RIVERBANKS ON ICE: MODELING THERMAL EROSION BY GRAVEL-BEDDED ARCTIC RIVERS

Permafrost soils contain more than twice as much carbon as the Earth’s atmosphere. In response to anthropogenic climate change, previously frozen soil organic carbon (SOC) is increasingly exposed by a deepening active layer. Arctic rivers release material from carbon-rich floodplain deposits through riverbank erosion, fluvial transport and possibly sequestration. In the absence of erosion, riverbank SOC is left to degrade by biogeochemical processes which release greenhouse gases. The rate at which rivers remove SOC is a key unknown in the carbon budgets of Arctic rivers draining icy terrain. Here we use field observations and numerical modeling to explore which physical processes drive Arctic riverbank erosion, specifically for coarse-grained rivers draining mountain fronts. Using the open-source Landlab toolkit, we model riverbank temperature over annual to decadal timescales. We track a riverbank profile as it evolves based on temperature-controlled erosion laws. Model parameters are derived from field measurements collected along the gravel-bedded Canning River, Alaska in the summer of 2021 and 2022, including geometries, grain size distributions, ice content, and thermal properties of soil and sediment. Characteristic river temperature, level, and discharge are based on USGS gauge data from 2008-2013, which we supplement with in-situ Acoustic Doppler Current Profiler transects collected in 2022. These parameters inform the erosion of riverbanks due to the temperature-controlled thawing and entrainment of subaqueous riverbank material. As river water thaws and undermines ice-rich sediment, ledges of strongly rooted grass and peat mats develop overhanging blocks. At some point these fail, then degrade in place or are transported downstream as rolling blocks. These processes are explored with preliminary subaerial erosion rules. We then use iPad Lidar scans and measured temperature profiles to validate modeled riverbank geometries and temperatures. Our results suggest the rate of SOC removal from the Arctic coastal plain is controlled by stage, but limited by riverbank and river water temperatures, rather than shear stresses exerted by river water as is typical in less icy river systems. This work highlights the power of ice in shaping landscapes and routing carbon through Arctic river systems.