Subglacial Floods Research Articles

Hydraulic damage in subglacial conduits: Evidence from rock hydrofracture and hydraulic jacking for high fluid pressures during rapid melt of the Fennoscandian Ice Sheet

When large volumes of meltwater arrive suddenly at the glacier bed, the capacity of subglacial conduits is exceeded, generating transiently high fluid pressures. Yet we currently lack reliable markers for where such conditions developed on the beds of former glaciers. Here, we describe hydraulic damage that developed at close to or above overpressure (Pw > Pi) at the bed of the last Fennoscandian Ice Sheet in Uppland, east-central Sweden. Hydraulic damage is mostly confined within sharp lateral and vertical limits at the boundaries of former large subglacial conduits. The conduits include large Röthlisberger- and Nye-channels - eskers, esker corridors, and meltwater channels – that operated at full pipeflow. Wider (0.4–1.5 km) meltwater corridors opened under sheetflow with broad, shallow Hooke-channels. Hydraulic damage found in Precambrian gneiss and granite bedrock includes hydrofracturing and hydraulic jacking, with fracture propagation, dilation, and block displacement. Dilated fractures are often plugged by massive sands and diamicts or laminated silts. The fracture fills represent brief, single and multi-phase, injections of viscous, sediment-laden fluids at the ice sheet bed. The most spectacular effects of hydraulic damage are seen in the distension, disruption, and bursting of 1–20 m high rock bumps and hills. We present a detailed taxonomy for hydraulic damage and the consequent glacitectonic damage sustained during glacial ripping.Industrial fracking provides a well understood analogue for hydrofracture in nature. In subglacial settings, pathways for the flow of pressurised fluids are determined by the tensile strengths and hydraulic transmissivity of ice, till, and rock. The easiest flow path is across the glacier bed beneath sliding ice. Abrupt increases in meltwater flow force ice-bed separation. Opening of conduits under high fluid pressures triggers further propagation of hydrofractures into ice, till, and rock. On the floors of subglacial conduits, pressurised fluids entered the hard rock bed via old, mineral coated fractures. Under Poiseuille's law, fracture flow was initially slow due to the narrow apertures, but flow accelerated rapidly as fluid wedging dilated fractures. Fluid pressures in old fractures are constrained by published measurements for near-surface fracture toughness of ≤0.7 MPa at Forsmark, east-central Sweden. Formation of new rock hydrofractures overcame gneiss tensile strengths of 8–18 MPa. Hydraulic jacking required accommodation space that developed during conduit opening. Due to buoyancy and ice bed separation, lower excess pressures of <0.3 MPa were required to lift rock sheets up to 10 m thick. Hydraulic bursting developed in large conduits under very high, but rapidly fluctuating fluid pressures as dilating rock fractures extended to the glacier bed. Distension of rock surfaces was brief due to rapid leak-off and the inherent instability of overpressured conditions in dilatant, fractured rock masses.Hydraulic damage developed in Uppland when accelerating ablation generated large subglacial meltwater fluxes in response to abrupt warming at the Younger Dryas-early Holocene transition (∼10.8 ka). The large volumes of subglacial meltwater, the sudden increases in subglacial water flow and fluid pressures, and the release and transport of 2 m b-axis boulders in esker tunnels suggest that hydraulic damage was triggered at the onset of large subglacial floods. Ice thicknesses at distances of up to 50 km behind the grounding line at the retreating subaqueous ice margin indicate that steady-state hydrostatic pressures were ∼2.9–4.5 MPa. Such pressures are consistent with (i) extensive, but brief ice-bed separation in meltwater corridors under overpressure and (ii) the development of hydraulic damage in conduits close to the ice margin. More extreme fluid pressures developed instantaneously under hydraulic shock, as rapid subglacial pipeflow was suddenly stopped or diverted. The taxonomy of hydraulic damage presented here can be applied to any hard glacier bed to identify where large volumes of subglacial meltwater reached high pressures. Similar markers are widely reported from modern and former glacier beds.

