Feedbacks between sediment and submersed aquatic vegetation (SAV) beds under flood conditions in the upper Chesapeake Bay

Since the late 2000s, there has been a resurgence of the large, dense beds of submersed aquatic vegetation (SAV) that historically colonized the Susquehanna Flats (the subaqueous delta of the Susquehanna River) in upper Chesapeake Bay. These beds play an important role in modulating sediment input to Chesapeake Bay from its major tributary via sediment trapping within the vegetation. During high river flows, modeling results indicate that vegetation can also direct the sediment plume, but direct observations have been lacking. The Susquehanna River floods in summer 2018 provided an opportunity to evaluate sediment transport and deposition under high-flow conditions within the context of previous work and sampling in smaller SAV patches to the west of the main bed a few weeks later. Sediments were analyzed for grain size, organic content, and ⁷Be (half-life 53.3 days; indicator of recently eroded terrestrial material), and samples were collected for above- and below-ground biomass. Results were compared with river discharge, turbidity, and satellite images from ongoing monitoring efforts. 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 outside the patches, highlighting the role of vegetation in modulating sediment input from the Susquehanna River to the upper Chesapeake Bay.