Meltwater flow through a rapidly deglaciating glacier and foreland catchment system

Virkisjökull is a rapidly retreating glacier in south-east Iceland. A proglacial lake has formed in the last ten years underlain by buried ice. In this study we estimate water velocities through the glacier, proglacial foreland and proglacial river using tracer tests and continuous meltwater ﬂ ow measurements. Tracer testing from a glacial moulin to the glacier outlet in September 2013 demonstrated a rapid velocity of 0.58 m s (cid:1) 1 . This was comparable to the velocity within the proglacial river, also estimated from tracer testing. A subsequent tracer test from the same glacial moulin under low ﬂ ow conditions in May 2014 demonstrated a slower velocity of 0.07 m s (cid:1) 1 . The glacier outlet river sinks back into the buried ice, and a tracer test from this sink point through the proglacial foreland to the meltwater river beyond the lake indicated a velocity of 0.03 m s (cid:1) 1 , suggesting that an ice conduit system within the buried ice is transferring water rapidly beneath the lake. Ground penetrating radar pro ﬁ les con ﬁ rm the presence of this buried conduit system. This study provides an example of rapid deglaciation being associated with extensive conduit systems that enable rapid meltwater transfer from glaciers through the proglacial area to meltwater rivers.


INTRODUCTION
. The IPCC () reports that increased glacier melt is resulting in changes to proglacial river systems with consequences for the management of water resources and hazards. In some catchments, an overall increase in river discharge has been observed due to the increase in meltwater volume (Björnsson & Pálsson ; Nolin et al. ). In particular, there is likely to be an increase in winter flows due to increased winter temperatures (Fountain & Tangborn ), and predictions have been made of a transition from ephemeral to perennial river flows ( Jóhannesson et al. ). Deglaciation also causes other changes in proglacial forelands. Buried ice may be left behind as glaciers retreat (French & Harry ; Evans & England ; Everest & Bradwell ). Proglacial lakes may also be rapidly formed and disappear (Kirkbride ; Ageta & Iwata

; Bennett & Evans ).
Virkisjökull is a rapidly deglaciating maritime glacier in south-east Iceland. Sequential field photographs and annual moraines show that there has been substantial retreat over the period 2008-2013 averaging 35 m a À1 (Bradwell et al. ). The glacier margin has retreated nearly 500 m since 1996, and there has been a decrease in the glacier surface elevation of 8 m a À1 in the lowest reaches since 2012. The rate of retreat is accelerating, with an increase from 14 m a À1 of retreat between 1990 and 2004 to 33 m a À1 of retreat between 2005 and 2011 (Bradwell et al. ). The proglacial foreland is changing in response to rapid deglaciation, and is characterised by extensive areas of buried ice and a growing proglacial meltwater lake, formed within the last ten years.
The aim of this study was to use tracer tests, river discharge measurements, and ground penetrating radar (GPR) to characterise the glacial and proglacial hydrology of this rapidly deglaciating system, and to determine the water velocity through the glacier and proglacial area. measuring mass balance (Flett ). The glacier is drained by the river Virkisá which has a catchment area of approximately 31 km 2 and extends southwestwards from the summit crater of Öraefajökull. There is a discrete river discharging from the terminus on the east side of the glacier (Point 3 on Figure 1), but on the west side there is no obvious channelised discharge.

Site description and overview of testing
Tracer tests were undertaken from glacial moulins on the east and west arms of the glacier (Points 1 and 2 on Beyond the terminus, the proglacial foreland is characterised by an area of buried ice. Daily photographs show that during typical flows, the river discharging from the terminus (Point 3 on Figure 1) flows intermittently across the proglacial foreland surface for approximately 50 m before it sinks into the buried ice via a collapse feature. This river does not resurface but collapse features in the sediment overlying the ice are prevalent. To the west the buried ice is overlain by a lake (Figure 1). The lake area is complex, with debris piles forming islands. Unstable ground prevents access to some areas of the lake shore. On the far side of the lake, an outlet feeds the proglacial river.
At the lake outlet, the proglacial area is geologically constrained by low permeability bedrock, therefore, the majority of melt water discharges through this area. A tracer test was undertaken to measure the transit time through the proglacial area. GPR surveys were undertaken prior to this study in the proglacial area to investigate the collapse of the glacier margin (Phillips et al. ). They are used here to determine whether conduits are present within the buried ice.

FIELD METHODS
Proglacial river discharge measurements River discharge was measured over three years at an automatic gauging station, 2.92 km downstream of the lake outlet (MacDonald et al. ; Point 6 on Figure 1). This was to determine temporal changes in meltwater discharge, and establish whether melting occurs on a perennial basis.
These data are also used to show river conditions during the tracer tests.
Water level was monitored continuously using submersible level transmitters attached to the adjacent road bridge.
An Ott Kalesto V surface velocity sensor was also deployed to indicate when changes in channel morphology were likely to have affected the stage-discharge rating. Velocities As ice affects the stage-discharge relationship, photographs were taken three times daily using an automated system, at 9:00, 12:00 and 15:00, to identify ice development in the channel. Periods when ice was present in the channel or around the banks were removed from the discharge record.