Okanogan lobe tunnel channels and subglacial floods into Moses Coulee, Channeled Scabland, northwestern United States

Abstract Outburst floods from glacial Lake Missoula largely explain erosion of the Channeled Scabland, a system of overfit, basaltic channels in Washington, northwestern United States. However, it is challenging to explain Missoula flood routing into Moses Coulee due to its topographic isolation from flood routes. To clarify flood pathways into Moses Coulee, we mapped channels that delineate a radial-anastomosing network connecting to Moses Coulee. Channels consist of coulee-like features eroded mainly in basalt. Channels climb adverse slopes and cross divides, and the network is draped with eskers and recessional moraines. These channel geometries and glacial landform associations suggest a tunnel channel network. Large channel dimensions and network anastomosis indicate formation in subglacial floods. The network connects to Moses Coulee with sufficient cross-sectional area to convey megafloods, hinting that subglacial floods may have been a significant source of Moses Coulee meltwater, in addition to possible diverted Missoula floods.

Subaerial and subglacial seismic characteristics of the largest measured jökulhlaup from the eastern Skaftá cauldron, Iceland

Abstract. Subglacial floods cause seismic tremors that can be located and tracked in space and time using a seismic array. Here, we shed light on the generating mechanisms of the seismic signals observed during the largest measured flood from the eastern Skaftá cauldron in the Vatnajökull ice cap, Iceland. We track the propagation of the flood in 2015 using two seismic arrays and a local seismic network in combination with GPS, hydrological, and geochemical data. We find that as the water drained from the subglacial lake beneath the cauldron, families of icequakes were generated in the area around the cauldron, while the glacier surface gradually subsided by more than 100 m. We detected a several-hours-long, non-harmonic tremor and high-frequency transient events migrating downglacier, following the subglacial flood front. We suggest that this tremor is composed of repeating, closely spaced icequakes that were generated as the glacier was being lifted, cracked, and deformed, thus enabling the subglacial water flow. When the lake had largely drained, the pressure within the underlying hydrothermal system dropped. At this time, we recorded minute-long tremor bursts emanating from the cauldron area, followed by an hour-long harmonic tremor each. We interpret these as being caused by hydrothermal explosions in the geothermal system within the cauldron and as being vigorous boiling in the crustal rocks, respectively, which is an interpretation corroborated by floodwater geochemical signals. Finally, the flood also led to detectable tremor due to more energetic flow in the rapids near Sveinstindur in the Skaftá river. We conclude that the flood generated five different seismic signal types that can be associated with five different geophysical processes, including the wide spectrum from brittle failure and explosions to boiling and turbulent flow.

Formation of murtoos by repeated flooding of ribbed bedforms along subglacial meltwater corridors

Fluctuations in meltwater discharge below modern glaciers and ice sheets due to diurnal, seasonal and long-term temperature variations are modulated by complex interactions between subglacial drainage, basal processes and bedform development. The bed of palaeo-ice sheets contains a variety of bedforms recording these modulations and provides an open window into the subglacial environment. Through the morphometric analysis of natural and experimental bedforms, respectively mapped along Scandinavian meltwater corridors and produced in a physical model simulating transitory subglacial water flow, we observe a morphological and genetic bedform continuum corresponding to the progressive transformation of ribbed bedforms into murtoos. Two alternating drainage configurations, related to repeated subglacial flooding events, are involved in this transformation: (i) significant meltwater discharge, high hydraulic connectivity and ice-bed decoupling during flooding events lead to hydraulic alteration of ribbed bedforms by erosion, sediment deposition and channel incision, while (ii) limited meltwater flow, low hydraulic connectivity and ice-bed recoupling that follow flooding events lead to their deformational reshaping into murtoos. The degree of transformation of ribbed bedforms into murtoos can be quantified by combining two dimensionless morphometric parameters (circularity and sinuosity) and provides a convenient proxy to constrain magnitudes, durations and/or frequencies of subglacial floods in palaeo-meltwater corridors.

Basal Crevasse Formation on Byrd Glacier, East Antarctica, as Proxy for Past Subglacial Flooding Events

AbstractLinear elastic fracture mechanics suggests that short‐lived flow accelerations, such as the one initiated by a flooding event beneath Byrd Glacier in 2006, can form abnormally large basal crevasses at the grounding line. Airborne radar measurements acquired in 2011 reveal hundreds of basal crevasses ranging in height from 40–335 m. Particle tracking results show that formation of the largest basal crevasse occurred at the grounding line during the 2006 flooding event. Very large basal crevasses form distinctive surface depressions directly overhead, which are observed along the Byrd Glacier flowline to the terminus of the Ross Ice Shelf. By using these surface depressions as proxies for abnormally large basal crevasses, we create a timeline of past subglacial flooding events on Byrd Glacier. Understanding the frequency of flooding events and their effect on glacier dynamics will help inform models of ice sheet stability and subglacial hydrology.

