ANTHROPOGENIC EFFECTS ON THE PROPAGATION OF DETRITAL PROVENANCE SIGNATURES: INSIGHTS FROM THE MISSOURI AND YELLOWSTONE RIVERS

The Missouri River, a major tributary of the Mississippi River, supplied roughly 80-90% of the sediment in the Mississippi making it to the Gulf of Mexico before European settlement of the United States. Currently the Missouri River accounts for only ~ 60% of the modern sediment load reaching the Gulf of Mexico, due to the cumulative effects of anthropogenic activities such as dam construction, agricultural water withdrawal, sand mining, and channel modification through civil engineering project such as levee stabilization. Recent analytical advancements in the rapid acquisition of geochronologic data has allowed detrital zircon U-Pb geochronology to emerge as a tool to quantitatively assess sediment budgets of modern sediment dispersal systems. This study compares detrital zircon U-Pb age spectra collected along down river transects of the Missouri and Yellowstone rivers from their respective headwaters in south western Montana to their confluence in western North Dakota. This stretch of the Missouri river has had significant anthropogenic modification through a series of constructed dams, including the world’s largest earthen dam at Fort Peck. In contrast this stretch of the Yellowstone river is entirely dam-free. Preliminary results indicate that the proportion of U-Pb age components systematically change downstream as a result of sediment mixing and drainage integration with tributaries along the Yellowstone River. Results from the Missouri River, however, indicate significant signal sequestration behind major dams which trap the sand fraction of the river’s sediment load. Samples collected downstream from major dams reveal DZ U-Pb distributions dominated by tributary mixing beneath the dams along with sediment contributed by cutbank and streambed incision. These results are significant to quantify the impact of anthropogenic activities in the headwaters of major river systems and how these artificial perturbations may propagate downstream. The anthropogenically imposed staging of sand in the upstream reaches of river systems has cascading effects for the sediment transport and depositional dynamics downstream. The decrease of sediment discharging at the river’s mouth has the potential to significantly impact communities living along the coast.