Andrew J. Russell

Controls on the formation and sudden drainage of glacier-impounded lakes: implications for jokulhlaup characteristics

Over the past few years there has been an increase in understanding of glacier-impounded or ‘ice-dammed’ lake behaviour. The spectacular jökulhlaup (catastrophic flood) from Grímsvötn, Iceland in November 1996 has both raised the profile of such events and emphasized the need for awareness of the processes involved. This review summarizes the extent of current knowledge of ice-dammed lakes, highlighting key developments and outlining areas of study still subject to difficulties. Controls on ice-dammed lake formation and persistence are identified, and cycles of jökulhlaup activity are related to glacier fluctuations. Ice-dammed lake drainage trigger mechanisms are reviewed and recent progress in the understanding of such mechanisms is emphasized. Controls on jökulhlaup routing and the development and character of jökulhlaup conduits are discussed and recent advances in jökulhlaup prediction, hydrograph modelling and peak discharge estimation are assessed. A process-based schematic model, drawing on published research, links ice-dammed lake occurrence and drainage to jökulhlaup characteristics. It is demonstrated that ice-dammed lake and ice-dam characteristics ultimately control seven key jökulhlaup attributes which determine the potential impact of jökulhlaups on both landscape and human activity in glaciated regions.

Ice fracturing during jökulhlaups: implications for englacial floodwater routing and outlet development.

Theoretical studies of glacial outburst floods (jokulhlaups) assume that: (i) intraglacial floodwater is transported efficiently in isolated conduits; (ii) intraglacial conduit enlargement operates proportionally to increasing discharge; (iii) floodwater exits glaciers through pre-existing ice-marginal outlets; and (iv) the morphology and positioning of outlets remains fixed during flooding. Direct field observations, together with historical jokulhlaup accounts, confirm that these theoretical assumptions are not always correct. This paper presents new evidence for spatial and temporal changes in intraglacial floodwater routing during jokulhlaups; secondly, it identifies and explains the mechanisms controlling the position and morphology of supraglacial jokulhlaup outlets; and finally, it presents a conceptual model of the controls on supraglacial outbursts. Field observations are presented from two Icelandic glaciers, Skeiðararjokull and Solheimajokull. Video footage and aerial photographs, taken before, during and after the Skeiðararjokull jokulhlaup and immediately after the Solheimajokull jokulhlaup, reveal changes in floodwater routing and the positioning and morphology of outlets. Field observations confirm that glaciers cannot transmit floodwater as efficiently as previously assumed. Rapid increases in jokulhlaup discharge generate basal hydraulic pressures in excess of ice overburden. Under these circumstances, floodwater can be forced through the surface of glaciers, leading to the development of a range of supraglacial outlets. The rate of increase in hydraulic pressure strongly influences the type of supraglacial outlet that can develop. Steady increases in basal hydraulic pressure can retro-feed pre-existing englacial drainage, whereas transient increases in pressure can generate hydraulic fracturing. The position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding. The development of supraglacial jokulhlaup outlets provides a new mechanism for rapid englacial debris entrainment.

Glaciohydraulic supercooling in Iceland

We present evidence of glaciohydraulic supercooling under jokulhlaup and ablation- dominated conditions from two temperate Icelandic glaciers. Observations show that freezing of sediment-laden meltwater leads to intraglacial debris entrainment during normal and extreme hydrologic regimes. Intraglacial frazil ice propagation under normal ablation-dominated conditions can trap copious volumes of sediment, which forms anomalously thick sections of debris-rich ice. Glaciohydraulic supercooling plays an important role in intraglacial debris entrainment and should be given more attention in models of basal ice development. Extreme jokulhlaup conditions can result in significant intraglacial sediment accretion by supercooling, which may explain the concentration of englacial sediments deposited in Heinrich layers in the North Atlantic during the last glaciation.

