over each runoff event. A tracing technique was utilized to establish the relative contributions from the uplands and the stream channel (i.e., channel banks and bed). This technique used the relationship between the naturally occurring radionuclide tracers 7Be and 210Pbxs to differentiate eroded upland soils from channel-derived sediments in the suspended sediment loads. A simple two end-member unmixing model was used to determine the relative contribution from each source. Results indicate that the upland source was the dominant contributor to the suspended load early in the runoff events, but channel contributions were more prevalent at later stages, reinforcing the conclusion drawn from the hysteresis observation. Further analysis of the results indicated that the uplands contributed the majority of the material to the suspended load because the sediment transport rate during the early stages of the event was much larger than during the later stages. Therefore, watershed managers wishing to minimize non-point source pollution resulting from erosion should first focus their efforts on reducing erosion of upland soils.

Coastal marshes serve important ecologic and economic functions, such as providing habitat, absorbing floodwaters and storm surges, and sequestering carbon. Throughout the northern Gulf of Mexico, coastal marshes are disappearing due to wave attack, sea-level rise, sediment export, and subsidence. Marsh area increases when sediments accumulate at the marsh shoreline, accrete vertically, and when non-marsh areas are colonized by marsh vegetation. Marsh shoreline erosion results in net marsh loss when transgression rates at the marsh-water edge exceed upland-marsh migration. The balance between marsh destroying and marsh creating processes determines the long-term survivability of a marsh system. Thus, processes of shoreline change are important considerations when evaluating the overall health and vulnerability of coastal marshes. Shoreline erosion can be measured using remotely sensed data in a geographic information system. Using shoreline position delineated from aerial imagery, historic maps, and field surveys, shoreline change analysis estimates long- and short-term shoreline movements to identify erosion or accretion for coastal marshes at the Grand Bay National Estuarine Research Reserve (GBNERR) and Wildlife Refuge (GBNWR) on the border of Mississippi and Alabama, USA. However, these techniques do not directly provide information on sediment deposition on the marsh surface. To understand sediment deposition, four study sites provide in-situ measurements of sediment deposition using sediment plates and sediment tracers (silica beads) that were collected every three months. Analysis of the shoreline change data and in-situ sediment data for the GBNERR showed that in 2017, all of the shorelines at the study sites are eroding at rates between -0.50 m/yr and -3.39 m/yr, an average rate of -1.45 m/yr. Positive sediment deposition rates were measured from 5-20 meters inland of the marsh shoreline during each season (3-month period) (0.19 ± 0.05 cm [Fall], 0.26 ± 0.11 cm [Winter], 0.48 ± 0.12 cm [Spring], 0.63 ± 0.15 cm [Summer]), indicating sediment deposition increased with every season. Sediment tracer (silica-bead) counts confirmed that sediment was transported onto the marsh surface from eroding marsh shorelines. Higher energy sites had more beads deposited on the sediment plates than the low energy tidal creek site, due to the different wave and tidal conditions between the sites. Increased wave and tidal energy correlated to increased sediment transport further into the marsh. The relative importance of this marsh cannibalism for the long-term marsh survival depends on factors, such as wave attack, as they control the rate of persistent lateral marsh loss. This findings in this thesis suggests that material from eroding marsh edges contributes to the ability of the interior of marshes to maintain their elevation with respect to rising sea levels.