The satellite image below shows glaciers flowing north (into Tibet) and south (into Nepal) of the main Himalayan mountain range. Many of these longer glaciers are debris-covered in their lower extents which changes their response to climatic warming. Rather than retreating up-valley as temperatures at lower elevations melt the glacier ice, thick debris cover insulates the underlying glacier which reduces melt and means the terminus of the glacier can remain stagnant on decadal timescales. Higher melt rates are often found higher up the glacier where the debris cover is thinner which can reduce the overall gradient of the glacier and promote supraglacial pond and lake development.
Supraglacial ponds that form on the surface of the glacier as melt water collects in depressions can increase the rate of ablation on debris-covered glaciers, along with exposed ice faces which protrude out of the debris. In addition to melt beneath the debris layer, these factors act to promote surface lowering on debris-covered glaciers. This vertical change is characteristic of debris-covered glaciers that are not retreating horizontally. Vertical lowering is harder to quantify using satellite imagery because of associated uncertainties over short timescales in the commonly used digital elevation models (DEMs), whereas horizontal change can be easily seen (e.g. in the examples below) and therefore measured more accurately. Debris-covered glaciers that have a glacial lake at their terminus can, however, retreat rapidly up-valley, in addition to experiencing surface lowering.
The Landsat satellite imagery time-lapse below, shows several glaciers located in the inset (a) which have glacial lakes at their terminus. The largest lake in the right of the image is no longer expanding; however, the two lakes on the left have retreated by ~800-900 m over the time period shown. The most northerly of these two lakes is a clean-ice glacier, whereas the most southerly has debris cover on its lower extent. Both glaciers are retreating by calving, where large sections of the glacier break off into the lake. This process speeds up the glacial retreat independent of the climatic warming and leaves a lake that can be over 100 m deep.
The glacial lakes formed at the terminus of glaciers can be hazardous and breach in a glacial lake outburst flood (GLOF), which releases the lake water downstream in a large flood event. There is high uncertainty surrounding the prediction of these events and assessing potentially hazardous lakes, since many factors must be considered (see GLOFs). Additionally, many of the lakes have existing drainage channels and are not considered hazardous.
Glacial lake expansion - inset (a) above. Landsat data available from the U.S. Geological Survey
The Landsat time-lapse below shows the retreat of Imja Glacier in inset (b). The lake that is expanding in front of the calving terminus is called Imja Tsho and has expanded by approximately 1 km over the time period shown. The lake first began to develop from several supraglacial ponds in the 1950s and is now over 100 m deep and over 2 km long. The second-time lapse from Google Earth imagery shows the expansion of the lake and a large calving event in 2013.
Imja Tsho expansion - inset (b) above. Data available from the U.S. Geological Survey
Google Earth imagery
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