Collaborative Research: RAPID Assessment of 2013 Flood Sedimentation, Button Rock Reservoir, North St. Vrain Watershed, CO

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This collaborative RAPID project will assess flood sedimentation in Button Rock Reservoir, North St. Vrain watershed in north-central Colorado following the September 2013 extreme event. In addition, it will fill knowledge gaps about event-based sedimentation from fires and floods, and expand knowledge to annual and decadal scales using a record of sediment accumulation since construction of the reservoir in 1950. The sedimentary record will inform on trends in basin-averaged erosion rates and sedimentation, as well as on trends in carbon storage in natural or artificial lakes, and on rates of carbon export from North St. Vrain Creek under flow scenarios from single, large events to flows over the past six decades. Button Rock Reservoir provides a unique setting to assess the effects of a very high magnitude, infrequent flood in the Front Range because (1) no intentional flushing events have been conducted by the reservoir operators since construction of the reservoir, (2) the reservoir stratigraphy chronicles uninterrupted delta deposition, and (3) this is the only on-channel reservoir with unimpeded, natural sediment flux from the Continental Divide to the mountain front in a basin with no significant historic timber harvests, flow regulation, or land use impacts. The team will collect four cores within the delta deposit during the December 2013 to March 2014 timeframe. A ground-penetrating radar survey will be completed of the exposed delta and subaqueous portions of the reservoir. Together with information from pre-dam topographic maps, information from these cores and the GPR survey will constrain the volume and spatial distribution of sediment deposition. Samples from the delta cores will be collected for radiocarbon analysis to constrain sediment accumulation and provide insight into landscape denudation through basin-averaged erosion rates. The carbon flux within the catchment will be determined from organic carbon content in delta sediments. Discharge and precipitation records will be compared with the core information to analyze trends in sediment flux into the reservoir in response to event-based and seasonal drivers.This project will collect a unique uniterrupted and undisturbed record of the impact of floods and fires on erosion in Colorado's Front Range over the past six decades. It will inform understanding of how the landscape responds to such major events and the role of these events in moving Carbon among storage points in the landscape. A near-term outcome of the work will be a refined estimate of the amount of material moved by the September 2013 weather event--an event that has necessitated the City of Longmont to remove the associated pulse of sediment from the Button Creek Reservoir in order to regain reservoir capacity. The project presents training experiences for graduate and undergraduate students, forges a new collaboration between colleagues at two universities, and will share information with entities that manage water and sediment flows along the Front Range.