Alluvial dynamics of a formerly glaciated Rocky Mountain headwater valley, Colorado

Following Pinedale deglaciation (~12 ka), unconfined valleys in the Rocky Mountains experienced periods of fluvial aggradation and incision, creating distinctive valley morphologies and substrates that influence present-day hydrological and ecological characteristics. Valley floors are thus physically important sediment storage sites that preserve alluvial records of past landscape dynamics. Using geologic mapping, ground-penetrating radar surveys, sediment coring, and radiocarbon and optically stimulated luminescence (OSL) geochronology, we investigated an unconfined portion of the South Fork Cache la Poudre River Valley, Colorado Front Range, to identify the dominant processes and temporal patterns of valley alluviation and incision following glacial retreat. We mapped a variety of glacial and fluvial deposits in the valley including two till deposits, distinct outwash terraces, fluvial terraces and an extensive floodplain. Abundant glaciofluvial outwash (65 m observed at one drill site) was deposited up-valley of the Last Glacial Maximum terminal moraine. Lateral bar migration, channel filling and vertical accretion of sediments were important processes of outwash aggradation and floodplain deposition. OSL dating of unconsolidated, laminated sand and silt suggests ponding up-valley of the terminal moraine between 13.4 and 11.5 ka, and the potential for an outburst flood(s). Channel incision occurred prior to 7.8–1.5 ka, creating outwash terraces that comprise over 30% of the valley floor area. Sedimentation occurred on the fluvial terrace and floodplain from at least 2.1 to 1.3 ka. The modern floodplain has been aggrading for at least 500 years. The South Fork Valley has anomalously thick post-glacial sediment from lateral migration and channel filling, whereas other Colorado headwater valleys are dominated by mass wasting deposits, beaver pond sediments or fluvial vertical accretion. The relict glacial topography and low hillslope-floodplain connectivity exerts the strongest control on alluvial dynamics in the South Fork Valley. Results of this study broaden the foundation for understanding post-glacial alluvial dynamics in unconfined mountain valleys. Knowledge of the processes that create and maintain alluvial fills is critical for effective management of these valleys.