160-2 Sediment Fingerprinting Reveals Event-Driven Shifts in Sediment Sources for a Large Federal Reservoir, Kansas, USA

Session: Advances in Fluvial Processes and Sediment Transport, Part I

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Abstract:

Erosion has caused major sedimentation in reservoirs, threatening water storage capacity and water quality demands. In Kansas, John Redmond Reservoir has lost more than 40% of its original capacity since impoundment. The current understanding that streambanks are the dominant source of sediment to the reservoir has resulted in extensive streambank stabilization along the Cottonwood River. Yet, it remains unclear whether such efforts have reduced sediment inputs at the watershed scale. This study uses sediment fingerprinting to quantify fractional contributions from streambanks, cropland, and grassland to sediment deposited in John Redmond Reservoir and to suspended sediment during flood events in the Cottonwood River. Sediment samples were collected in spring and summer 2024 and analyzed using geochemical, organic, and isotopic tracers. A Bayesian mixing model, incorporating five informative tracers (OC, N, δ¹⁵N, Ba, and ^239+240Pu), identified cropland as the dominant source of reservoir sediment (43%), followed by streambanks (34%) and grassland (23%). In contrast, for river samples, streambanks were the dominant source. Streambank contributions were 40% in discrete storm-event samples and 54% in time-integrated sediment trap samples, suggesting ongoing bank erosion during storm recession. Reservoir sediment reflects sourcing during large out-of-bank floods, storm-event samples represent moderate in-channel flows, and time-integrated samples reflect more frequent, lower-magnitude events. Together, these patterns indicate that larger floods shift sediment sourcing away from banks toward cropland as out-of-bank flows mobilize sediment from cultivated floodplains, primarily via chute channels. Disproportionally large sediment loads are transported during these infrequent large magnitude events. Longitudinal sampling along an 80 km stabilized reach revealed a downstream increase in bank-derived sediment, suggesting that streambank stabilization has not fully curtailed in-channel erosion. These findings emphasize the need for integrated watershed management that addresses both streambank and upland (cropland) erosion. Sediment fingerprinting offers a critical, basin-wide perspective to guide and evaluate restoration and conservation strategies.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7927

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