Sensitivity of Permafrost Dominated River and Stream Banks to Climate Change
Abstract
River systems in permafrost environments may be particularly sensitive to climate-induced changes in hydrology, water temperature, and air temperatures. In these systems, the rate of bank erosion may be significantly influenced by the presence of permafrost and the rate the permafrost thaw. Using analysis of remote sensing imagery and field studies, we attempt quantification of the spatial and temporal patterns of river bank erosion to understand the controls on river planform dynamics in permafrost settings. The rate that river banks erode in permafrost settings represents a balance between the rate at which bank material may thaw and become mobile and the rate at which mobile material may be transported away from the river bank. Along steep banks in large river systems with a significant discharge throughout the open water season, such at the Yukon River, fine-grained (silt and sand) sediments may be removed as quickly as they are thawed. In these systems, the maximum rate of bank retreat is effectively thermally limited and may be sensitive to climate changes that increase the rate of permafrost thaw, such as water and air (to a lesser degree) temperature increases. In contrast, in smaller river systems, like the Selawik River in northwest Alaska, highly seasonal discharge, heterogeneous floodplain deposits, and bank vegetation all lead to significant variability in the spatial and temporal rates of bank erosion. At one riverbank site monitored between 2010 and 2012, yearly retreat rates varied from 0 m/yr to 5 m/yr (the long term average measured from aerial photos and satellite imagery). However, in the year with a retreat of 5 m the erosion occurred over a 5-day time period corresponding to peak snow melt discharges. The year with no observable bank retreat followed a winter season with very little snow pack and reduced spring discharge. These observations suggest that in small systems with highly seasonal flows, small changes in river hydrographs, such as changes in the duration of peak flows by a few days, have the potential to significantly change riverbank erosion rates. In the Selawik, spring bank erosion often leads to bank undercutting that produces large mats of tundra vegetation that drape or fall onto the bank face. It appears that these vegetation mats may protect the banks from further erosion over the course of the summer, even during discharge events with stages close to those observed during snow melt. At locations where the vegetation mats do not re-freeze to the bank face, spring ice out appears to remove the prior season's mats thereby exposing the banks to fresh erosion. Mats that become frozen within the winter ice cover may be carried away with the ice, while mats higher on the bank or attached to the surface tundra may be abraded by ice rafts carried in the flow. Hydrological changes that affect the timing and magnitude of spring ice out flows may also alter this erosional dynamic with the riparian vegetation. Our observations suggest that the response of river planform dynamics to climate-induced changes in arctic freshwater systems may be quite complex and exhibit significant spatial and temporal heterogeneity.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.H41B1234R
- Keywords:
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- 1815 HYDROLOGY Erosion;
- 0744 CRYOSPHERE Rivers;
- 0702 CRYOSPHERE Permafrost;
- 1625 GLOBAL CHANGE Geomorphology and weathering