Climatic warming and permafrost thaw are predicted to increase Arctic riverbank erosion, threatening communities and accelerating sediment, carbon and nutrient cycling between rivers and floodplains. Existing theory assumes that pore‐ice thaw sets riverbank erosion rates, but overpredicts observed erosion rates by orders of magnitude. Here, we developed a simple model that predicts more modest rates due to a sediment‐entrainment limitation and riverbank armoring by slump blocks. Results show that during times of thaw‐limited erosion, the river rapidly erodes permafrost and undercuts its banks, consistent with previous work. However, overhanging banks generate slump blocks that must thaw and erode by sediment entrainment. Sediment entrainment can limit bank and slump block erosion rates, producing seasonally averaged rates more consistent with observations. Importantly, entrainment‐limited riverbank erosion does not depend on water temperature, indicating that decadal erosion rates may be less sensitive to warming than predicted previously.
Whether 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 data set of published riverbank erosion rates. We found that erosion rates in rivers influenced by permafrost are on average nine 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 Wm−1. On smaller rivers, however, hydrology rather than thaw rate may be the 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.
more » « less- Award ID(s):
- 2127442
- NSF-PAR ID:
- 10441497
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Earth Surface
- Volume:
- 128
- Issue:
- 7
- ISSN:
- 2169-9003
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract How will bank erosion rates in Arctic rivers respond to a warming climate? Existing physical models predict that bank erosion rates should increase with water temperature as permafrost thaws more rapidly. However, the same theory predicts much faster erosion than is typically observed. We propose that these models are missing a key component: a layer of thawed sediment on the bank that buffers heat transfer and slows erosion. We developed a 1D model for this thawed layer, which reveals three regimes for permafrost riverbank erosion. Thaw‐limited erosion occurs in the absence of a thawed layer, such that rapid pore‐ice melting sets the pace of erosion, consistent with existing models. Entrainment‐limited erosion occurs when pore‐ice melting outpaces bank erosion, resulting in a thawed layer, and the relatively slow entrainment of sediment sets the pace of erosion similar to non‐permafrost rivers. Third, the intermediate regime occurs when the thawed layer goes through cycles of thickening and failure, leading to a transient thermal buffer that slows thaw rates. Distinguishing between these regimes is important because thaw‐limited erosion is highly sensitive to water temperature, whereas entrainment‐limited erosion is not. Interestingly, the buffered regime produces a thawed layer and relatively slow erosion rates like the entrainment‐limited regime, but erosion rates are temperature sensitive like the thaw‐limited regime. The results suggest the potential for accelerating erosion in a warming Arctic where bank erosion is presently thaw‐limited or buffered. Moreover, rivers can experience all regimes annually and transition between regimes with warming, altering their sensitivity to climate change.
-
more » « less
Abstract Permafrost thaw is hypothesized to increase riverbank erosion rates, which threatens Arctic communities and infrastructure. However, existing erosion models have not been tested against controlled flume experiments with open‐channel flow past an erodible, hydraulically rough permafrost bank. We conducted temperature‐controlled flume experiments where turbulent water eroded laterally into riverbanks consisting of sand and pore ice. The experiments were designed to produce ablation‐limited erosion such that any thawed sediment was quickly transported away from the bank. Bank erosion rates increased linearly with water temperature, decreased with pore ice content, and were insensitive to changes in bank temperature, consistent with theory. However, erosion rates were approximately a factor of three greater than expected. The heightened erosion rates were due to a greater coefficient of heat transfer from the turbulent water to the permafrost bank caused by bank grain roughness. A revised ablation‐limited bank erosion model with a heat transfer coefficient that includes bank roughness matched our experimental results well. Results indicate that bank erosion along Arctic rivers can accelerate under scenarios of warming river water temperatures for cases where the cadence of bank erosion is set by pore‐ice melting rather than sediment entrainment.
-
more » « less
Abstract Rapid warming in the Arctic threatens to destabilize mercury (Hg) deposits contained within soils in permafrost regions. Yet current estimates of the amount of Hg in permafrost vary by ∼4 times. Moreover, how Hg will be released to the environment as permafrost thaws remains poorly known, despite threats to water quality, human health, and the environment. Here we present new measurements of total mercury (THg) contents in discontinuous permafrost in the Yukon River Basin in Alaska. We collected riverbank and floodplain sediments from exposed banks and bars near the villages of Huslia and Beaver. Median THg contents were 49+13/−21ng THg g sediment−1and 39+16/−18ng THg g sediment−1for Huslia and Beaver, respectively (uncertainties as 15th and 85th percentiles). Corresponding THg:organic carbon ratios were 5.4+2.0/−2.4Gg THg Pg C−1and 4.2+2.4/−2.9Gg THg Pg C−1. To constrain floodplain THg stocks, we combined measured THg contents with floodplain stratigraphy. Trends of THg increasing with smaller sediment size and calculated stocks in the upper 1 m and 3 m are similar to those suggested for this region by prior pan-Arctic studies. We combined THg stocks and river migration rates derived from remote sensing to estimate particulate THg erosional and depositional fluxes as river channels migrate across the floodplain. Results show similar fluxes within uncertainty into the river from erosion at both sites (95+12/−47kg THg yr−1and 26+154/−13kg THg yr−1at Huslia and Beaver, respectively), but different fluxes out of the river via deposition in aggrading bars (60+40/−29kg THg yr−1and 10+5.3/−1.7kg THg yr−1). Thus, a significant amount of THg is liberated from permafrost during bank erosion, while a variable but generally lesser portion is subsequently redeposited by migrating rivers.
-
more » « less
Abstract Arctic coastal environments are eroding and rapidly changing. A lack of pan-Arctic observations limits our ability to understand controls on coastal erosion rates across the entire Arctic region. Here, we capitalize on an abundance of geospatial and remotely sensed data, in addition to model output, from the North Slope of Alaska to identify relationships between historical erosion rates and landscape characteristics to guide future modeling and observational efforts across the Arctic. Using existing datasets from the Alaska Beaufort Sea coast and a hierarchical clustering algorithm, we developed a set of 16 coastal typologies that captures the defining characteristics of environments susceptible to coastal erosion. Relationships between landscape characteristics and historical erosion rates show that no single variable alone is a good predictor of erosion rates. Variability in erosion rate decreases with increasing coastal elevation, but erosion rate magnitudes are highest for intermediate elevations. Areas along the Alaskan Beaufort Sea coast (ABSC) protected by barrier islands showed a three times lower erosion rate on average, suggesting that barrier islands are critical to maintaining mainland shore position. Finally, typologies with the highest erosion rates are not broadly representative of the ABSC and are generally associated with low elevation, north- to northeast-facing shorelines, a peaty pebbly silty lithology, and glaciomarine deposits with high ice content. All else being equal, warmer permafrost is also associated with higher erosion rates, suggesting that warming permafrost temperatures may contribute to higher future erosion rates on permafrost coasts. The suite of typologies can be used to guide future modeling and observational efforts by quantifying the distribution of coastlines with specific landscape characteristics and erosion rates.
