Four Ferocious Fighting Forces: Glaciers, Meltwater, Debris, and Rivers

Abstract:

Glacier damming of rivers is a rarer than landslide damming, because glaciers make slower additions of potentially damming mass into the river, and the fact that some of that mass is ice, which can melt as fast as it can be delivered to the river.

Meltwater (or rain) may cause glaciers to melt back or to surge. What elapses depends on how rapidly— and how— water is introduced into the system.

A river met by an advancing glacier either slices through the glacier like a hot knife through butter, or if the river is at low flow stage and cold, then an advancing glacier might create a delicate dam, and then a lake when the river flows again-- followed by an outburst flood.

If debris is supplied to a river by an advancing glacier, the debris-rich ice may dam the river or not-- it depends on the particle size-frequency distribution, ice content, and river temperature and flow. Large mono-size boulders generally won’t make an effective dam, and fine-grained silt immediately washes away.

Debris added onto glacier surfaces may cause glaciers to be insulated and advance and maybe dam a river; or to retain water, thin and retreat.

A perfect confluence of circumstances may result in river damming and an outburst flood disaster, or a stable new geological feature; or no significant event.

I will review some disaster events in High Mountain Asia (2010 Atabad Landslide, 2014 Bhote Koshi landslide, and the 2019 glacier-lake-river dynamics of the Shispare glacier surge, Hunza River damming and outburst flood. I will compare and contrast different types of mass movements, their dynamics, materials, and dam efficiency. The distinct natures of glacier versus rock debris dammed rivers often is stark, but there can be circumstances where, despite differing mass movement dynamics, the outcome is similar. The widely varied outcomes of river dammings, and of mass movements that might have dammed a river but don’t, can be understood. Changing climate and the changing cryosphere— such as shifts in freeze-thaw, melting of glaciers and permafrost, and changing glacier behavior— impacts our understanding, as the recent past is no longer a key to the present or future. However, we can anticipate some of these changes and how they may affect the likelihoods and outcomes of river-damming events. So how might the next century of river damming events differ from the past century?