Aufeis as a Major Forcing Mechanism for Channel Avulsion and Implications of Warming Climate

Abstract

Aufeis derives from a German word for "ice on top" and is now commonly used to refer to all forms of frozen overflow (Ensom et al., 2020). Icing is sometimes used to describe the process in which water emerges from the subsurface and freezes, whereas aufeis or the Russian term naled refer to the resulting bodies of ice (Ensom et al., 2020). Aquitards such as permafrost can promote movement of water toward the surface and conduits for flow occur in fractured bedrock, carbonate lithologies with karst weathering, and zones of high effective porosity in alluvium (Ensom et al., 2020). Typical aufeis locations include valleys in proximity to mountains with high elevations; limestone areas with springs; glacial morphology such as terminal moraines and glacial deposits that create strong spatial heterogeneity in hydraulic conductivity; and fault systems (Ensom et al., 2020). Aufeis commonly re-forms each winter in the same locations (Morse & Wolfe, 2015;Yoshikawa et al., 2007), can extend for greater than 4 km 2 (Morse & Wolfe, 2015), and averages 1-2 m thick (Alekseyev, 2015) but can exceed 3 m in thickness (Li et al., 1997).The water source for aufeis can be a spring, river, or active-layer water expelled to the surface during seasonal freezing (K. L. Carey, 1973). Writing of aufeis in the braided channel of Jarvis Creek, Alaska, Daly et al. (2011) describe how individual channels accumulate frazil ice at the surface and anchor ice over the channel substrate during initial freezeup. As water level in each channel rises in response to the formation of ice cover, channels with higher water-surface elevations spill across gravel bars into lower channels and the spillover freezes. Pressurized conduit-type flow can occur between channels, as evidenced by upwelling of water through openings in the ice cover at the downstream ends of conduits. This movement of water across the river corridor promotes the progressive formation of aufeis via spatially extensive but shallow sheet flow that may be fed by tiny fractures in the ice, and via narrower active channels atop the ice that are fed by visible openings with upwelling water (Daly et al., 2011).

Scale-dependent influence of permafrost on riverbank erosion rates

“…The annual break up of winter ice on high latitude rivers plays a major role in flooding (Prowse & Beltaos, 2002) and has also been observed to have dramatic local impacts on banks and riparian vegetation (Ettema, 2002;Gautier et al, 2021;Prowse & Culp, 2003;Scrimgeour et al, 1994) (Figure 1E). Regions of widespread surface ice (aufeis) have been attributed to flow diversion and channel widening along Alaskan north slope rivers (Wohl & Scamardo, 2022). At reach-to watershed-scale, however, the effect of ice on bank erosion remains uncertain with studies concluding that ice has minimal influence (Eardley, 1938;Williams, 1952Williams, , 1955, to ice protecting banks (Costard et al, 2014;Miles, 1976;Prowse & Culp, 2003), to ice increasing bank erosion (Brown et al, 2020;Chassiot et al, 2020;Prowse & Culp, 2003) while other studies suggest the available data is inconclusive (Ettema, 2002).…”

Scale‐Dependent Influence of Permafrost on Riverbank Erosion Rates

“…The annual break up of winter ice on high‐latitude rivers plays a major role in flooding (Prowse & Beltaos, 2002) and has also been observed to cause localized bank erosion and destruction of riparian vegetation (Boucher et al., 2009; Ettema, 2002; Gautier et al., 2021; Prowse & Culp, 2003; Scrimgeour et al., 1994) (Figure 1e). Studies of small rivers on the north slope of Alaska have documented localized widening of river channels in response to the transitions from bedfast to floating ice in channels (Best et al., 2005; Boucher et al., 2009; McNamara & Kane, 2009; Wohl & Scamardo, 2022). At reach‐ to watershed‐scale, however, the effect of ice on bank erosion remains uncertain with studies concluding that ice has minimal influence (Eardley, 1938; Williams, 1952, 1955), to ice protecting banks (Costard et al., 2014; Miles, 1976; Prowse & Culp, 2003), to ice increasing bank erosion (Best et al., 2005; Brown et al., 2020; Chassiot et al., 2020; Prowse & Culp, 2003; Vandermause et al., 2021) while other studies suggest the available data are inconclusive (Ettema, 2002).…”

Giant Aufeis in the Pangong Tso Basin: Inventory of a Neglected Cryospheric Component in Eastern Ladakh and Western Tibet

“…Broad valleys and braided river systems are mostly located in the northern portion of the PTB. Such geomorphological settings are considered favorable for the development of large aufeis fields [74,75]. The wide and shallow valley bottoms, where discharge can fluctuate between surface and subsurface flow, favor freezing to the bottom of the stream, leading to a blockage of the streamflow, which results in an increase in hydrostatic pressure and overflow of water to the surface.…”