Scale-dependent influence of permafrost on riverbank erosion rates

Abstract

Whether the presence of permafrost systematically alters the rate of riverbank erosion is a fundamental geomorphic question with significant importance to infrastructure, water quality, and biogeochemistry of high latitude watersheds. For over four decades this question has remained unanswered due to a lack of data. Using remotely sensed imagery, we addressed this knowledge gap by quantifying riverbank erosion rates across the Arctic and subarctic. To compare these rates to non-permafrost rivers we assembled a global dataset of published riverbank erosion rates. We found that erosion rates in rivers influenced by permafrost are on average six times lower than non-permafrost systems; erosion rate differences increase up to 40 times for the largest rivers. To test alternative hypotheses for the observed erosion rate difference, we examined differences in total water yield and erosional efficiency between these rivers and non-permafrost rivers. Neither of these factors nor differences in river sediment loads provided compelling alternative explanations, leading us to conclude that permafrost limits riverbank erosion rates. This conclusion was supported by field investigations of rates and patterns of erosion along three rivers flowing through discontinuous permafrost in Alaska. Our results show that permafrost limits maximum bank erosion rates on rivers with stream powers greater than 900 W/m-1. On smaller rivers, however, hydrology rather thaw rate may be dominant control on bank erosion. Our findings suggest that Arctic warming and hydrological changes should increase bank erosion rates on large rivers but may reduce rates on rivers with drainage areas less than a few thousand km2.

Scale‐Dependent Influence of Permafrost on Riverbank Erosion Rates

“…Prior to installing the temperature arrays, we removed all thawed sediment down to frozen materials. Over the course of 2 weeks in the late June and early July 2018, the banks thawed between 40 and 124 mm/day with rates generally decreasing with higher ice contents (Rowland et al., 2023) https://data.ess-dive.lbl.gov/datasets/doi:10.15485/1922885. In all locations, the thawed materials remained in place and created a thermal buffer between the diurnally fluctuating air temperatures and the advancing thaw front.…”

“…Riverbank temperatures were monitored at five locations along 3 riverbanks using an array of iButton loggers installed in a custom rod manufactured by Alpha Mach Inc. The temperature loggers were located at the bank face, 5, 10, 20, and 40 cm depths (Rowland et al., 2023) https://data.ess-dive.lbl.gov/datasets/doi:10.15485/1922885.…”

“…Soil and air temperatures on the Selawik were recorded at hourly time intervals (Rowland et al, 2023) https:// data.ess-dive.lbl.gov/datasets/doi:10.15485/1922885. Soil temperature sensors were placed 50 cm below the ground surface at 66.48°N, 157.71°W.…”

“…The temperature sensor data from the rapidly eroding riverbank (Rowland et al, 2023) https://data.ess-dive. lbl.gov/datasets/doi:10.15485/1922885 led us to infer that the bank remained frozen until the bank materials collapsed into the river.…”

“…Volumetric ice content at five bends we sampled on the Koyukuk River ranged from 0.41 to 0.76 (Rowland et al, 2023) https://data.ess-dive.lbl.gov/datasets/ doi:10.15485/1922885. Erosion rates decreased with increasing ice content at four of the five locations (Figure 6c); the fifth location had the highest ice content and exhibited the highest erosion rates.…”