Morphology and sedimentology of a giant supraglacial, ice-walled, jökulhlaup channel, Skeiðarárjökull, Iceland: implications for esker genesis

Abstract This paper examines the sedimentary infill of a spectacular, 500-m-long, 100-m-wide ice-walled supraglacial channel, excavated into the snout of Skeiðararjokull, Iceland during the November 1996 jokulhlaup. The ice-walled channel developed in an area of the glacier, which was extensively fractured during the jokulhlaup. Sculpting of the ice-walled channel into the active snout of Skeiðararjokull suggests that the presence of stagnating glacier ice is not a prerequisite for the development of ice-walled channels. The ice-walled channel occupied an inter-lobate location, which acted as a focus for meltwater during the November 1996 jokulhlaup. The geometry of the supraglacial ice-walled channel system acted as a major control on the morphology and sedimentology of jokulhlaup deposits, through the tremendous spatial variability of resultant flow conditions. Maximum calculated jokulhlaup powers and shear stresses for the supraglacial ice-walled channel reached 40,000 W m −2 and 5000 N m −2 , respectively, with associated mean flow velocities between 7 and 11 m s −1 . Within the main ice-walled channel, Ground Penetrating Radar and outcrop exposure provide evidence of an ∼8-m-thick progradational and aggradational gravel macroform succession. The supraglacial ice-walled channel system is therefore analogous to a bedrock-confined fluvial system. This study provides a new analogue for the interpretation of ice-contact glaciofluvial deposits associated with former ice margins in Iceland and other areas subject to high magnitude discharges. Former supraglacial ice-walled channels resulting from tunnel collapse and ice margin break-up during high magnitude jokulhlaups will be associated with extensive coarse-grained, heavily kettled proglacial outwash surfaces. It is clear that the relationship between the characteristics of former ice-walled channels labeled as eskers and the prevailing glaciological and hydrological conditions needs to be modified in light of our knowledge of a modern flood-related large-scale supraglacial channel and its sedimentary infill. Such re-evaluation may provide a valuable new insight into former ice margin positions, modes of glacier retreat, and the role of high magnitude floods within the sedimentary record of former proglacial areas. This study therefore improves our understanding of the meltwater magnitude and frequency regime of former glaciers.

A new cycle of jokulhlaups at Russell Glacier, Kangerlussuaq, West Greenland

Jokulhlaups in 2007 and 2008 from an ice-dammed lake at the northern margin of Russell Glacier, West Greenland, marked the onset of a renewed jokulhlaup cycle after 20 years of stability. We present a record of successive ice-dammed lake drainage events and associated ice-margin dynamics spanning 25 years. Robust calculations of lake volumes and peak discharges are made, based on intensive field surveys and utilizing high-spatial-resolution orthophotographs of the lake basin and ice margin. These data enable identification of controls on the behaviour of the ice-dammed lake and provide the first field-based examination of controls on jokulhlaup magnitude and frequency for this system. We find that Russell Glacier jokulhlaups have a much higher peak discharge than predicted by the Clague-Mathews relationship, which we attribute to an unusually short englacial/subglacial routeway and the presence of a thin ice dam that permits incomplete sealing of jokulhlaup conduits between lake drainage events. Additionally, we demonstrate that the passage of jokulhlaups through an interlinked system of proglacial bedrock basins produces significant attenuation of peak discharge downstream. We highlight that improved understanding of jokulhlaup dynamics requires accurate information about ice-dammed lake volume and ice-proximal jokulhlaup discharge.

Glacier surging as a control on the development of proglacial, fluvial landforms and deposits, Skeiðarársandur, Iceland

Abstract Glacier-hydrological processes are one of the main factors controlling proglacial fluvial systems. It has been proposed that where jokulhlaups occur they play a dominant role in the evolution of proglacial outwash plains. However, extraordinary meltwater and sediment discharge associated with glacier surging can also play a crucial role in the proglacial system. The interplay of surge-related and jokulhlaup floods has been investigated at Skeiðararjokull, a jokulhlaup type-site where surging is also known to occur, allowing the geomorphological and sedimentological effects of these events to be differentiated. Skeiðararsandur contains a spectacular assemblage of landforms and deposits associated with the 1991 surge of Skeiðararjokull. The impact of the 1991 surge was felt mainly on the western half of the glacier where the ice advanced up to 1 km between September and November. The surge limit is marked by a push-moraine complex up to 5 m in height and 10 m in breadth. Proglacial fluvial sediments were deposited as a series of outwash fans adjacent to the glacier, up to 400 m in diameter, as the glacier advanced during the surge. Glaciotectonic structures associated with ice pushing inter-finger with undisturbed proglacial fluvial fan sediments, constraining timing of deposition of proglacial fans to the period during and immediately following the glacier surge. The study of landforms and sedimentary successions associated with the 1991 surge provides an excellent modern analogue for larger-scale push moraines and proglacial fans on Skeiðararsandur, which are related to similar processes. Surge-related outflows operating over timescales of months–years, together with jokulhlaup flows play a major role in the creation of distinctive proglacial fluvial landforms and deposits. Examination of the sedimentary and landform records of areas presently subject to surging will allow the development of models which can be used to differentiate glacier surging from rapid glacier response to abrupt climate change.

