Kniss, Sylvia, Earth & Environmental Sciences, Susquehanna University, 514 University Ave., Selinsgrove, PA, 17870-116, knisss@susqu.edu; Weaver, Olivia, M., Earth & Environmental Sciences, Susquehanna University, 514 University Ave., Selinsgrove, PA, 17870-116, weavero@susqu.edu; Elick, Jennifer, M. Earth & Environmental Sciences Susquehanna University Susquehanna Univ Earth & Environmental Sciences 514 University Ave Selinsgrove PA, 17870-1164, elick@susqu.edu.
Several islands in the Susquehanna River formed solely from anthropogenic activity and major flood events and are therefore proposed here to be described as “legacy islands”. These islands are composed of both glacial and legacy sediments deposited upstream from man-made fish weirs that were produced and used by Native Americans and early European settlers. They are distinct from other alluvial islands due to their low elevation, V-shaped structure, sparse vegetation, length to width ratio, and nearly complete association with anthropogenic activities.
Historical aerial imagery (1939-present) was used to locate fish weir locations and islands that formed from the weirs. This study examined the transformation of an island near Beach Haven, PA in the North Branch of the Susquehanna River. It was surveyed, and 6 cores were drilled to determine the stratigraphy. Samples were analyzed for grain size, mineralogy, and chemistry. Sand to sandy loam was deposited on top of medium to coarse-grained gravel; the gravel represented high-energy flood events. This sediment was periodically rooted between floods. The finest sediment (clay and silt) was composed of vermiculite, chlorite, muscovite, and quartz. Sand size sediments considered glacial consisted of quartz and quartz with magnetite inclusions, andradite, pyrope, chromite, enstatite, zircon, and magnetite. Sand sediments from coal production included anthracite, magnetic glass, metallic industrial waste, shale fragments, ferric oxyhydroxide, hematite, and coke. The distribution of fine grain sizes corresponded to trends observed in some of the major oxides (Fe2O3, K2O, Na2O, MgO, and TiO2). Increases in these major oxides may be due to frequent small-scale floods or hydromorphic processes. This zone is represented by mottled and iron-rich layers in the cores. Trees on the island were cored; the oldest tree was 46 years in age.
Based on the aerial imagery and vegetation data, a timeline for the transition from bar to island was established. Since 1939, the gravel bar was impacted by many floods resulting in deposition and erosion. Periodic small-scale vegetation covered parts of the bar. After multiple, major flood events (1972 and 1975), vegetation colonized and stabilized the bar, resulting in the formation of the current island. Trees have been growing since at least 1978, stabilizing the island and causing it to grow in elevation. The major flood of 2011 flowed over the island, deeply scouring the surface, producing parallel-to-flow ruts (~80 cm) in the island soil with a layer of gravel 1.5 m above the river surface. Trees caught debris flowing downstream forming debris dams.
Historical aerial images help resolve how these islands form: 1. gravel bars form upstream from fish weirs, 2. vegetation colonizes gravel and trap sediment, allowing the island to grow and stabilize, and 3. significant floods may reset the process. There may be more “legacy islands” in the Susquehanna, and they may not be restricted to this river. Despite being an unintentional manmade feature comprised of mining waste, these islands provide a valuable habit for many types of organisms. These habits may be threatened by future global climate change due to stronger floods and weather events.
legacy islands, legacy sediments, Susquehanna River, Beach Haven