Downstream sediment transport in river systems is impacted by anthropogenic modifications, such as dams or levees. This study used detrital zircon U-Pb geochronology and sediment mixture modeling to investigate the effects of dams on provenance signatures and sediment transport along the upper Missouri and Yellowstone Rivers. The Yellowstone River is the longest dam-free river in North America, while the upper Missouri River has several major reservoir-forming dams. We present 4777 new individual detrital zircon U-Pb ages from 32 sand samples collected from sand bars of the Missouri and Yellow-stone Rivers and their major tributaries. These new data along the Missouri River track downstream modification of detrital zircon U-Pb age signatures due to sediment sequestration in reservoirs, bank erosion, and mixing at stream confluences. In contrast, detrital zircon U-Pb age data evolve more progressively downstream along the Yellowstone River, which displays less anthropogenic influence and muted tributary mixing and dilution. U-Pb age component proportions along the extensively dammed Missouri River vary up to 27% downstream, with amplified changes occurring at river confluences downstream from dams. These dramatic changes are attributed to storage of sediment and preferential capture of heavy minerals (including zircon) in reservoirs, resulting in a reduced sediment load in the river downstream from dams. Consequently, detrital zircon mixture models show a disproportionate provenance contribution from tributaries compared to the trunk river downstream from the dams. Dams not only reduce the sediment flux from a river but also change the locations where sediment is generated by initiating erosion in a river downstream from a dam.

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