FLOODS ON THE SUSQUEHANNA RIVER: CAUSES OF HISTORIC FLOODS AND FLOODPLAIN EVOLUTION

As the longest river on the east coast of North America, the Susquehanna River collects runoff from many tributaries to ultimately discharge into Chesapeake Bay. High discharge events on both the West Branch and the North Branch have resulted in loss of human life as well as significant damage to public and private property. Mapping of floodplain and river terrace landforms enables specific estimates for the magnitudes of floods which have inundated those surfaces when indexed to their nearest gaging station. Gaging stations on the North Branch of the Susquehanna River in Pennsylvania at Towanda, Wilkes-Barre, and Danville have particularly long and complete records dating from 1913 to the present. Individual large flood magnitudes have also been estimated for floods prior to the first systematic gaging, and there was much consideration of flooding and weather during construction of the North Branch Canal beginning in 1830. Partitioning the flood populations based upon meteorological cause, tropical cyclones including Agnes (1972) and Eloise (1975) do emerge as particularly significant in both number of deaths and property damage. However, the most prevalent cause for discharges which exceed bankfull is actually late winter to early spring events in which snowmelt contributes additional runoff. Changes in floodplain topography are evident from mapping of a Holocene first terrace with remnants on the valley walls as well as on islands where the river is braided. Vertical accretion of river silt in these terrace remnants began sometime during the early Holocene following an earlier river regime more characterized by incision, lateral channel migration, and bedload transport.