Ice-dammed lakes are a common feature of glaciated mountain ranges. They form wherever glacial ice blocks the drainage of rivers or meltwater. This includes:
- where a glacier blocks a trunk or tributary valley; and
- where a glacier fills an overdeepened valley created by glacial erosion
Today, ice-dammed lakes exist at the margins of many mountain valley or icefield glaciers. During the last Ice Age, when glaciers were expanded globally, huge ice-dammed lakes formed when continental ice sheets advanced and blocked the flow of river systems, causing water to pond up against their margins1,2.
Ice-dammed lakes create a unique landsystem that reflects the action of both glacial ice and water on the landscape3. The main landform and sediment assemblages related to ice-dammed lake activity are described below.
Landforms of ice-dammed lakes
The most characteristic landforms of ice-dammed lakes are features created at lake margins, which result from the erosional and depositional action of waves.
Some of the most common landforms related to ice-dammed lakes are wave-cut shorelines4,5. Shorelines are seen as distinct benches or terraces in glaciated landscapes that dip towards a current or former glacial lake and run unbroken for hundreds of metres up to tens of kilometres where large glacial lakes once existed. Shorelines are useful as they mark out the extent and elevation of ice-dammed lakes that no longer exist4,5.
At the very largest glacial lakes that formed in the last Ice Age, shorelines are seen to tilt upwards when moving upvalley from a former glacier terminus4-7. This is caused by the rebound of Earth’s crust after ice has retreated and no longer weighs down on the land surface6,7. Glacial lake shorelines can, therefore, be used to work out the rate of Earth surface rebound (known as postglacial rebound) caused by the weight of former ice sheets.
Deltas are another common landform related to ice-dammed lakes. Deltas are masses of sediment that build out into lakes at the point where a river meets standing water. In glaciated areas, rivers often carry large sediment loads that allow deltas to grow rapidly in size8.
There are many types of delta, but the most common at ice-dammed lakes are known as Gilbert-type deltas (after the American geologist Grove Gilbert)9. Gilbert-type deltas have three main parts10,11: topsets, fluvial sediments deposited at the delta surface, foresets, sediments deposited underwater on the steep delta front that dips downward into the lake, and bottomsets, sediments deposited in deeper water at the base of the delta.
Similar to shorelines, the surface of a delta (the topsets) marks the water level of a former ice-dammed lake. Often, a ‘staircase’ of deltas will form as the level of a lake (and the river that flows into it) drops over time (see photo above)5,12. Ice-dammed lakes can also partially or completely refill after being drained, and this may lead to new shorelines being cut into the front of older deltas by wave action5,12.
Beaches are commonly found in close proximity to raised deltas and lake shorelines5,12, and form in shallow water near the lake edge3. Like coastal beaches, those formed at the edges of ice-dammed lakes are the product of wave action and longshore drift that deposits sand, gravel and cobbles around the lake margin3.
It is also common to observe beach ridges that closely mirror the shape of the lake shoreline and reflect short periods of time when waves moved sediment up the beach to a specific elevation13,14.
Unique to glacial lakes are features created by icebergs15,16. Icebergs broken off from the glacier drift across the lake pushed by wind and lake currents. As they drift, their keels may scour long grooves and plough marks into the sediment at the lake floor. When an iceberg becomes grounded on the lake bottom (usually in shallower water near the lake edges), it sinks down into soft lake sediments, creating craters and hollows that remain in the landscape after the icebergs have melted and lake drained3.
Grounding line fans
While moraines can form at the margins of glaciers that terminate in lakes, more common are subaqueous grounding line fans3,9. These are fan-shaped deposits that build up around meltwater channels at the base of a glacier as when meltwater drops the sediment load it is carrying as it enters deep lake water. When a glacier remains stable for some time, it is common for fans to link up along the base of the glacier margin, forming a chain of connected fans3,9 that – much like a moraine – record a former glacier position.
Sediments of ice-dammed lakes
Ice-dammed lakes are sinks for the sediments transported by glacial meltwater or rivers.
Close to the glacier margin
As we have already seen with deltas, the largest particles are dropped from rivers at the lake edges, where they enter relatively still water3,9. In a similar way, the largest grains (sand, gravel and cobbles) entering the lake in meltwater plumes directly from a glacier are deposited close to the ice margin, often forming fans along the ice front (see above)3,9. Along with fans, debris (such as boulders) may fall from the glacier surface into the water and accumulate at the base of the terminus9.
At the lake bottom
Further away from the ice margin, fine silt and clays settle out of the water column to the lake bottom3. This material is moved to deeper parts of the lake in meltwater currents that flow from the ice margin known as underflows (which travel along the lake bed) interflows (which travel through the lake at intermediate levels) or overflows (which travel across the lake surface)9,17.
It is common for this material to settle on the lake bottom as coarse (silt) and fine (clay) couplets known as varves18. This happens as only the heaviest material (silt) can fall to the lake floor during the summer period, when glacial meltwater disturbs the water column. The lightest material (clay) falls from suspension in winter, when meltwater stops entering the lake, and when the lake surface freezes over preventing disturbance of the water column by winds18.
A final unique feature of glacial lake sediments is ice-rafted debris, material that is contained in or on icebergs and which falls to the lake bottom when icebergs roll, tip, break up, or melt16,19.
Case study: Ice Age glacial lakes in Patagonia
During the cool climate of the last Ice Age, glaciers of the North and South Patagonian Icefields expanded and joined together to form a large mountain ice sheet21. This barrier of ice blocked the flow of rivers to the ocean, and huge volumes of water ponded at the ice sheet edge. The best known of these lakes are the Lago Buenos Aires and Lago Pueyrredón ice-dammed lakes that formed around the expanded North Patagonian Icefield5.