Seismic ground vibrations give advanced early-warning of subglacial floods

Glacier runoff and melt from volcanic and geothermal activity accumulates in glacier dammed lakes in glaciated areas around the world. These lakes eventually drain, creating hazardous subglacial floods that are usually only confirmed after they exit the glacier and reach local river systems, which can be many tens of kilometres from the flood source. Once in the river systems, they travel rapidly to populated areas. Such delayed detection represents a potentially lethal shortcoming in early-warning. Here we demonstrate how to advance early-warning potential through the analysis of four such floods in a glaciated region of Iceland. By comparing exceptional multidisciplinary hydrological, GPS and seismic ground vibration (tremor) data, we show that array analysis of seismic tremor can be used for early location and tracking of the subglacial flood front. Furthermore the timing and size of the impending flood can be estimated, prior to it entering the river system. Advanced warnings of between 20 to 34 hours are achieved for large (peak discharge of more than 3000 m3/s, accumulation time of ~ 5.25 years) to small floods (peak discharges from 210 to 380 m3/s, accumulation times of ~ 1.3 years) respectively.

Estimation of the water discharge during the development of the glacial and subglacial outburst floods.

Estimation of the water discharge during the development of the glacial and subglacial outburst floods.

Оценка расхода воды в процессе развития прорывного паводка ледниковых и подледниковых водоемов

Оценка расхода воды в процессе развития прорывного паводка ледниковых и подледниковых водоемов

Velocity increases at Cook Glacier, East Antarctica, linked to ice shelf loss and a subglacial flood event

Abstract. Cook Glacier drains a large proportion of the Wilkes Subglacial Basin in East Antarctica, a region thought to be vulnerable to marine ice sheet instability and with potential to make a significant contribution to sea level. Despite its importance, there have been very few observations of its longer-term behaviour (e.g. of velocity or changes at its ice front). Here we use a variety of satellite imagery to produce a time series of ice front position change from 1947 to 2017 and ice velocity from 1973 to 2017. Cook Glacier has two distinct outlets (termed East and West), and we observe the near-complete loss of the Cook West Ice Shelf at some time between 1973 and 1989. This was associated with a doubling of the velocity of Cook West Glacier, which may also be linked to previously published reports of inland thinning. The loss of the Cook West Ice Shelf is surprising given that the present-day ocean climate conditions in the region are not typically associated with catastrophic ice shelf loss. However, we speculate that a more intense ocean climate forcing in the mid-20th century may have been important in forcing its collapse. Since the loss of the Cook West Ice Shelf, the presence of landfast sea ice and mélange in the newly formed embayment appears to be important in stabilizing the glacier front and enabling periodic advances. We also show that the last calving event at the larger Cook East Ice Shelf resulted in the retreat of its ice front into a dynamically important portion of the ice shelf and observe a short-lived increase in velocity of Cook East between 2006 and 2007, which we link to the drainage of subglacial Lake Cook. Taken together, these observations suggest that the velocity, and hence discharge, of Cook Glacier is highly sensitive to changes at its terminus, but a more detailed process-based analysis of this potentially vulnerable region requires further oceanic and bathymetric data.

The influence of a poroelastic till on rapid subglacial flooding and cavity formation

We develop a model of the rapid propagation of water at the contact between elastic glacial ice and a poroelastic subglacial till, motivated by observations of the rapid drainage of supraglacial lakes in Greenland. By treating the ice as an elastic bending beam, the fluid dynamics of contact with the subglacial hydrological network, which is modelled as a saturated poroelastic till, can be examined in detail. The model describes the formation and dynamics of an axisymmetric subglacial cavity, and the spread of pore pressure, in response to injection of fluid. A combination of numerical simulation and asymptotic analysis is used to describe these dynamics for both a rigid and a deformable porous till, and for both laminar and turbulent fluid flow. For constant injection rates and laminar flow, the cavity is isostatic and its spread is controlled by bending of the ice and suction of pore water in the vicinity of the ice–till contact. For a deformable till, this control can be modified: generically, a flexural wave that is initially trapped in advance of the contact point relaxes over time by diffusion of pore pressure ahead of the cavity. While the dynamics are found to be relatively insensitive to the properties of the subglacial till during injection with a constant flux, significant dependence on the till properties is manifest during the subsequent spread of a constant volume. A simple hybrid turbulent–laminar model is presented to account for fast injection rates of water: in this case, self-similar turbulent propagation can initially control the spread of the cavity, but there is a transition to laminar control in the vicinity of the ice–till contact point as the flow slows. Finally, the model results are compared with recent geophysical observations of the rapid drainage of supraglacial lakes in Greenland; the comparison provides qualitative agreement and raises suggestions for future quantitative comparison.

