INCISION AND LATERAL MIGRATION ALONG TERRACES OF THE LOWER CHIPPEWA RIVER VALLEY, WI, USA: A GPR INVESTIGATION

The Lower Chippewa River Valley in west-central Wisconsin displays numerous terraces formed by episodes of incision and followed by lateral migration. Sediments deposited during the lateral migration of the river are primarily composed of sand while the underlying sediments consist of a mix of sand and gravel deposited by glaciofluvial process as the paleo-Chippewa River aggraded during the late Wisconsinan. Using high resolution ground penetrating radar (GPR) data, the depth to which the Chippewa River laterally migrated following each episode of incision was determined. GPR data was collected along numerous terraces using Sensors and Software PulseEkko 100 and 1000 GPR systems while laser leveling was used to determine topographic changes. High frequency antennae were used to attain the level of detail needed to determine the depth of deposits related to lateral migration. Step size varied from 0.1 m to 0.5 m, while antennae separation ranged from 0.5 m to 1.0 m. Common midpoint surveys (CMP) were conducted and resulted with a near surface velocity of 0.10-0.11 m/ns at most sites. The radar facies of the top most depositional layer were examined in order to better understand the nature of the river during times of lateral migration. The reflections show horizontal to sub-horizontal, semi-continuous reflections to a depth of three to four meters. The radar stratigraphic analysis suggests a well-defined episode of incision followed by lateral migration [braided river process] on all investigated terraces.