POST-GLACIAL ALLUVIAL VALLEY DYNAMICS IN A HEADWATER RIVER OF THE ROCKY MOUNTAINS

Valley bottoms are physically important as sediment storage sites where alluvial records of past landscape dynamics may be preserved. Following the Last Glacial Maximum (LGM) retreat, alpine 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. Here we examine the processes and chronology of post-glacial evolution of the South Fork Cache la Poudre River (South Fork) Valley, Colorado Front Range, to investigate the dominant processes, and temporal patterns of valley alluviation following LGM glacial retreat. Methods include Quaternary mapping, ground penetrating radar (GPR) surveys, coring of valley bottom sediments, and radiocarbon geochronology. Mapping indicates a variety of glacial and fluvial deposits in the South Fork Valley, including moraines, two distinct outwash terraces (8m and 6m above the channel, respectively), fluvial terraces 1-2m high, and an extensive floodplain. Well logs indicate over 10m of outwash was deposited upstream of the LGM terminal moraine, and GPR reflections suggest that point bar migration, channel filling, and vertical accretion of sediments were important processes of outwash aggradation in the valley. The South Fork channel has since incised into the outwash, and overbank-deposited silt-to-sand sized sediments cover the valley bottom. Radiocarbon samples of valley sediments indicate that outwash is at least 13.9 ka, with 8-10m incision occurring after 13.9 ka and prior to 2.1 ka. Fine-grained sedimentation occurred in the valley bottom from least 2.1 ka to 1.4 ka. The modern floodplain has been vertically accreting for at least the last 485 years. Our results highlight the complex patterns of sediment storage and removal in unconfined valleys, with broader implications for maintaining riparian biodiversity and storing water and sediment in alpine environments.