Why Rock-Covered Glaciers Still Melt Fast: Streams and Ice Cliffs on Kennicott Glacier

In their lower reaches, many of Alaska's biggest valley glaciers are covered by a thin but conspicuous layer of rock debris. The rock starts out in lateral moraines along the sides of tributary glaciers upstream. Where tributaries join, those moraines are squeezed between the lobes of ice and become the dark stripes of medial moraines. Carried further downhill, the debris — only 1 to 24 inches deep — insulates the ice beneath it, so that where it is thicker the surface melts more slowly and stands higher than the bare white ice beside it. Gradually the debris spreads out until it blankets the glacier's end.

That much is well understood. What long puzzled glaciologists is a contradiction: debris slows surface melt, yet averaged over the long term, the ends of debris-covered glaciers generally melt about as fast as glaciers with no debris at all.

Ice cliffs

Part of the answer is ice cliffs. Debris-covered glaciers are usually lumpy, because small differences in debris thickness lead to different rates of melt. Among the hummocks are ice cliffs too steep for rock to rest on. These cliffs melt back quickly, with rock sliding almost constantly from top to bottom — and their fast melting helps debris-covered glaciers waste away at a pace comparable to clean ones.

The study abstract calls ice cliffs melt "hot spots" that account for a disproportionate share of melt. But what makes the cliffs in the first place, and why don't they quickly bury themselves under the rock that keeps tumbling down them?

The role of streams

To find out, researchers from Alaska, Norway and the National Park Service worked on the debris-covered — and much-visited — end of Kennicott Glacier in Wrangell-St. Elias National Park and Preserve. They focused on supraglacial streams, meltwater streams that flow on the glacier's surface rather than beneath or beside it. Combining field observations, stream flow measurements, remote sensing and a simple conceptual model of how cliffs melt back, they found that streams are central to the life of an ice cliff.

Streams do two main jobs:

• they cut the cliffs. Flowing water melts into the ice, both by wandering sideways and by cutting downward, which creates cliffs and keeps them steep; it leaves an undercut lip at the base.
• they clear the rubble. Streams carry away rocks up to tens of centimetres across that fall to the foot of a cliff, so the cliff is never buried.

Stream shapes even show up in the ice. Meanders, like those of rivers flowing through sediment, and cut-off loops like oxbow lakes, produce curving and crescent-shaped cliffs. When a stream suddenly jumps to a new course, its cliff can collapse quickly and the old channel is abandoned — and cliffs that streams abandon are seen to be buried again by debris, which shows how much their survival depends on the water.

Not every cliff comes from a stream. At the glacier's western edge, a localised, surge-like event opened crevasses that formed cliffs of another kind: sets of straight, parallel cliffs, unlike the crescents that streams carve.

The researchers suggest that models of how the landscape evolves could help measure the overall effect of these processes on how much of a glacier is covered in cliffs, and on its mass balance, alongside other factors such as surface ponds, uneven melt, ice flow and the collapse of hollows inside the ice.

Why it matters

The work gives a clearer picture of how debris-covered glaciers behave — which matters more each year, because rising summer melt is increasing the amount of debris-covered ice in Alaska's national parks.

Sources

Based on "The connection between glacier debris and melt," published by the National Park Service, Wrangell-St. Elias National Park and Preserve, summarising Petersen, E., R. Hock, and M. G. Loso (2024), "Stream hydrology controls on ice cliff evolution and survival on debris-covered glaciers," Earth Surface Dynamics 12(3): 727–745; rewritten in hubnx's own words.

• The Park Service summary says more than 90% of ice cliffs are influenced by streams, while the study's abstract gives a different share of cliffs actively influenced by streams; neither figure is given here.
• The photograph of ice cliffs on the Park Service page is credited to one of the study's university authors and is not reproduced.
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.