As glaciers retreated at the end of the last Ice Age, these lakes expanded greatly, forming shorelines, deltas and beaches that extend over one hundred kilometres upvalley of the maximum ice extent5,22-24.
The gradual retreat of ice opened up new valleys over time, causing water to drain away and lower the lake surface5. This left great staircases of shorelines and raised deltas in the landscape, which record several large (about 100 m) drops in the lake level and the escape of meltwater along river valleys5,12.
Eventually, as glaciers broke up and the North and South Patagonian Icefield split apart, a huge flood of meltwater was released5,25. This sped along the Río Baker river and out to the Pacific Ocean, eroding deep gorges into bedrock and depositing huge bars topped with house-sized boulders as it went.
Today, glacial geologists use the landforms and sediments of these vast ice-dammed lakes to work out when and how glaciers changed during the demise of the last Ice Age5,26, how outburst floods changed the landscape25, and how meltwater released to the ocean may have altered regional climate24.
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 Thorndycraft, V.R., Bendle, J.M., Benito, G., Davies, B.J., Sancho, C., Palmer, A.P., Fabel, D., Medialdea, A. and Martin, J.R., 2019. Glacial lake evolution and Atlantic-Pacific drainage reversals during deglaciation of the Patagonian Ice Sheet. Quaternary Science Reviews, 203, 102-127.
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 Østrem, G., Haakensen, N. and Olsen, H.C., 2005. Sediment transport, delta growth and sedimentation in Lake Nigardsvatn, Norway. Geografiska Annaler: Series A, Physical Geography, 87, 243-258.
 Benn, D.I. and Evans, D.J.A., 2010. Glaciers and Glaciation (pp. 570-573)Routledge, London.
 Nemec, W., 1990. Aspects of sediment movement on steep delta slopes. In Coarse-grained deltas (Vol. 10, pp. 29-73).
 Smith, D.G. and Jol, H.M., 1997. Radar structure of a Gilbert-type delta, Peyto Lake, Banff National Park, Canada. Sedimentary Geology, 113, 195-209.
 Bell, C.M., 2009. Quaternary lacustrine braid deltas on Lake General Carrera in southern Chile. Andean Geology, 36, 51-66.
 Fisher, T.G., 2005. Strandline analysis in the southern basin of glacial Lake Agassiz, Minnesota and North and South Dakota, USA. Geological Society of America Bulletin, 117, 1481-1496.
 Lepper, K., Buell, A.W., Fisher, T.G. and Lowell, T.V., 2013. A chronology for glacial Lake Agassiz shorelines along Upham’s namesake transect. Quaternary Research, 80, 88-98.
 Woodworth-Lynas, C.M.T. and Guigné, J.Y., 1990. Iceberg scours in the geological record: examples from glacial Lake Agassiz. Geological Society, London, Special Publications, 53, 217-233.
 Eyles, N., Eyles, C.H., Woodworth-Lynas, C. and Randall, T.A., 2005. The sedimentary record of drifting ice (early Wisconsin Sunnybrook deposit) in an ancestral ice-dammed Lake Ontario, Canada. Quaternary Research, 63, 171-181.
 Ashley, G.M., 2002. Glaciolacustrine environments. In Modern and past glacial environments (pp. 335-359). Butterworth-Heinemann.
 Palmer, A.P., Bendle, J.M., MacLeod, A., Rose, J. and Thorndycraft, V.R., 2019. The micromorphology of glaciolacustrine varve sediments and their use for reconstructing palaeoglaciological and palaeoenvironmental change. Quaternary Science Reviews, 226, 105964.
 Ovenshine, A.T., 1970. Observations of iceberg rafting in Glacier Bay, Alaska, and the identification of ancient ice-rafted deposits. Geological Society of America Bulletin, 81, 891–894.
 Wilson, R., Glasser, N.F., Reynolds, J.M., Harrison, S., Anacona, P.I., Schaefer, M. and Shannon, S., 2018. Glacial lakes of the Central and Patagonian Andes. Global and Planetary Change, 162, 275-291.
 Hein, A.S., Hulton, N.R., Dunai, T.J., Sugden, D.E., Kaplan, M.R. and Xu, S., 2010. The chronology of the Last Glacial Maximum and deglacial events in central Argentine Patagonia. Quaternary Science Reviews, 29, 1212-1227.
 Turner, K.J., Fogwill, C.J., McCulloch, R.D. and Sugden, D.E., 2005. Deglaciation of the eastern flank of the North Patagonian Icefield and associated continental‐scale lake diversions. Geografiska Annaler: Series A, Physical Geography, 87(2), pp.363-374.
 Bell, C.M., 2008. Punctuated drainage of an ice‐dammed quaternary lake in southern south america. Geografiska Annaler: Series A, Physical Geography, 90(1), pp.1-17.
 Glasser, N.F., Jansson, K.N., Duller, G.A., Singarayer, J., Holloway, M. and Harrison, S., 2016. Glacial lake drainage in Patagonia (13-8 kyr) and response of the adjacent Pacific Ocean. Scientific Reports, 6, p.21064.
 Benito, G. and Thorndycraft, V.R., 2019. Catastrophic glacial-lake outburst flooding of the Patagonian Ice Sheet. Earth-Science Reviews, p.102996.
 Bendle, J.M., Palmer, A.P., Thorndycraft, V.R. and Matthews, I.P., 2017. High-resolution chronology for deglaciation of the Patagonian Ice Sheet at Lago Buenos Aires (46.5°S) revealed through varve chronology and Bayesian age modelling. Quaternary Science Reviews, 177, 314-339.