Curley, Peyton, Earth & Environmental Sciences, Susquehanna University, 514 University Ave., Selinsgrove, PA, 17870-1164, curley@susqu.edu; Elick, Jennifer, Earth & Environmental Sciences, Susquehanna University, 514 University Ave., Selinsgrove, PA, 17870-1164, elick@susqu.edu.
A recently produced island in the mainstem of the Susquehanna River near Dalmatia, PA represents a new landform tentatively described as a “legacy island”. This island is situated on the Keyser and Tonoloway Formations (Silurian-Devonian) near McKees Half Falls. Historical aerial imagery from the Pennsylvania Spatial Data Access (PASDA) website depicts a series of fish weirs spanning the river near Dalmatia from 1939 until 1970. Between 1970 and 1983, a small, vegetated landform develops over one of the fish weirs. Since 1970, the Susquehanna River has experienced many floods related to tropical storms and nor’easters. The floods from 1972 (TS Agnes) and 1975 (TS Eloise) may have been large contributors to the formation of this island. During these floods, the fish weir may have been located near or at the thalweg of the North Branch flow. The weir was an obstacle that caught bedload gravel and sediments. Later floods added sediment to the growing feature. By 1983, high altitude imagery depicts a tiny bar that is vegetated. This bar grows into the small island that today is 68 m long and 35 m wide; it has an area of 165 m2 with a larger surrounding gravel apron (D50 size of 31 mm).
The island soil is vegetated. The soil was examined by drilling two cores until gravel was reached, approximately 1.2 and 1.17 m depth. Grain size of the soil samples (collected in 10 cm intervals) was analyzed using a gravity settling technique. The soil was predominantly composed of sandy loam to loam. Sediments were also analyzed using X-ray Diffractometry (XRD). Elutriated slides of fine sediment revealed a mineralogy of quartz, magnetite, muscovite, kaolinite and illite. Using microscopy, the composition of the sand was determined to be composed of quartz, magnetite, garnet, muscovite, and small rock fragments. Other grains associated with coal waste included hematite, anthracite coal fragments, magnetic slag, and ferric oxy hydroxide flakes. Plastic was identified in the soil approximately 40 cm below the surface. The vegetation on the island consisted of three species of trees: sycamore (42), silver maple (14), and river birch (9). Many of the trees exhibited scarring at 1 to 1.5 m above the soil. An understory composed of forbs and sedges, including Japanese knotweed, smart weed, and stilt grass was also identified.  Recent flooding (2011?) has deposited debris dams in the trees at the head of the island.
This island is different from other typical alluvial bar islands in how it formed. It developed from a fish weir and is composed of legacy sediment composed of coal waste and glacial sediments. Fish weirs were known to have been constructed and used by both Native Americans and early European settlers to catch shad and eel. This island is one of several islands known to have formed this way and represents a new form of alluvial bar island formation. The island resulted from two of the greatest floods to affect the region in recent time.