Rutledge, Abigail, Department of Earth and Environmental Science, Susquehanna University, 514 University Ave, Selinsgrove, PA 17870, rutledgea@susqu.edu; Elick, Jennifer, Department of Earth and Environmental Science, Susquehanna University, 514 University Ave, Selinsgrove, PA, elick@susqu.edu.
Some islands in the Susquehanna River have recently formed and developed from human activities. A small island cluster located downstream from the Selinsgrove Railroad Bridge (circa 1871) formed due to 1) an obstacle- the bridge, 2) coal and industrial legacy sediments, and 3) the removal of vegetation by locals. Aerial photographs and personal accounts reveal how the island cluster transitioned from a series of bars to vegetated landforms up to 0.5 acres in area and 2.2 m in elevation. We consider these fluvial islands to be “legacy islands”.
In this research we modeled the processes and formation of legacy islands. The island cluster initiated from an accumulation of gravel. The gravel formed a bar that was colonized by a pioneer species (American water-willow community). The willow caused finer sediment to fall out of suspension, and the substrate to aggrade. This promoted the growth of successive woody vegetation (riverine scour community) which colonized and stabilized the bar. Finally, a permanent silver maple-sycamore forest grew, transforming the bar into an island. One key factor in island growth was the capture of flood debris by multi-trunk trees which produced debris dams. Debris dams deflect high energy flood water, causing finer sediments to be deposited. The occasional loss of vegetation due to man resulted in the island transforming back to a bar.
The base of the island was composed of cobbles to gravel that decreased downstream (30 to 19 mm); sand occurred on the island’s tail end. Bituminous coal, quartzite, quartz/wacke sandstone, siltstone, brick, glass, and other human-caused debris formed some of the sediment. Much of this sand was composed of quartz, coal, and industrial waste. Minor amounts of the sand contained garnet, quartz with magnetite and muscovite (Pleistocene source). X-ray Fluorescence (XRF) of the cored sediments indicated that slight increases in silt corresponded to increases in certain oxides and metals (Fe2O3, Al2O3, K2O, MgO, Pb, Zn, Zr, and Ni). CaO increases were attributed to Corbicula and Pleurocera found between 100 and 150 cm in the core. Initial values of Pb (20 to 74 ppm) measured higher than local background levels.
Anthropogenic activities like construction, mining and deforestation have resulted in the formation and destruction of islands. Many of these island clusters have formed within the last 30 years and support a wide range of wildlife (mammals, migrating birds and waterfowl, amphibians, and reptiles). We need to understand how legacy islands form because they represent new fragments of the riparian ecosystem that may require future conservation and restoration. Additionally, the erosion of these floodplain sediments has consequences downstream. They add to the naturally produced sediments that accumulate behind dams, and they may be transported by major floods to lowland ecosystems like the Chesapeake Bay.
Fluvial Island, Legacy sediments, riverine scour, debris dams