Piedmont Glaciers Explained
Most glaciers are constrained by the valleys they occupy. Valley glaciers flow between rock walls that direct their course, confine their width, and channel their ice from accumulation zone to terminus in a more or less linear path. But occasionally a glacier flowing from a mountain range encounters relatively flat lowland terrain at the base of the mountains, and without topographic walls to constrain it, the ice spreads laterally in all directions, forming a broad, lobe-shaped mass at the mountain front. This distinctive glacier form is called a piedmont glacier — piedmont meaning foot of the mountain, from the French and Italian — and it represents a transitional type between the confined valley glacier and the spreading ice cap.
Piedmont glaciers are comparatively rare because they require a specific combination of conditions: a large, high-accumulation glacier or system of glaciers in a mountain range, feeding ice rapidly enough to fill a valley and push ice out onto lower terrain; and a relatively flat lowland adjacent to the mountain front with sufficient space for the ice lobe to spread. Where these conditions are met, piedmont glaciers can be among the largest glaciers outside the polar ice sheets, covering areas measured in thousands of square kilometres.
The Malaspina Glacier: World's Largest Piedmont Glacier
The Malaspina Glacier in southeastern Alaska is the largest piedmont glacier on Earth, covering approximately 3,900 square kilometres on the coastal plain south of the Saint Elias Mountains between Yakutat Bay and Icy Bay. It is fed by ice flowing from the Saint Elias and Chugach Mountains through a series of valley glaciers — the Agassiz, Seward, and Marvine Glaciers among them — that converge at the mountain front and then spread across the broad coastal lowland, each tributary lobe visible in satellite imagery as distinct bulging fans that together form the single great piedmont expanse.
The Malaspina is sometimes called a fossil delta of ice: its surface is covered with supraglacial debris, vegetation in places, ponds, and streams that give it the appearance of a degraded outwash plain from the air. The marginal lobes are heavily forested in places where the ice surface is below the maritime snowline and debris has accumulated thickly enough to support tree growth. Occasional small forests of Sitka spruce are visible on the more stable parts of the debris-covered margin.
The Malaspina was observed and named by the Italian navigator Alejandro Malaspina during his 1791-1794 expedition to the North Pacific, making it one of the earlier-named glaciers in Alaska. Its size and accessibility to aerial and satellite observation have made it a frequent subject of research into surge dynamics, since portions of the Malaspina have exhibited surge behaviour — rapid pulsed flow episodes — that produce characteristic folded and contorted medial moraine patterns visible from satellite.
The Bering Glacier System
The Bering Glacier in southern Alaska is technically a piedmont glacier or piedmont glacier complex, depending on the definition applied: it consists of the Bering Glacier proper plus the Bagley Icefield that feeds it, together covering approximately 5,175 square kilometres including the Bagley Icefield accumulation zone. The Bering Glacier itself, spreading from the Chugach Mountains onto a coastal lowland near Cordova, covers roughly 1,900 square kilometres and is one of the world's most extensively surging glaciers. Major surges in 1993-1995 and 2008-2011 produced dramatic advances of the terminus, temporarily overriding forests and ponds at the glacier margin.
Piedmont Glaciers in Patagonia and Greenland
While Alaska has the world's largest examples, piedmont glaciers exist in other glaciated regions where comparable topographic settings occur. The Viedma Glacier in Argentine Patagonia, fed from the Southern Patagonian Ice Field, spreads into a broad lobe as it enters the flat terrain above Lago Viedma, exhibiting a piedmont-like widening of its terminus area, though it ultimately calves into the lake rather than spreading freely. Several outlet glaciers of the Greenland Ice Sheet that emerge from narrow fjords onto broader coastal lowlands develop piedmont-like spreading at their margins.
In the Russian Arctic, some of the outlet glaciers of the Novaya Zemlya ice caps spread onto coastal lowlands or develop broad lobes at the mountain front, though their extent is smaller than the Alaskan examples.
Ice Dynamics in Piedmont Glaciers
The spreading of ice onto a piedmont lobe changes the glacier's dynamics significantly compared to a confined valley glacier. Without valley walls to constrain lateral spreading, the ice can move radially outward from the point where it exits the confining mountains. This radial spreading reduces the ice thickness at any given point, decreasing the driving stress and slowing flow. Piedmont glaciers typically have lower surface slopes than valley glaciers and therefore lower driving stress, which means their ice moves more slowly despite the large volume being discharged from the mountain glacier above.
The radial flow pattern in a piedmont lobe also produces characteristic structural features: medial moraines from the tributary mountain glaciers fan out radially from the mountain front, and the annual flow lines are curved, reflecting the spreading motion. These structures are often beautifully preserved as folded or looped moraines visible in satellite imagery when surge events have occurred, as at the Malaspina.
Mass Balance and Retreat
Piedmont glaciers are particularly sensitive to climate warming because their termini, which occupy low-elevation lowland terrain, are in the warmest part of the glacier's altitudinal range. As climate warms, the equilibrium line altitude rises, reducing the accumulation zone area relative to the ablation zone. For piedmont glaciers, whose extensive low-elevation lobes are all below the equilibrium line, a rise in the ELA disproportionately increases the ablation area. Both the Malaspina and the Bering Glacier have thinned and in some sectors retreated over recent decades, consistent with the general trend of Alaskan glacier mass loss, though surge dynamics at the Bering complicate the interpretation of simple retreat measurements.
Browse the map to locate piedmont glaciers in Alaska, Patagonia, and other glaciated regions worldwide and to understand their positions at the interface between high mountain ice fields and coastal lowlands.