RIVER ICE DIRECTS PERMAFROST THAW ACROSS AN ARCTIC DELTA: FIELD OBSERVATIONS AND MODELING INSIGHTS FROM THE CANNING RIVER DELTA IN ARCTIC ALASKA

Arctic river discharge is rising in concert with increasing air and permafrost temperatures. These trends could accelerate river bank permafrost thermal erosion. Ice-rich permafrost river banks thermally erode when river water supplies enough heat to thaw. Still, river ice occupies Arctic rivers and deltas for over six months. River ice cools water and diverts flow to secondary channels by preferentially filling distributary channels for some time after ice breakup at the delta apex.

We explore how river ice impacts thermal erosion of ice-rich permafrost riverbanks along the Canning River Delta in Arctic Alaska. We focus on three sites: a site at the delta apex, upstream from a perennial ice field, a site downstream from the ice field in the Staines distributary, and one site in the Canning distributary channel with no ice field. The Canning distributary channel receives a channel-filling discharge while Staines is partially blocked by ice. In Aug. 2023, the Staines branch transmitted 70% of discharge at the delta apex with little river ice. Based on satellite imagery, banks retreated at 10 m/year between Aug. 2020 and Aug. 2024.

We calculate thermal riverbank erosion rates by coupling numerical models for river water temperature, ablation, and cantilever collapse. In-situ hydrologic and meteorologic records define the problems boundary conditions. We calculate an erosion rate along a discretized vertical bank profile at sub-daily timesteps. Over ten years with 45 days of ice diversion in spring, we track daily erosion along with peak erosion rate time and magnitude from the initiation of snowmelt (when air temp. = –5° C) through the end of the hydrologic year.

The model predicts 0.1 m²/day of bank area loss per unit distance downstream along distributaries (min = 0, max = 2.5). Over ten years, the model estimates annual erosion rate of 27.2 m² from the 2.5 m-tall bank along Staines (or 10.0 m/year of lateral erosion), and almost twice as much in the Canning. Bank erosion peaks in the Canning channel on 6/22, 42 days before it peaks in Staines.

Permafrost thaw across Arctic deltas is sensitive to river ice diversion. Under projected warming, the model suggests increased shoulder season and winter streamflow could accelerate thaw across the delta, especially where flow is diverted by river ice. Estimates of modern and future thermal erosion in Arctic deltas should account for river ice.