Modelled subglacial floods and tunnel valleys control the life cycle of transitory ice streams

Abstract. Ice streams are corridors of fast-flowing ice that control mass transfers from continental ice sheets to oceans. Their flow speeds are known to accelerate and decelerate, their activity can switch on and off, and even their locations can shift entirely. Our analogue physical experiments reveal that a life cycle incorporating evolving subglacial meltwater routing and bed erosion can govern this complex transitory behaviour. The modelled ice streams switch on and accelerate when subglacial water pockets drain as marginal outburst floods (basal decoupling). Then they decelerate when the lubricating water drainage system spontaneously organizes itself into channels that create tunnel valleys (partial basal recoupling). The ice streams surge or jump in location when these water drainage systems maintain low discharge but they ultimately switch off when tunnel valleys have expanded to develop efficient drainage systems. Beyond reconciling previously disconnected observations of modern and ancient ice streams into a single life cycle, the modelling suggests that tunnel valley development may be crucial in stabilizing portions of ice sheets during periods of climate change.

Excavation of subglacial bedrock channels by seasonal meltwater flow

AbstractSubglacial water flow drives the excavation of a variety of bedrock channels including tunnel valleys and inner gorges. Subglacial floods of various magnitudes – events occurring once per year or less frequently with discharges larger than a few hundred cubic metres per second – are often invoked to explain the erosive power of subglacial water flow. In this study we examine whether subglacial floods are necessary to carve bedrock channels, or if more frequent melt season events (e.g. daily production of meltwater) can explain the formation of substantial bedrock channels over a glacial cycle. We use a one‐dimensional numerical model of bedrock erosion by subglacial meltwater, where water flows through interacting distributed and channelized drainage systems. The shear stresses produced drive bedrock erosion by bed‐ and suspended‐load abrasion. We show that seasonal meltwater discharge can incise an incipient bedrock channel a few tens of centimetres deep and several metres wide, assuming abrasion is the only mechanism of erosion, a particle size of D=256 mm and a prescribed sediment supply per unit width. Using the same sediment characteristics, flood flows yield wider but significantly shallower bedrock channels than seasonal meltwater flows. Furthermore, the smaller the shear stresses produced by a flood, the deeper the bedrock channel. Shear stresses produced by seasonal meltwater are sufficient to readily transport boulders as bedload. Larger flows produce greater shear stresses and the sediment is carried in suspension, which produces fewer contacts with the bed and less erosion. We demonstrate that seasonal meltwater discharge can excavate bedrock volumes commensurate with channels several tens of metres to a few hundred metres wide and several tens of metres deep over several thousand years. Such simulated channels are commensurate with published observations of tunnel valleys and inner gorges. Copyright © 2018 John Wiley &amp; Sons, Ltd.

Glacial curvilineations found along the southern sector of the Laurentide Ice sheet and a hypothesis of formation involving subglacial slope failure in tunnel valleys and subglacial lakes

AbstractFirst discovered in Poland, glacial curvilineations (GCLs) are enigmatic landforms comprising parallel sets of sinuous ridges and troughs of metres amplitude and around 150 m wavelength, found within kilometres‐wide valleys interpreted as being produced by meltwater flowing subglacially. Their morphological and sedimentary characteristics and association with tunnel valleys has been described for some prominent Polish examples. From these observations the existing hypothesis is that they form as a consequence of erosion by longitudinal vortices that develop in subglacial floods. Here we report, for the first time, GCLs found along the southern sector of the Laurentide Ice Sheet in three northern states of the USA. Using mapping and topographic analysis from high resolution digital elevation models we report observations on their morphological properties and landform associations. We find aspects of their context and morphology difficult to explain using the existing hypothesis. We instead suggest that these glacial curvilineations are produced by subglacial bank and slope failures that locally widen tunnel valleys, or that occur near subglacial lake shorelines. Further investigation is required to test this hypothesis and to ascertain the mechanisms of proposed mass movements, which may have occurred by rotational or translational slope failure or by creep deformation. Our preferred mechanism is that such movements occurred where subglacial water was emplaced over previously perma‐frozen ground. Under such circumstances, sediment blocks thawed by the water may then easily glide over a frozen décollement at low slope angles; analogous to subaerial active‐layer glides in permafrost environments. Permafrost spring sapping may have provided lines of weakness for slope failure. If the requirement for permafrost is found to hold, then GCLs may become an important indicator of the palaeo‐distribution of permafrost. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Subglacial flood path development during a rapidly rising jökulhlaup from the western Skaftá cauldron, Vatnajökull, Iceland

