Sediment Transport Processes over the Seasonally Vegetated Bayhead Delta of the Susquehanna River, Upper Chesapeake Bay

Susquehanna Flats (SF) in upper Chesapeake Bay is the subaqueous bayhead delta of the Susquehanna River. It is a large (>50 km²) predominantly sandy feature, with finer sediments towards the southern end. The SF received large sediment loads during the 19^th and early 20^thcenturies, but sediment loads were curtailed by construction of the Conowingo Dam (CD) several km upstream in 1928. The large beds of submerged grasses (SAV) that historically occupied the SF effectively disappeared following Hurricane Agnes in 1972, but have recovered dramatically in the last decade. The CD also has recently reached sediment storage capacity and episodic sediment loads to the SF are increasing. We carried out observations of physical and sedimentological processes over the SF from 2013-2015, with SAV observations starting several years previously. In combination with nearby continuous monitoring stations, these data provide a glimpse of the processes that control sediment delivery, retention, and bypassing in the vicinity of the SF.

The dominant sediment transport feature over the SF is the seasonal influence of the SAV beds. While the shallow depths of the flats tend to focus flow and sediment fluxes into the navigation channel even in winter, seasonal increases in SAV abundance greatly reinforce this pattern. By late summer, both fine sediment erodibility and water column turbidity are significantly lower inside the beds than outside. However, the SAV beds also increase sediment retention. The most dramatic example of this was during a large flow event after a tropical storm in 2011, when large amounts of fine sediment were deposited in the beds. Late spring turbidity levels did not return to normal until the 3^rd year after the storm. ⁷Be distributions in the surface sediments of the bed also show evidence of enhanced sediment retention during the growing season. An idealized flow and sediment transport model of the SF system replicates and expands on these observational results.