FLUVIAL ADJUSTMENT TO PLEISTOCENE BASE-LEVEL FALL AFFECTS SEDIMENT BUDGETS OF MINNESOTA RIVER TRIBUTARIES

As society strives to address turbidity-impaired rivers, sediment budgets can be a useful decision-making tool. An inventory of sediment source extents and measurements of supply rates can quantify sediment loading from watershed features and help landowners, land managers and other interested parties allocate resources effectively. When geologic history, watershed hydrology, land use patterns, surficial geology and high suspended sediment loads are similar in a group of adjacent basins, can information from one carefully-crafted sediment budget also transcend watershed boundaries? The Minnesota River provides the perfect test case, where turbid tributaries draining extensively row-cropped basins are adjusting to a profound (70m) base-level fall 13.4 ka. A previous sediment budget addressed the tributary with the highest sediment yield with particular focus on the “knickzone:” reaches below knickpoints where adjustment to base-level fall drives erosion of near-channel features like bluffs. This work revealed near-channel sediment sources supply more sediment to channels than upland sources like agricultural fields. To explore how sediment source inventories and supply rates change nearby, we extended that budget upstream and to adjacent watersheds in the Blue Earth River basin. Our budgets use historic aerial photos and lidar-derived DEMs in ArcGIS to measure bluff retreat rates and delineate source extents. We also adjust for sediment source texture and bulk density, explore how different erosion rate extrapolation techniques affect estimated suspended load, and estimate sediment storage in upland lakes.

Our work improves sediment budget precision basin-wide and confirms the importance of bluffs in the knickzone as sediment sources. We found no statistically significant correlations between decadal bluff retreat rates and parameters such as bluff vegetative cover, slope, size, aspect, sediment texture or stream power. However, we found strong spatial trends in rates of bluff sediment yield in all subwatersheds. High sediment yield near the mouth of the basin is a response, over geologic timescales, to geologic history. The basin continues to adjust to a spectacular base-level fall, priming the knickzone to deliver high sediment yields for thousands of years to come.