ABSTRACTDischarge and water temperature measurements in the Skaftá river and measurements of the lowering of the ice over the subglacial lake at the western Skaftá cauldron, Vatnajökull, Iceland, were made during a rapidly rising glacial outburst flood (jökulhlaup) in September 2006. Outflow from the lake, flood discharge at the glacier terminus and the transient subglacial volume of floodwater during the jökulhlaup are derived from these data. The 40 km long initial subglacial path of the jökulhlaup was mainly formed by lifting and deformation of the overlying ice, induced by water pressure in excess of the ice overburden pressure. Melting of ice due to the heat of the floodwater from the subglacial lake and frictional heat generated by the dissipation of potential energy in the flow played a smaller role. Therefore this event, like other rapidly rising jökulhlaups, cannot be explained by the jökulhlaup theory of Nye (1976). Instead, our observations indicate that they can be explained by a coupled subglacial-sheet–conduit mechanism where essentially all of the initial flood path is formed as a sheet by the propagation of a subglacial pressure wave.

A spectrum of jökulhlaup dynamics revealed by GPS measurements of glacier surface motion

ABSTRACTGPS campaigns on glaciers during jökulhlaups show how subglacial floods affect glacier motion and shed light on the dynamics of such floods. Three such campaigns have been carried out on southern and western Vatnajökull, southeast Iceland, over known jökulhlaup paths. Two slowly rising jökulhlaups from Grímsvötn and two rapidly rising jökulhlaups from the western and eastern Skaftá cauldrons were captured in these campaigns, with maximum discharge ranging from 240 to 3300 m3 s−1. Glacier surface movements measured in these campaigns are presented along with the corresponding discharge curves. The measurements are interpreted as indicating: (1) initiation of rapidly rising jökulhlaups with a propagating subglacial pressure wave, (2) decreased glacier basal friction during jökulhlaups, (3) subglacial accumulation of water in slowly rising jökulhlaups and (4) lifting of the glacier caused by subglacial water pressure exceeding overburden in both rapidly and slowly rising jökulhlaups. The latter two observations are inconsistent with assumptions that are typically made in theoretical and numerical modelling of jökulhlaups. Both viscous and elastic deformation of the glacier as well as turbulent hydraulic fracture at the ice/bedrock interface are important in the dynamics of the subglacial pressure wave at the front of rapidly rising jökulhlaups.

Episodic ice velocity fluctuations triggered by a subglacial flood in West Antarctica

AbstractHeight change anomalies in satellite altimeter data have been interpreted as the surface expressions of basal water moving into and out of subglacial lakes. These signals have been mapped throughout Antarctica on timescales of months to years, but only broad connections have been made between active lakes and ice dynamics. We present the first high‐frequency observations of ice velocity evolution due to a cascading subglacial lake drainage event, collected over 5 years (2010–2015) using Global Positioning System data on Whillans and Mercer ice streams, West Antarctica. We observed three episodic ice velocity changes over 2 years, where flow speed increased by up to 4%, as well as an 11 month disruption of the tidally modulated stick‐slip cycle that dominates regional ice motion. Our observations reveal that basal conditions of an Antarctic ice stream can rapidly evolve and drive a dynamic ice response on subannual timescales, which can bias observations used to infer long‐term ice sheet changes.

Catastrophic flooding, palaeolakes, and late Quaternary drainage reorganization in northern Eurasia

