Estimations of paleochannel geometry and discharge using Ground Penetrating Radar (GPR) on terraces of the Le Sueur River, south-central Minnesota

Abstract:

Paleo channels, preserved on terraces via meander cutoffs during an incisional period, record the channel geometry and thus discharge throughout a river’s history. We measured paleochannel geometry on terraces throughout the Le Sueur River in south-central Minnesota, to track how channel geometry has changed throughout the last 13,400 years. A rapid drop in base level 13,400 yr B.P. triggered knickpoint migration and valley incision that is ongoing today. Since the 1800’s, the area has developed rapidly with an increase in agriculture and the amount of tile drainage, directly impacting river discharge by increasing water input to the river. Five paleochannels were identified on terraces along the Le Sueur River from 1m-resolution lidar data. Ground Penetrating Radar (GPR) was used to obtain a subsurface image across paleo-meanders to estimate the geometry of paleochannels. By measuring the geometry of paleochannels, we can compare the channel geometry and effective discharge at the time the terrace was being carved to today’s conditions. Three lines were run across each paleo-channel perpendicular to the historic water flow. Each of the 15 lines were “processed” using the EKKO Project 2 software supplied by Sensors and Software to sharpen the images, making it easier to identify the paleo channel geometry. OSL samples were collected from overbank deposits to determine the time of channel abandonment. Paleo discharge accompanied with depositional ages provide a history of flow conditions on the Le Sueur River. Preliminary results suggest the river channel has widened and increased in depth with time. This implies that the increase in agriculture and tile drainage since the area’s development has negatively impacted the Le Sueur River resulting in a change in channel morphometry more conducive to erosion along the bluffs and banks. This increase in erosion has directly impacted the amount of sediment delivered to the rivers from banks and bluffs, increasing the turbidity in this turbidity-impaired river system.