Science Serving Maryland's Coasts

R/E-22a

Exploring the Connectivity of Sediment Transport in Upper Chesapeake Bay

Principal Investigator: 

Cindy Palinkas

Start/End Year: 

2015 to 2017

Institution: 

Horn Point Laboratory, University of Maryland Center for Environmental Science

Co-Principal investigator: 

Emily Russ, Horn Point Laboratory, University of Maryland Center for Environmental Science

Strategic focus area: 

Resilient ecosystem processes and responses

Description: 

Since the early twentieth century, the Conowingo Dam has captured a significant portion of the Susquehanna River sediment load bound for the upper Chesapeake Bay. However, the Conowingo Reservoir is nearing its sediment capacity, which has resulted in decreased reservoir sedimentation and increased suspended sediment delivery to the Bay. Additionally, sediment scoured from behind the dam during high flow events contributes to the increased sediment input into the Bay. Sediment-transport models, developed with the aid of sediment budgets, are used to guide management of this excess sediment input; however, these models were calibrated assuming that the Conowingo Reservoir would maintain a constant sediment capacity. Also, recently resurged submersed aquatic vegetation (SAV) beds affect sediment deposition, and their effects were not included in sediment-transport model calibration. The Bay sediment-transport model also needs to take into account the contribution from various sources of sediment (suspended fluvial sediment, Conowingo Reservoir bottom sediments, and sediment from shoreline erosion). Sediment provenance analyses can reveal the different geochemical characteristics between sources, and their relative contribution to the depositional environments.

This research investigates how sediment transport is connected from the lower Susquehanna River into the upper Bay by developing event- and decadal-scale sediment budgets and exploring the potential of sediment provenance techniques to differentiate sediment sources in the upper Bay. This study is driven by the following hypotheses: 1) Highest sediment deposition rates occur on the Susquehanna Flats due to the presence of SAV beds. 2) Recently eroded Susquehanna River sediments will have different geochemical characteristics, such as detectable 7Be activities and greater nutrient bioavailability, than bottom sediments in the Conowingo Reservoir. 3) Coal deposits can be used to distinguish between the coal-rich Susquehanna River sediments and coastal erosion sediments. 4) Sediments deposited in the Susquehanna Flats and upper Bay during lower flow are a mixture of suspended fluvial sediments and sediments from shoreline erosion; during high flow events, Conowingo Reservoir bottom sediments are present as well.

Data from separate research projects on sediment characteristics in the Conowingo Reservoir and Susquehanna Flats will be integrated with sediment data collected from the upper Bay for this project. Decadal- and event-scale accumulation rates will be calculated and compared with sediment load information in order to construct decadal and event sediment budgets. The sediments from the three depositional environments will also be analyzed for the sediment provenance analysis. Radioisotopes, nutrients, and coal signatures in the source sediments will be compared with those in the target sediments to determine the relative sediment contribution from each source.

The results from this study are expected to help improve sediment-transport models and their predictive capabilities, in response to changing reservoir capacity. This study will inform the 2017 Chesapeake Bay Program mid-point assessment, and help guide management decisions that improve water quality. We will directly work with a watershed restoration specialist in order to effectively communicate our results with local governments and agencies, as well as engage the general public to promote coastal resilience.

Impact/Outcome: 

RelevanceThe reservoir behind the Conowingo Dam, on the Susquehanna River at the top of the Chesapeake Bay, historically trapped nutrients and sediments from the river, but the reservoir is filling with mud and losing its ability to do so. Maryland public officials are highly concerned about the reservoir and dam's future and looking to its owner, a utility company, to manage the sediments before issuing it a multi-year license. Controlling the sediment coming into the bay is important for Pennsylvania and Maryland to meet Environmental Protection Agency Total Maximum Daily Load requirements to reduce sediment pollution. Managers need information to understand if the recent expansion of sea grass beds below the dam helps remove these sediments from the water.

ResponseA team of researchers with the University of Maryland Center for Environmental Science was curious about the potential rebound and impact of reduced sediment trapping by the Conowingo Dam. They knew that elsewhere in the Chesapeake Bay the presence of grasses can correlate with clear water and increase the amount of sunlight reaching shallow bay bottom. The researchers designed a study to learn how the amount and type of sediment coming over the dam might affect submerged aquatic grasses, water clarity and the flow of sediments below the dam. The study focused on a large seagrass bed known as the Susquehanna Flats (SF) located downstream of the Conowingo Dam.

ResultsFour of the five sites in the SF seagrass bed showed higher sedimentation rates when grasses were present. Scientists determined muddy sediments are trapped by the grasses because of the reduced current and wave energy in grass beds, suggesting that the Susquehanna Flats helps prevent mud from traveling farther down the Bay. The study, among the first to consider sediment transport from the Susquehanna River to upper Chesapeake Bay since the reduced trapping by the Conowingo Dam began, will help public officials protect grass beds and advise management decisions regarding the dam and its reservoir.