Permafrost slows Arctic riverbank erosion

The rate of river migration affects the stability of Arctic infrastructure and communities^1,2 and regulates the fluxes of carbon^3,4, nutrients⁵ and sediment^6,7 to the oceans. However, predicting how the pace of river migration will change in a warming Arctic⁸ has so far been stymied by conflicting observations about whether permafrost⁹ primarily acts to slow^10,11 or accelerate^12,13 river migration. Here we develop new computational methods that enable the detection of riverbank erosion at length scales 5-10 times smaller than the pixel size in satellite imagery, an innovation that unlocks the ability to quantify erosion at the sub-monthly timescales when rivers undergo their largest variations in water temperature and flow. We use these high-frequency observations to constrain the extent to which erosion is limited by the thermal condition of melting the pore ice that cements bank sediment¹⁴, a requirement that will disappear when permafrost thaws, versus the mechanical condition of having sufficient flow to transport the sediment comprising the riverbanks, a condition experienced by all rivers¹⁵. Analysis of high-resolution data from the Koyukuk River, Alaska, shows that the presence of permafrost reduces erosion rates by 47%. Using our observations, we calibrate and validate a numerical model that can be applied to diverse Arctic rivers. The model predicts that full permafrost thaw may lead to a 30-100% increase in the migration rates of Arctic rivers.