Influence of Sediment-Vegetation Feedbacks on Sediment Transport and Fate in the Upper Chesapeake Bay

Feedbacks between sediment and plants in the submersed aquatic vegetation (SAV) beds of the Susquehanna Flats play an important role in modulating sediment delivery into the upper Chesapeake Bay from its major tributary (Susquehanna River). Recent modeling work has shown that SAV can steer the river plume, directly controlling sediment transport and deposition. However, the likely transport mechanisms differ between flood and non-flood conditions, and the fate of sediment in the upper Bay depends on event timing. An opportunity to evaluate these insights in the field occurred with several flood events in summer 2018. Sediment and biomass samples were collected after the flood in the large, continuous main bed of the Flats, as well as smaller patches to the west of the main bed. Sediments were analyzed for grain size and organic content, and deposition rates were calculated with 7Be (half-life 53.3 days). Results were compared to previous observations during non-flood conditions in the main bed in 2014-2015 and in the patches in 2019. Overall, we found that sediment deposition in the main bed of the Susquehanna Flats after the July 2018 flood event was remarkably similar to observations from previous years with much lower river discharge. However, field sampling occurred on the rising limb of a second flood event in August 2018, which could have maintained sediment in suspension and/or eroded recently deposited flood sediment. It is also possible that vegetation on the Flats blocked the river plume and/or steered it to the west, as suggested by satellite imagery and recent model results. Sedimentation within vegetated patches west of the main bed was similar to observations in the main bed and much higher than unvegetated areas outside the patches, highlighting the role of vegetation in modulating sediment input from the Susquehanna River to the upper Chesapeake Bay.