MEANDER EVOLUTION IN THE LOWER THE WABASH RIVER OF POSEY COUNTY, INDIANA

A series of four cross cutting, inset meander scars on terrace treads within the lower Wabash River valley in western Posey County, Indiana show evidence of successive cut off events in a small geographic area. Additionally, the modern channel has meander pattern consistent with the older meander scars. These data suggest external controls on channel morphology such as tectonic (fault) influences or climate change. Utilizing LIDAR imagery, aerial photos, sediment cores and OSL dates, the channel/meander patterns and terrace morphology are correlated to external forcing factors.

The pattern on the oldest terrace is non-sinuous, compared to the two younger terraces, suggesting formation by a braided stream consistent with older terraces present elsewhere in the Wabash valley that are associated with the late Pleistocene Maumee torrent. Four sediment cores consist of a surface soil (0 to 1.5 m) developed in clay and silt, underlain by pebble rich fine to medium sand at depths from 3 to 5.5 m. Soils consist of Ap, Bt, C-horizons that extend to depths of 105 to152 cm, and have silt loam textures at the surface. An abrupt boundary exists between the overlying fine material and the unaltered, pebbly sand below. The surface fines are interpreted as eroded and reworked loess from the uplands deposited as channel fills on sand and pebble rich channel sediments. Coarse sediments underlying the silt and clay are interpreted as channel sands.

Sand was collected from three of the terraces below the channel fill/channel sand boundary for Optically Stimulated Luminescence (OSL) dating, analysis of which are nearly completed. However, soil development on, and previously reported radiocarbon ages from channel fills associated with younger terraces indicate the they are at least 800 years old, and are likely no older than Holocene. Regardless, circumstances indicate that meander abandonment occurs episodically at the same specific location, most likely thoroughout the Holocene. If correct, the propensity for meander cut offs at this location, could be related to its proximity to known faults associated with the Wabash valley seismic zone and/or to Pleistocene/Holocene climate related changes in discharge that triggered river incision, terrace formation, meander abandonement and channel scar backfilling.