MAJOR FLOOD EVENTS IN THE SUSQUEHANNA RIVER VALLEY CONTRIBUTE TO “LEGACY ISLAND” FORMATION: DALMATIA, PA

A relatively “new” island in the Susquehanna River formed in the 1970’s from flood deposits composed of both natural and legacy sediments trapped upstream by a man-made fish weir. This “legacy” island is distinct from typical alluvial bar islands in that it formed nearly completely from anthropogenic activities. It is located near McKees Half Falls, upstream from Dalmatia, PA.

Historical aerial imagery from the Pennsylvania Spatial Data Access (PASDA) was used to identify fish weirs in the river from 1939 to present. Between 1970 and 1986, a small gravel bar developed upstream from a weir near Dalmatia. The floods from 1972 (TS Agnes) and 1975 (TS Eloise) were large contributors to the formation of this landform. During these floods, the weir was located in the thalweg of the North Branch and was an obstacle that caught bedload gravel and sediments. This produced a bar that later became a vegetated island. Today this island measures 68 m long and 35 m wide, and an area of 165 m2 with a larger surrounding gravel apron (D50 size of 31 mm).

Two sediment cores were drilled to the gravel base that were 1.17 (mid-island) and 1.2 (end of island) m long. Grain size (collected in 10 cm intervals) was analyzed using gravity settling. The soil was predominantly composed of sandy loam to loam. Sediments were also analyzed using X-ray Diffraction and Fluorescence (XRD and XRF). Elutriated slides of fine sediment contained a mineralogy of quartz, muscovite, kaolinite and illite. Using microscopy, the sand was determined to be composed of quartz, magnetite, feldspar, garnet, muscovite and small rock fragments. Other legacy sediment grains from coal waste included, anthracite coal fragments, magnetic slag, and ferric oxy hydroxide flakes. Plastic, plaster, and glass bottle shards were also found. The vegetation on the island consisted of three species of trees: sycamore, silver maple, and river birch; trees were age dated to be 21 to 26 years old. Flooding in 2011 deposited debris dams in the trees at the head of the island.

The Susquehanna River has experienced many floods due to tropical storms and nor’easters. Such floods can be responsible for the formation and destruction of islands. Several legacy islands in the Susquehanna are now known to have formed from major flood events. They represent a new form of alluvial bar island and habitat on the floodplain.