Tracer tests
In all tracer tests a 40% sodium fluorescein or rhodamine WT dye solution was used. Sodium fluorescein has a high     Velocity is calculated using the distance and time to peak.  A marked zone of muted or absent reflections was observed in a number of profiles where they crossed a distinct linear zone that had been particularly affected by collapse holes.

River discharge
The exact reason for poor reflection in this zone is not known, but it may be related to a turbulent subterranean river that was observed sinking underground in this zone at the time of survey (Figure 7(a)). Field observations indicate that kettle holes and collapse features in the proglacial zone intercept a freely draining system as meltwater from the terminus is regularly redirected into one of these features. Collectively, the radar data and observations from the proglacial area demonstrate the presence of an extensive mass of ice buried, with numerous conduits and voids, beneath the outwash sands and gravels.   This may be because flows in moulins are so much smaller than those in the main conduit system, therefore they are more affected by pooling and debris effects. The tracer test through this area also suggests that drainage is via a conduit system. Water emerging from the Virkisjökull glacier terminus rapidly sinks into the buried ice in the foreland via a large kettle hole and flows through a conduit system in the buried ice to re-emerge within the proglacial lake. The presence of dye tracer specifically on the western side of the lake outlet channel, and not on the eastern side, suggests that discharge occurred at a localised point rather than in a dispersed manner, and that once meltwater emerged from the conduit system into the lake, dispersion within the lake was minimal. If there was a substantial reduction in flow within the lake the tracer would have become too dispersed and diluted through the lake area to be able to detect it at the outlet. This was supported by visual observations of a fast flowing channel that was visible, within the lake, and on the east side which seemed to supply the east side of the outlet channel, where the fluorometer was stationed (Point 5 on Figure 1). It seems likely that at the time of the tracer test there was a conduit within the buried ice which discharged beneath the lake.

DISCUSSION
Although it is unclear how permanent this drainage configuration is, its location in a stagnant area of buried ice, the continuous yearly meltwater supply and GPR profiles suggest that it could be a feature that is exploited for melt water flow throughout the year.
Tracer injected into moulins on the western side of the glacier was not detected in the glacier terminus outlet or at the lake detection points. The flow of water observed in this terminus outlet stream (Point 3 on Figure 1) is substantially less than the flow of water from the lake (Point 4/5 on Figure 1). This suggests that the drainage from the western arm of the glacier may be connected to the proglacial area through a different route. It is possible that this meltwater discharged in a dispersed manner into the lake, diluting the dye to below detection at the lake outlet. However, the injection quantity was much smaller than that in the successful proglacial test which resulted in relatively low tracer concentrations, so even if tracer was discharged into the lake at a specific point, it could have been diluted to below the detection threshold.
The GPR and dye tracing results from the proglacial lake suggest that there is a conduit within buried ice in the proglacial area. This conduit may be the remains of the original subglacial conduit that has been buried in the foreland after terminus retreat. A simplified conceptual model of the evolution of the proglacial area is presented in Figure 8.
Ice is buried by the accumulation of debris transported from higher reaches (where ice is still flowing as a result of the steep gradient of the ice fall) (Figure 8(1)). The remains of active meltwater channels, that exploit planes of weakness in the ice, begin to collapse back due to being covered by only a thin layer of ice and sediment. This exposes the water moving through the area (Figure 8(2)) and a lake begins to form where ponding of water and the formation of surface pools occur (Figure 8(3)). The current proglacial region has a surface river that sinks back below buried ice into a conduit that was formerly connected to the active glacier system. This conduit connects to the lake that formed as a result of the collapse and decay of the ice on the far side of the proglacial area (Figure 8(4)). Rapidly deglaciating catchments such as Virkisjökull create a proglacial setting which is transitional, resulting in an extensive system of buried ice containing the relic conduits of the former ablation zone through which meltwater can be transferred rapidly to the river. Tracer testing and GPR at Virkisjökull have shown that despite the presence of a large lake, meltwater is rapidly transported through the proglacial area to the river. Buried ice in proglacial forelands is likely to become more common as a result of deglaciation, and understanding the hydrology of these areas is important to enable appropriate catchment modelling and hazard mitigation. the terminus of the glacier that is subsequently buried by the accumulation of debris transported by the active glacier margin. Meltwater is input into this system from conduits that remain active in the stagnant ice (1). The remains of these active conduits within the buried ice will then begin to collapse to expose water moving through the proglacial buried-ice area (2). This process allows the formation of a proglacial lake that sits upon ice as the active glacier margin continues to retreat (3). The unmoving buried ice is insulated from rapid melting by the accumulation of debris. In the final stage (currently observed at Virkisjökull), the collapse of the active ice margin has exposed an englacial conduit.

CONCLUSION
The meltwater, rather than flowing across the surface, exploits a collapse feature within the foreland to sink back into the conduit system within the buried ice to resurface within the newly formed proglacial lake system (4).