Post-glacial alluvial valley dynamics of the South Fork Cache la Poudre River Valley at the Colorado State University Mountain Campus
Date
2022
Authors
Suhr, Jens Christoph, author
Rathburn, Sara, advisor
McGrath, Daniel, advisor
Morrison, Ryan, committee member
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Abstract
Wide valley bottoms are physically important sediment storage sites where alluvial records of past landscape dynamics may be preserved. Following deglaciation after the Last Glacial Maximum (LGM), unconfined valleys in the Colorado Front Range experienced periods of fluvial aggradation and incision, creating distinctive valley bottom morphologies and the substrates which influence present-day hydrological and ecological characteristics. The objectives of this study are to investigate the processes and chronology of post-glacial geomorphic evolution of an unconfined portion of the South Fork Cache la Poudre River (South Fork) Valley, Colorado Front Range, to identify the dominant processes and temporal patterns of valley alluviation and incision following LGM retreat at the Colorado State University Mountain Campus (Mountain Campus). Methods used include geologic mapping, ground-penetrating radar (GPR) surveys, coring of valley bottom sediments, radiocarbon geochronology, and analysis of historical aerial images. Mapping of the Quaternary sediments indicates a variety of glacial and fluvial deposits occur in the South Fork Valley, including moraines, two distinct outwash terraces (approximately 8 m and 6 m above the present-day channel, respectively), fluvial terraces 1–2 m high, and an extensive floodplain. Well logs indicate over 10 m of glaciofluvial outwash sediment was deposited upstream of the LGM terminal moraine, and GPR reflections suggest that lateral bar migration, channel filling, and vertical accretion of sediments were important processes of outwash aggradation in the valley. The South Fork River has since incised into the outwash. A fluvial terrace and the modern floodplain are inset within the outwash sediments and are composed of overbank-deposited silt-to-sand sized sediments. Radiocarbon samples of valley sediments indicate that outwash was deposited at least 16.8 ka, with 8–10 m incision occurring after 16.8 ka and prior to 7.8 ka. Fine-grained sedimentation occurred on the fluvial terrace and floodplain from at least 2.1 ka to 1.3 ka. The modern floodplain has been vertically accreting for at least the last 500 years. Historical aerial images show that the South Fork channel was relatively stable from 1938 to the present; the channel planform area changed by no more than 2.5% per year during this period. Additionally, in the last ~80 years, the channel has largely occupied the center of the unconfined valley, reducing connectivity between the channel, terraces, and the valley sides. My results highlight the complex patterns of sediment storage and removal in unconfined valleys, with at least two phases of aggradation and one phase of incision following deglaciation. In addition, the South Fork Valley is relatively geomorphically stable: large volumes of Quaternary sediments have been stored for over 16.8 ka years, and the modern fluvial system has not responded drastically to local disturbances because of low connectivity between hillslopes and the valley bottom. The South Fork Valley is an effective site of fluvial sediment storage following deglaciation despite a long-term trend of sediment removal from the valley in the Holocene. Broader implications of assessing valley bottom stability and long-term sediment storage in mountains include managing unconfined valleys where development pressures, proposed water diversions, and climatically forced changes to the hydrology are occurring. Findings presented herein may provide insights for maintaining riparian biodiversity and surface-subsurface water exchange in formerly glaciated environments.
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Subject
geologic mapping
ground-penetrating radar
valley bottom evolution
glacial geomorphology
fluvial geomorphology
Quaternary geology