R/SD-1

Strategic focus area:

Description:

The Conowingo Dam has historically trapped a significant fraction of the sediments and particulate nutrients carried by the Susquehanna River bound for Chesapeake Bay (CB). However, the effective trapping capacity of the dam may be decreasing, such that more of these materials reach the CB than in the past. However, the role of the extensive beds of submersed aquatic vegetation (SAV) that occupy the Susquehanna Flats (SF) in modulating these inputs has not yet been addressed. The resurgence of these SAV beds, and their cold- season senescence, may significantly mitigate the ecosystem effects of inflowing materials from behind the dam through seasonal trapping, re-release, burial, and transformation. However, very little data exist on the basic form and function of SF, especially from a sediment dynamics perspective.

This study focuses on the sedimentary history of the SF over the last ~100 years, and on the modern, seasonally varying, dynamics of sediment trapping and release on the SF. Four hypotheses drive this study: 1) the SF have been sediment starved since dam construction (with some lag), leading to changes in sediment texture and reduced sedimentation rates; however, further changes have occurred with recent dam infilling and SAV resurgence; 2) the SF trap fines during the growing season and release fines after senescence, introducing a lag in the input of fines to CB that reduces environmental impacts; 3) large events are critical to sedimentation on the SF because of large sediment inputs, fast currents, and waves; whether or not large events are directly observable the record of previous events should be present in sediment cores; 4) the SF SAV beds seasonally damp surface waves and concentrate flow and particle fluxes into the adjacent unvegetated channels.

Three field trips will occur each year - early spring when the SF are bare and runoff is high, at the height of the growing season in summer, and in the fall after SAV has mostly senesced and winds have increased. Observations will be made of sediment texture and seasonal- and decadal-scale sedimentation rates within and outside of the plant bed. An additional set of push cores will be collected at these sites for sediment erodibility testing. Seasonal changes in sediment elevation will be measured with graduated dowels. Various instruments will be deployed (1-week deployments) to measure tidal height and suspended particle size; velocity, salinity, temperature, turbidity, and waves over SF and in the adjacent main channel. Ancillary data will be obtained from MD-DNR water quality monitoring stations and the NOAA CBIBS Susquehanna buoy. Bathymetry data will be also be collected.

The results of this study will directly inform resource managers and regulators and will feed into the next generation of CB water quality models. We will continue our productive relationships with agency contacts to ensure that our results are directly communicated to resource managers. We will also create a project website, geared toward the general public, that includes links to all products related to the project.