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Browse  abstracts

Icy Landscapes: modeling sediment transport in permafrost regions

Irina Overeem, University of Colorado Boulder , United States. irina.overeem@colorado.edu

Arctic rivers play a crucial role in transporting sediment and nutrients from permafrost landscapes to the Arctic Ocean, influencing both landscape evolution and biogeochemical cycles. These river systems are undergoing significant transformations due to decreasing snow, intensified summer precipitation, altering vegetation, and permafrost thaw. Over a seasonal cycle the thermal state of Arctic rivers changes as their beds and banks thaw. Long-term observations indicate a rise in Arctic river discharge. However, our understanding of the complex mechanisms governing sediment transport in these rivers remains limited.

To address this gap, we focus on the Canning River, a gravel-bed river situated in continuous permafrost in Alaska. Previous studies on small nearby rivers during the 1970s suggested that sediment transport is hindered during the ice break-up flood because the channel bed remains frozen while cold river water starts running, slowing the sediment bed from thawing. This would imply a decoupling of sediment transport from water discharge, at least seasonally, in Arctic rivers. To investigate this hypothesis, we conducted fieldwork during the summers of 2022 and the spring of 2023, representing periods of high river discharge with differing thermal states. Our data collection included measurements of discharge, temperature, suspended sediment fluxes, grain size distributions, seismic signals, ground temperature, and river ice thickness, which we compared to a historical 5-year river discharge record.

We model how the river freezes to its bed over extended stretches during winter, and how it forms aufeis up to 2 meters thick despite limited water flow. Observed water temperature around the ice break-up period hovers around 0°C, potentially requiring several days to thaw the matrix sands and prohibiting pebble movement, according to our thermal model. Conversely, by mid-summer, water temperature rises to approximately 12°C. Although mid-summer river discharge peaks are lower, suspended sediment increases substantially during intense rainfall events, indicating a strong coupling with river discharge. These initial findings suggest that annual sediment transport might amplify with warming conditions, as the river water may no longer freeze to the channel bed and as summer flows intensify.