632 SEDIMENTARY PROCESSES/Catastrophic Floods the breccia dam Such events have been reported from locations such as the Alps, Greenland, and the Canadian Arctic and commonly result in floods with peak discharges up to an order of magnitude higher than ‘normal’ diurnal meltwater flows These floods occur within either ice-marginal river systems or englacial tunnels, and they are more frequent earlier in the melt season when increasing meltwater discharges undermine and remove glacier ice from winter advance positions Glaciers are able to store water within ice-dammed lakes over time-periods of 101–102 years, generating glacier outburst floods or joă kulhlaups when they drain Lakes can form in a number of locations: beneath, within, on top of, and at the edge of glaciers (Figure 4) Subglacial lakes can exist either within pre-existing topographic depressions, such as volcanic calderas and valleys, or within up-turned bowl-shaped lakes protruding upwards into the glacier Such subglacial lakes are common in Iceland where high geothermal heat fluxes and frequent volcanic activity are common In Iceland, subglacial Lake Grmsvoătn drains every few years as meltwater takes time to accumulate, raising the lake to a level where drainage will occur In 1996, rapid influx of meltwater from the nearby Gja´lp subglacial eruption site resulted in the filling of Grmsvoătn within one month Approximately 3.4 km3 of meltwater drained subglacially from the lake as a joăkulhlaup, which lasted for three days Although Grmsvoătn is one of the largest modern subglacial lakes known to drain, it is dwarfed by Late Quaternary joăkulhlaups from glacial Lake Missoula, which drained over 2000 km3 of water within just a few days The release of water from sub- or englacial pockets has also been invoked as an explanation for sudden outburst floods or ‘debacles’ from Alpine glaciers Large supraglacial lakes are also known from modern glaciers and ice-sheets and can involve the drainage of up to $0.1 km3 of meltwater subglacially over distances of tens of kilometres Ice-marginal, icedammed lakes are relatively common at modern glaciers, and the presence of shorelines, deltas, and lacustrine deposits provide widespread evidence for the existence of large ice-marginal lakes associated with Palaeo ice-margins Large proglacial lakes formed during the deglaciation of North America, and drained to the North Atlantic and Arctic oceans via a series of enormous spillway channels For example, glacial Lake Agassiz is known to have drained 160 000 km3 of meltwater within only a few years Although ice-dammed lakes occupy a variety of locations relative to the impounding glacier, the most significant control on joăkulhlaup characteristics is whether the glacier enters a main valley as an outlet and has the potential to pond a lake within the main valley upstream of the glacier dam In this case, the ice-dammed lake volume would be much larger than that of ice-marginal, ice-dammed lakes relative to the glacier dam A number of mechanisms to account for the drainage of ice-dammed lakes have been proposed In general terms, lake drainage will take place if there is a hydraulic gradient allowing lake water to flow into the glacier However, a connection must also exist between the lake and the glacier for drainage to commence In most cases ice-dammed lake drainage leaves the glacier intact, but complete ice-dam removal may take place where the lake volume is large relative to ice dam volume The timing of ice-dammed lake drainage is frequently controlled by the retreat and advance of glaciers Glacier thinning and retreat often allow the initiation of cycles of lake drainage (Figure 6) Joăkulhlaup magnitude decreases during each cycle as impounded lake volume decreases during glacier retreat (Figure 6) Glacier retreat within high mountain regions often creates highly unstable moraine-dammed lakes, which are prone to sudden failure and flood generation Calving of glacier ice into these lakes can generate waves, which overtop and then incise the moraine dam, allowing the extremely rapid development of a breach, facilitating catastrophic evacuation of lake water Melt out of buried ice within moraine dams may take decades, and it is common for morainedammed lakes to exist for considerable periods before drainage Once a moraine dam has been breached, only a subsequent glacier advance can reinstate the dam Landslides can generate floods in two ways Firstly, rapid input of landslide debris to lakes can generate displacement waves The best-known example of such a flood was at the Vaiont reservoir in Italy, where a landslide generated a large displacement wave that washed up a hillside and over the dam In Iceland, a rockfall generated a displacement wave known as the Steinholtshlaup, which constituted one of the most rapid flood-rising stages experienced in Iceland Landslides frequently dam major valleys allowing the formation of large lakes Dam failure may occur immediately after formation, or may be delayed, possibly for decades, until reservoir volume has built up to a sufficient level for drainage to occur Bedrock and sediment dams are also known to fail via catastrophic enlargement of their spillways, leading to large outbursts The drainage of Lake Bonneville in the USA during the Late Quaternary generated an outburst with a peak discharge of approximately  106 m3 s 1, whilst draining 4800 km3 of water over several months In New Zealand the