A comparison of energy balance calculations, measured ablation and meltwater runoff near Søndre Strømfjord, West Greenland

Abstract This paper presents a comparison of measured ablation and calculated ablation based on an energy balance approach. The meteorological measurements for this study were conducted on the ice sheet in the Sondre Stromfjord area, West Greenland, during the 1990 and 1991 melt seasons as a part of the Greenland Ice Margin Experiment (GIMEX). The calculated ablation was used to force a model to predict proglacial meltwater runoff in the same area. Results are presented from a comparison between calculated and measured ablation as well as a comparison between measured and modelled discharge.

GPR derived architecture of November 1996 jökulhlaup deposits, Skeiðarársandur, Iceland

Abstract Skeiðarársandur in southeastern Iceland, with an area of > 1000 km2, is the world’s largest active proglacial outwash plain. In July–August 2000, a total of over 10 km of ground penetrating radar (GPR) profile data (at 50 MHz and 100 MHz) was collected from a variety of proglacial outwash sediments across the Gígjukvísl channel region of the Skeiðarársandur plain. GPR-profile results and their corresponding facies interpretations are presented for the flood deposits of a single supraglacial outwash fan and its associated source-proximal ice-walled canyon created entirely by the November 1996 jökulhlaup event. By combining the GPR data with ground surveying, photogrammetry and detailed sedimentary outcrop evidence, this study adds a new perspective to the large-scale analysis of single, high-magnitude flood events and the sedimentary record of former, ice-proximal outwash plains. The GPR derived architectures point to a higher degree of sediment reworking than predicted by previous sedimentary models and may provide a useful analogue for the study of sedimentation within similar bedrock fluvial and alluvial fan feeder systems.

Subglacial jökulhlaup deposition, Jotunheimen, Norway

Abstract The aim of this paper is to discuss the origin of a linear boulder deposit near the Mjolkedalsbreen, Jotunheimen, Norway. Subglacial fluvial deposition is suggested by palaeoflow indicators showing flow directions uphill along the axis of a ridge-like deposit and the close association of this deposit with an ice-contact boulder delta. Peak discharges of repeated jokulhlaups from the former ice-dammed Ovre Mjolkedalsvatn were estimated at 1188 m3 s-1 using evidence derived from the proglacial jokulhlaup routeway. Former ice-tunnel dimensions and flow conditions associated with the ridge-like deposit were derived from a simple pipe-flow equation. Flow velocities of 9.8–11.6 m s-1 and shear stresses of 1440–2018 N m2 were reconstructed for a tunnel less than 3 m in average height and 45 m wide. Such flows would have been capable of transporting boulders of greater than 3 m in diameter as traction load and material of gravel-cobble size in suspension. Material of this size has been deposited downflow as an ice-contact delta. Linear ridges of boulder gravel with up-slope gradients found in association with ice-contact deposits in other formerly glaciated areas may similarly be of jokulhlaup origin.

The sedimentary architecture of outburst flood eskers: A comparison of ground-penetrating radar data from Bering Glacier, Alaska and Skeiðarárjökull, Iceland

We present ground-penetrating radar (GPR) profiles that reveal the sedimentary architecture of an esker deposited during a surge-associated outburst flood at the Bering Glacier, Alaska. The wide, up-flow end of the esker contains a transition from large backset beds to large foreset beds interpreted to reflect composite macroform development in an enlarged part of the conduit. By contrast, the narrow, down-flow portion of the esker is dominated by plane beds interpreted to have been deposited where the conduit was constricted and the flow was faster. A previously studied outburst esker at Skeidararjokull, Iceland, has a similar morphology and stratigraphic architecture. This suggests that outburst floods generate distinct depositional signatures in eskers, both in terms of morphology and sedimentary architecture. Identification of these distinct signatures in ancient eskers will help assess the paleohydraulic conditions under which ancient eskers formed and, by extension, the nature of meltwater drainage systems beneath the Laurentide and Eurasian ice sheets.