Changes in channel geometry through the Holocene in the Le Sueur River, south-central Minnesota, USA

Abstract

Paleochannels preserved on terraces via meander cutoffs during an incisional period record the channel geometry and thus discharge at distinct points in time 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 over 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 associated 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 paleomeanders to estimate the geometry of paleochannels. Paleochannel geometry and estimated discharge were then compared to modern conditions to assess how much change has occurred. Three lines were run across each paleochannel 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 paleochannel geometry. Paleodischarge was determined using the Law of the Wall and Manning's Equation, using modern slope and roughness conditions. OSL samples were collected from overbank deposits on terraces to determine the time of channel abandonment, and supplemented with terrace ages obtained from a numerical model of valley incision. Paleodischarge coupled with depositional ages provide a history of flow conditions on the Le Sueur River. Results show an increase in channel widths from the time paleochannels were occupied to modern channel dimensions from an average of 20 meters to 35 meters. The change was not constant through time, as all paleochannels analyzed on terraces had similar-sized channels. The best way to determine paleogeometry was using the 'best interpretation' of GPR data couple with coring data; and paleodischarge was best estimated using Manning's equation with an n value of 0.035. Results show an increase in discharge compared to paleochannels of a factor of two. Uncertainty estimates in GPR-based paleogeometry can change paleodischarge calculations by 50 %. Incremental flood frequency analyses, based on data obtained from the Red Jacket stream gage at the outlet of the Le Sueur, suggest a 1.5- and 2-year flood of 102 m3/s and 154 m3/s, respectively, which is comparable to estimations of bankfull based on current channel geometry at the Red Jacket gage, validating the methodology. Problems associated with paleogeometry estimations are primarily due to meander bend preservation in the subsurface, challenging GPR interpretation. The increase in channel geometry and discharge implies that the increase in flow associated with drainage and climate change since the area’s development has greatly impacted the Le Sueur River. This resulted in a change in channel morphometry through increased erosion along the bluffs and banks, widening channels. This increase in erosion has directly impacted the amount of sediment delivered to the rivers from banks and bluffs, increasing the fine sediment load in this turbidity-impaired river system.