Proglacial lake. Ice-contact lake occurring adjacent to the frontal margin of a glacier.

A proglacial lake abuts and extends beyond the glacier terminus. Many proglacial lakes are moraine-dammed, whereas others form in basins created by isostatic depression near the ice margin. These latter lakes commonly are buttressed by the ice itself. Not all ice-dammed lakes, however, are proglacial, so these terms are not strictly synonymous. Some ice-dammed lakes, for instance, form laterally, such as when a glacier dams river flow in a side valley. Possibly the best-known ice-dammed lake was Glacial Lake Missoula, which occupied thousands of square kilometers of western Montana and Idaho before draining catastrophically (many times) to the Pacific.

Proglacial lakes can be ephemeral in nature or persist hundreds, if not thousands of years. The transient nature of some lakes results from several processes, including drainage (catastrophic or otherwise) caused by breaching of the moraine dam or by gradual isostatic uplift. Proglacial lakes also infill with sediment over time and can be replaced by outwash plains or bogs.

The sometimes catastrophic nature of proglacial lake drainage presents a significant problem to society. Rapid outbursts caused by moraine failure or tectonic activity lead to property destruction and loss of life (i.e., Lliboutry et al., 1977; Richardson and Reynolds, 2000; Allen et al., 2009). Even without complete drainage, the surface-level of proglacial lakes tends to vary, because of the changing elevation of outlets (i.e., due to moraine degradation or isostatic rebound), advance or retreat of glaciers, and periods of large recharge (such as the summer melt season or rainfall events).

Proglacial lakes are a product of both glacial and interglacial times. For example, rapid ice recession over the past century has led to the development of new, commonly moraine-dammed proglacial lakes (Figure 1) associated with retreating alpine glaciers. During the last deglaciation, large proglacial lakes abutted the melting ice sheets in both North America and Europe. One of the most famous, Glacial Lake Agassiz, covered more than half a million square kilometers and occupied basins that sloped down toward the retreating Laurentide Ice Sheet over central Canada. Catastrophic drainage of this lake at ∼8,200 years before present (Barber et al., 1999) occurred when lake water was able to float the remnant ice sheet and flow out through Hudson Strait. Some have argued that large injections of fresh meltwater into the climatically sensitive North Atlantic Ocean from lakes such as Agassiz caused shutdowns of thermohaline circulation and consequent cooling of the Northern Hemisphere several times during the last deglaciation (i.e., Teller, 2004; Teller and Leverington, 2004; although see Lowell et al., 2005 for an alternate interpretation).

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Proglacial lakes affect both the adjacent glacier and its sedimentation. Calving is a significant ablation process in proglacial lakes, with rates increasing as water deepens (Warren and Kirkbride, 2003). Thus, glaciers attached to proglacial lakes may retreat more rapidly than those terminating on land. Conversely, readvances occur more slowly, because the glaciers must overcome ice loss caused by calving.

Proglacial lakes can produce thick sedimentary sequences. Commonly, the sediments consist of rock flour, fine-grained glacial silt produced by subglacial abrasion. Such flour gives many proglacial lakes a characteristic milky or blue color. Strong seasonal differences in meltwater and sediment input lead to the formation of varves or other rhythmic bedding in some proglacial lakes. The sediments also can include ice-rafted dropstones. In polar regions, rare types of proglacial lakes, known as “proglacial lake-ice conveyors” can transport glacial till on their perennial ice cover kilometers beyond the actual glacier margin (Hendy et al., 2000) and deposit it in seasonally open water near the lake edge.

Proglacial lake sediments afford an opportunity for reconstructing glacier mass balance and climate (i.e., Leemann and Niessen, 1994). Different proxies, such as sediment grain size, preserved in annual layers, can be used to estimate changes in meltwater volume and glacier proximity. In some well-constrained cases, it is possible to estimate not only ice-marginal fluctuations, but (potentially) former equilibrium-line and temperature changes (i.e., Nesje et al., 2001; Shakesby et al., 2007; Thomas and Briner, 2009).

Proglacial lakes are common features of glacial landscapes. Drainage of large lakes that formed during the last deglaciation alongside the southern margin of the Laurentide Ice Sheet in North America have been implicated in abrupt climate change, although this is an area of ongoing research. The long records preserved in many proglacial lakes are receiving increasing attention as researchers look for high-resolution, continuous proxies of past glacier and climate fluctuations.

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