Changing patterns in sediment delivery in incising agricultural watersheds

Background/Question/Methods

Management and restoration of aquatic ecosystems in sediment-impaired streams requires quantification of sediment sources.  The focus here is on determining magnitude and sources of suspended sediment in a tributary of the turbidity-impaired Minnesota River, USA.  In the predominantly-agricultural Le Sueur River, primary sources of fine-grained sediment are classified as upland field vs. near-channel sources, with near-channel sources including stream banks, bluffs, and ravines.  Our research group has been exploring the relative importance of near-channel vs. upland sediment sources in both modern times and throughout the Holocene.  The modern sediment budget was assembled from historic aerial photography, aerial lidar topographic data, repeat terrestrial lidar scans of bluffs, auto-sampling of ravines, and geochemical fingerprinting of suspended sediment during storms. 

Erosional patterns in the Le Sueur are fundamentally controlled by events near the end of the last glaciation.  The Minnesota River valley was carved through catastrophic drainage of glacial Lake Agassiz 13.4 ka BP.  As the valley incised, knickpoints began migrating upstream on tributaries like the Le Sueur, carving deep valleys where most near-channel sediment sources are derived.  The history of incision is recorded in hundreds of strath terraces, cut into glacial till through channel migration and incision.  Numerical modeling of valley evolution constrained by dated strath terraces was conducted to compare the modern sediment budget to that of the river prior to anthropogenic modification including widespread conversion of prairie and wetland to agricultural fields.

Results/Conclusions

The modern sediment budget shows that most fine sediment in the Le Sueur River comes from bluffs, banks, and ravines in the deeply-incised knick zone.  Modern decadal-scale near-channel erosion rates are 3-5 times higher than “pre-settlement” millennial-scale erosion rates determined from numerical modeling.  Depositional records from Lake Pepin downstream show a more dramatic 10-fold increase in deposition rates from pre-settlement times to the present.  Sediment fingerprinting shows that pre-settlement sediment loads were dominated by near-channel sediment sources.  As deposition rates rose in the late 1800s and early 1900s, the sources included an increasing percent coming from fields.  In the last few decades, deposition rates have remained high, but sediment loads are increasingly dominated by near-channel sources once again.  On-going changes in basin hydrology, from both installation of agricultural drainage systems and on-going climate change, have put more water in rivers, increasing rates of near-channel erosion.  This shift in sediment sources has significant implications for turbidity management to improve aquatic ecosystem health in the Minnesota River basin.