Photo of a riverbank in Alaska.
Photo of a 1.5-meter-tall riverbank along the Koyukuk River near the community of Huslia, Alaska. The river erodes laterally into permafrost ground, exposing frozen soil and sediment to relatively warm water and air temperatures that cause it to thaw. This bank exposure shows a layer of brown, carbon-rich peat overlying frozen silt with an icy matrix. Credit: Douglas et al. [2023], Figure 1b
Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Journal of Geophysical Research: Earth Surface

Arctic communities are experiencing rapid physical, chemical, and biological changes in their river systems due to warming air and river water temperatures and thawing permafrost ground. Many Alaskan communities are situated on the banks of meandering rivers, which provide access for subsistence fishing but present a major risk to community infrastructure as the rivers erode laterally into their floodplains. Predicting whether riverbank erosion rates will accelerate in a warming climate is challenging as several processes drive erosion at different times of the year, including ice jams and river ice break up in the spring, undercutting during high flow events, and ablation by warming air and river water throughout the summer.

In some settings, riverbank erosion rates are limited by ablation (heat transfer and pore-ice melting), while in other settings erosion can be limited by the river’s ability to transport sediment off its banks where thawed soil and slumped blocks of peat accumulate and armor the riverbank. In a changing Arctic climate with increasing river discharge and water temperatures, ablation-limited rivers are likely to experience accelerated riverbank erosion rates, such that communities inhabiting their banks are at great risk of losing land and infrastructure.

To elucidate the mechanics of ablation-limited riverbank erosion, Douglas et al. [2023] designed frozen flume experiments in the laboratory to simulate the erosion of a hydraulically rough and erodible permafrost riverbank under open-channel flow. By embedding temperature sensors within a model riverbank composed of frozen water and sand, the researchers measured how rapidly heat is conducted from the flowing water through the bank material. Their results indicate that erosion rates scale linearly with water temperature, and that bank ice content plays an important role in modulating erosion rates. Because the phase change from ice to water requires a large amount of heat, banks with greater ice content will erode more slowly than riverbanks with less ice in their matrix. Ultimately these experiments suggest that erosion rates in permafrost riverbanks will accelerate as river water temperatures increase across the Arctic.

While this study clearly demonstrates the relationship between river water temperature and permafrost riverbank erosion, the experimental erosion rates were much higher than those measured in natural rivers. This discrepancy reveals the complexity of permafrost riverbank erosion in natural settings, where bank armoring by cohesive vegetative mats or bank strengthening by shrubification may act to slow bank erosion. Despite this complexity, the results from the frozen flume experiments described here provide a foundation to more accurately predict future changes in Arctic riverbank erosion.

Citation: Douglas, M. M., Miller, K. L., Schmeer, M. N., & Lamb, M. P. (2023). Ablation-limited erosion rates of permafrost riverbanks. Journal of Geophysical Research: Earth Surface, 128, e2023JF007098.

—Marisa Repasch, Associate Editor, JGR: Earth Surface

Text © 2023. The authors. CC BY-NC-ND 3.0
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