FLUVIAL HISTORY OF MEANDER ABANDONMENT IN THE WABASH RIVER OF POSEY COUNTY, INDIANA

The Indiana side of the Wabash River valley, just northwest of Mount Vernon, Indiana contains meander scars that indicate former channel positions that can be identified on aerial photographs, topographic maps and from field observations. This research examined four sediment cores collected from meander scars within the valley so that fluvial and lacustrine depositional processes and the timing of river channel migration/abandonment could be determined. Two meander scars, chosen for cross-cutting relationships that allowed for relative age determination, were cored and sediment samples collected. Cores 1 and 2, located on former point bars associated with the channel from which Core 3 was collected, did not penetrate subsurface sand. Core 3, located within a channel scar 2.4 km (1.5 mi) from the modern river, contained clayey silt interbedded with sand, interpreted as rising- (sand) and falling-stage (silt/clay) flood deposits. A thinly-bedded clayey silt layer with laminar organics was observed from -1.96 to -3.25 m, and had an upper interval (21 cm) that was bioturbated, indicating a lacustrine environment (oxbow lake). Core 4, located 3.7 km (2.3 mi) from the river, also exhibited clayey silt interbedded with sand. However, the laminar organics observed in Core 3 were absent in Core 4. Radiocarbon dating of wood and organic rich sediments within the laminar organics zone from Core 3 produced age dates of 660 ±30 years before present and 810 ±30 years before present (A.D.1350 and A.D.1200 respectively). These data likely represent minimum ages for abandonment of the youngest meander, as Core 3 penetrated only 3.43 m to sandy sediments. The basal sand likely represents point bar or channel bar sands, as it lacks the depth to be interpreted as the channel base for such a large river. Additional cores, sediment samples and GPR data are needed to produce a more detailed interpretation of river behavior.