THE FORMATION OF “LEGACY ISLANDS” IN THE SUSQUEHANNA RIVER: AN UNINTENTIONAL CONSEQUENCE OF INDUSTRIALIZATION

Recent fluvial islands in the Susquehanna River have formed from anthropogenic sediments, specifically coal waste, deposited upstream from manmade fish weirs. The V-shaped stone structures, produced by Native Americans and early European settlers, produce obstacles during flood events that may fill with sediments. Through a series of steps these man-made structures may form “legacy islands”. We recognize this progression involving 1) fish weir construction, 2) shoaling and gravel bar development, 3) pioneer vegetation colonization, 4) woody vegetation growth and debris dams, and 5) continued island growth. Strong river currents and lack of woody vegetation may prevent islands from forming. Major floods also have the potential to erode any previously deposited sediment, transforming an island into a bar.

Examples of each step in the legacy island process were observed throughout the river system. These locations were identified through comparison of aerial images (1938 to present) and represent where weirs were progressively infilled with sediment, producing bars that sometimes became islands. Sediment core samples were collected at each location and analyzed for grain size, mineralogy, and chemistry. Tree type, tree cores, and gravelometer measurements were also evaluated. Islands were found to be recent in age (up to 45 yrs), forming after major floods. Legacy islands were significantly impacted by floods from tropical storms (1972, 1975, and 2011) which added to or eroded the islands, depending on the amount and type of vegetation. Islands formed from glacial and naturally produced sediments, as well as those composed of coal, brick, magnetic slag, iron oxyhydroxide, coal ash, metallic industrial waste and plastics from upstream sources. These anthropogenically produced sediments may be toxic.

Legacy islands represent new floodplain landforms and habitat. Their occurrence, sediment composition, and structure require additional examination in order to understand man’s overall impact on the river system.