Sediment Sources to the Le Sueur River, Minnesota, Over Multiple Temporal and Spatial Scales

The incising Le Sueur River in south-central Minnesota exports high suspended sediment loads to the turbidity-impaired Minnesota River. Watershed management requires a solid understanding of sediment sources, sinks, and transport pathways to control excess sediment loading. We are developing an integrated sediment budget for the Le Sueur River watershed using a combination of LiDAR DEMs, aerial photographs, field observations, geochemical fingerprinting, and stream gaging.

Rapid base-level fall at the outlet of the Le Sueur River 11.5 ka BP triggered up to 70 m of channel incision at the mouth, spawning knickpoints that now separate the basin into an upper watershed, receiving sediment primarily from uplands and streambanks, and a lower incised zone which receives additional sediment from high bluffs and ravines. Paired stream gages installed above and below knick zones show dramatic increases in sediment loading above that expected from increases in drainage area, indicating substantial inputs from bluffs and ravines, potentially as high as 61-74% of the total suspended sediment loads.

We are measuring sediment contributions from bluffs and ravines over multiple temporal and spatial scales to compare with modern gaging on mainstem channels and upland ditches. LiDAR DEMs were used to determine total volume of sediment removed from the lower valleys since the initial incision event 11.5 ka BP. Aerial photographs from 1938 – 2006 were used to determine bluff retreat rates and ravine tip elongation over the past 70 years. Ground-based, side-scanning LiDAR is being used to track modern bluff retreat rates annually starting in 2007. Modern sediment loading from select ravines is being monitored through a network of gaging stations with auto samplers. Cosmogenic radionuclides are being used to track cumulative inputs from bluffs and deep ravines, taking advantage of the unique ratio of Be¹⁰:Al²⁶ found in tills through which the river is downcutting.