ABSTRACTWe review the long history of Russian scientific study of the major late Quaternary palaeodrainages and palaeolakes in northern Eurasia to provide overviews of (1) past research efforts (mainly published in the Russian language) and (2) recent studies that include contributions from those outside of the former Soviet Union region. There are two major groups of catastrophic flooding phenomena and related palaeodrainage and palaeolake systems in this region: (1) those involving large-scale lowland basins and (2) those involving upland intermontane basins. Many of these palaeodrainages and palaeolakes were active during the late Quaternary, though the available age data are inconclusive in regard to more exact timing and synchronicity. These palaeodrainages and palaeolakes in the late Quaternary are strongly linked with climatic change, most notably to ice age. For the megalakes in large-scale basins, spillover events probably occurred because of impoundment by the late Pleistocene Arctic ice sheets. In some cases, gradual transgressions of the megalakes caused by varying climatic conditions could also have contributed to the spillovers. Some of these spillover events may have been catastrophic. Ice-dammed lakes formed when drainage into intermontane basins was impounded by late Pleistocene alpine-type glaciers. The collapses of the glacier dams caused downstream catastrophic flooding, producing a characteristic suite of flood-related landforms. In the Azas Plateau volcanic field, there is evidence for Icelandic-type subglacial flooding.

A model for subglacial flooding through a preexisting hydrological network during the rapid drainage of supraglacial lakes

AbstractIncreasingly large numbers of supraglacial lakes form and drain every summer on the Greenland Ice Sheet. Some of these lakes drain rapidly within the timescale of a few hours, and the vertical discharge of water during these events may find a preexisting film of water potentially within a distributed drainage system of linked cavities. Here we present a model for subglacial flooding applied specifically to such circumstances. One of many interesting results we find is that water flows in from the far field prior to the arrival of flooding. We systematically evaluate the effect of initial ice/bed opening on the degree of perturbation to the subglacial system. Of particular importance, we find that floods propagate much faster and vertical displacements are much greater for larger openings. For 10 and 1 cm of initial opening, for example, floods travel about 68% and 50% farther than in the fully coupled ice/bed scenario after 2 h of drainage, respectively. For the same choices of initial opening, the elastostatic displacement at the injection point is about 1.39 and 1.26 times that of the fully coupled scenario. Using the framework with a preexisting water film results in avoiding the pressure singularity that is inherent to classical hydrofracture models, thus opening an avenue for integrating the likes of our model within continuum subglacial hydrological models. Furthermore, we foresee that the theory presented can be used to potentially infer subglacial hydrological conditions from surface observables.

The geomorphic signature of massive subglacial floods in Victoria Land, Antarctica.

We describe a landscape in the Dry Valleys area consisting of channels, potholes, plunge pools, stripped bedrock, and scabland created by subglacial outburst floods on a scale similar to that of the massive outbursts of Lake Missoula in Washington, United States. The features are dated to the mid-Miocene and occurred when a thicker East Antarctic ice sheet overrode this part of the Transantarctic Mountains. Ice sheet modeling is used to reconstruct potential source areas for meltwater discharge. Results suggest that meltwater from Lake Vostok, or other interior basins, would have bypassed the Dry Valleys area and instead flowed out beneath the troughs of David and Byrd glaciers, north and south of the Dry Valleys, respectively. A more likely source might have been lakes on the inner flank of the Transantarctic Mountains, with periodic outbursts associated with local breaches in the cold-based rim of the ice sheet. The efficacy of erosion by such outbursts is remarkable. The most concentrated effects were in the labyrinth on the bed of the former Wright outlet glacier. Such outbursts are likely to contribute to the erosion of present-day outlet glacier troughs.

Antarctic Subglacial Lake Discharges

Antarctic subglacial lakes were long time supposed to be relatively closed and stable environments with long residence times and slow circulations. This view has recently been challenged with evidence of active subglacial lake discharge underneath the Antarctic ice sheet. Satellite altimetry observations witnessed rapid changes in surface elevation across subglacial lakes over periods ranging from several months to more than a year, which were interpreted as subglacial lake discharge and subsequent lake filling, and which seem to be a common and widespread feature. Such discharges are comparable to jokulhlaups and can be modeled that way using the Nye-Rothlisberger theory. Considering the ice at the base of the ice sheet at pressure melting point, subglacial conduits are sustainable over periods of more than a year and over distances of several hundreds of kilometers. Coupling of an ice sheet model to a subglacial lake system demonstrated that small changes in surface slope are sufficient to start and sustain episodic subglacial drainage events on decadal time scales. Therefore, lake discharge may well be a common feature of the subglacial hydrological system, influencing the behavior of large ice sheets, especially when subglacial lakes are perched at or near the onset of large outlet glaciers and ice streams. While most of the observed discharge events are relatively small (101-102 m3 s -1), evidence for larger subglacial discharges is found in ice free areas bordering Antarctica, and witnessing subglacial floods of more than 106 m3 s-1 that occurred during the middle Miocene. Copyright © 2010 by the American Geophysical Union.