STREAM PROCESSES, LAND-USE CHANGE AND SEDIMENT SUPPLY, SWARR RUN, LANCASTER COUNTY, PA

Sediment pollution is one of the most important contributors to the degradation of the Chesapeake Bay. The Susquehanna River is one of the main sources of sediment to the Bay and this source could dramatically increase in the future. The reservoirs on the Susquehanna River are almost at sediment-storage capacity. The lowest reservoir (Conowingo) could be at capacity within a few decades (Langland and Hainly, 1997). Once this occurs little sediment will be trapped and prevented from entering the Bay. Sediment continues to be supplied to the river system from across the watershed. Understanding the sources of this sediment, as well as the processes contributing to the continued flow of sediment, are crucial if efforts to restore the Bay’s ecosystem are to be successful.

Our study has focused on one of the sub-basins of the Susquehanna River located in Lancaster County, PA. We have examined the spatial and temporal distribution of sediment in the stream channel and floodplain and linked this information to changes in the landscape over the last several thousand years.

Typically we see coarse-grained channel lag deposits formed by lateral migration of streams which are overlain by predominately fine-grained organic-rich (including large logs) sediment that accumulated slowly over several thousand years (based on radiocarbon ages). Deforestation resulting from European occupation beginning in the early 1700’s caused a >2x increase in the rate of sediment deposition in the floodplain and a decrease in the organic content of the sediment. Debris flows from the surrounding hillslopes resulted in matrix-supported conglomerates within the floodplain sediments. Modifications to the stream channels to facilitate agriculture were also common.

More recent landscape changes resulting from suburbanization have resulted in complex spatial and temporal responses in the stream system. Initially, sediment deposition in the channels increased, however changes in land development practices have resulted in increased peak stream flows and the remobilization of the sediment from the stream channels and floodplains. Most streams are incising and /or widening their channels causing high sediment yields. The resulting sediment pollutes the local streams and contributes to ongoing degradation of the Chesapeake Bay.