We provide the first synoptic view of the drainage system of a Himalayan debris-covered glacier and its evolution through time, based on speleological exploration and satellite image analysis of Ngozumpa Glacier, Nepal. The drainage system has several linked components: (1) a seasonal subglacial drainage system below the upper ablation zone; (2) supraglacial channels, allowing efficient meltwater transport across parts of the upper ablation zone; (3) sub-marginal channels, allowing long-distance transport of meltwater; (4) perched ponds, which intermittently store meltwater prior to evacuation via the englacial drainage system; (5) englacial cut-and-closure conduits, which may undergo repeated cycles of abandonment and reactivation; and (6) a “base-level” lake system (Spillway Lake) dammed behind the terminal moraine. The distribution and relative importance of these elements has evolved through time, in response to sustained negative mass balance. The area occupied by perched ponds has expanded upglacier at the expense of supraglacial channels, and Spillway Lake has grown as more of the glacier surface ablates to base level. Subsurface processes play a governing role in creating, maintaining, and shutting down exposures of ice at the glacier surface, with a major impact on spatial patterns and rates of surface mass loss. Comparison of our results with observations on other glaciers indicate that englacial drainage systems play a key role in the response of debris-covered glaciers to sustained periods of negative mass balance.
Debris-covered glaciers in many parts of the Himalaya have
undergone significant surface lowering in recent times (Kääb et
al., 2012), with net losses of several tens of metres since the 1970s (Bolch
et al., 2008a, 2011). Glacier thinning and reduced surface gradients have
resulted in lower driving stresses and ice velocities, and large parts of
many glaciers are now stagnant or nearly so (Bolch et al., 2008b; Quincey et
al., 2009). These morphological and dynamic changes have encouraged formation
of supraglacial ponds and lakes and increased water storage within glacial
hydrological systems (Quincey et al., 2007; Benn et al., 2012). Where lakes
form behind dams of moraine and ice, volumes of stored water can be as high
as
Several studies have shown that the development and enlargement of englacial conduits play an important role in the evolution of debris-covered glaciers during periods of negative mass balance (e.g. Clayton, 1964; Kirkbride, 1993; Krüger, 1994; Benn et al., 2001, 2009, 2012; Gulley and Benn, 2007; Thompson et al., 2016). The collapse of conduit roofs can expose areas of bare ice at the glacier surface, locally increasing ablation rates. Additionally, areas of subsidence associated with englacial conduits create closed hollows (dolines) that can evolve into supraglacial ponds, further increasing ice losses by calving. Conversely, supraglacial ponds can drain if a connection is made with the englacial drainage system, provided the pond is elevated above hydrological base level (“perched lakes” in the terminology of Benn et al., 2001, 2012). Drainage of relatively warm water through the glacier leads to conduit enlargement, which in turn increases the likelihood of roof collapse, surface subsidence, and ultimately new pond formation (Sakai et al., 2000; Miles et al., 2015). Because ablation rates around supraglacial pond margins are typically 1 or 2 orders of magnitude higher than those under continuous surface debris, ponds contribute disproportionately to overall rates of glacier ablation (Sakai et al., 1998, 2000, 2009; Thompson et al., 2016). By controlling the location and frequency of surface subsidence and pond drainage events, englacial conduits strongly influence overall ablation rates and the volume of water that can be stored in and on the glacier (Benn et al., 2012).
Speleological investigations in debris-covered glaciers in the Khumbu Himal have demonstrated that englacial conduits can form by three processes: (1) “cut and closure”, or the incision of supraglacial stream beds followed by roof closure; (2) hydrologically assisted crevasse propagation, or hydrofracturing, which may route water to glacier beds; and (3) exploitation of secondary permeability in the ice (Gulley et al., 2009a, b; Benn et al., 2012). The relative importance of these processes in the development of glacial drainage systems, however, has not been investigated in detail. Furthermore, there are no data on the large-scale structure of englacial and subglacial glacial drainage systems in the Himalaya or on how they evolve during periods of negative mass balance. In this paper, we investigate the origin, configuration and evolution of the drainage system of Ngozumpa Glacier, using three complementary methods. First, speleological surveys of englacial conduits are used to provide a detailed understanding of their formation and evolution. Second, historical satellite imagery and high-resolution digital elevation models (DEMs) are used to identify past and present drainage pathways, glacier-wide patterns of surface water storage and release, and regions of subsidence. Finally, feature tracking on TerraSAR-X imagery is used to detect regions of the glacier subject to seasonal velocity fluctuations, as a proxy for variations in subglacial water storage. Taken together, these methods provide the first synoptic view of the drainage system of a large Himalayan debris-covered glacier and its influence on glacier response to recent warming.
Ngozumpa Glacier, showing the location of the three branches and Spillway Lake. Image: orthorectified GeoEye-1 from December 2012.
Ngozumpa Glacier is located in the upper Kosi River catchment, Khumbu Himal, Nepal (Fig. 1). It has three confluent
branches: a western (W) branch flowing from the flanks of Cho Oyu
(8188
The lower ablation zone of the glacier is effectively stagnant, with little
or no detectable motion on most of the E branch, or on the main trunk for
The lower tongue of the glacier has a concave surface profile, with the
overall gradient declining from 5.8 to 2.4
Longitudinal surface profile of the W branch and main trunk of Ngozumpa Glacier, showing downglacier changes in gradient and relative relief (see Fig. 3a for location). “Upper channeled”, “transitional”, and “hummocky perched lake” refer to the drainage zones described in Sects. 4.2 and 4.3.
We surveyed 2.3
Satellite imagery used in the paper.
A range of optical imagery was used to map indicators of the large-scale structure of the drainage system (Table 1). The location of supraglacial channels and ephemeral supraglacial ponds was mapped using declassified Corona KH-4 (1965), Landsat 5 Thematic Mapper (TM, 2009), GeoEye-1 (9 June 2010 and 23 December 2012), and WorldView-3 (5 January 2015) imagery. The Corona and Landsat imagery was not co-registered or orthorectified beyond the standard terrain correction of the product and was used to identify the presence/absence of larger ponds or channels, not to quantify rates of change.
Examples of surface topography, supraglacial meltwater channels, and
englacial conduit locations on Ngozumpa Glacier:
GeoEye-1 imagery from June 2010 and December 2012 and WorldView-3 imagery
from January 2015 were acquired for a region covering 17.4
The 2010 DEM was used to define the extent of individual surface drainage basins on the glacier surface. This was achieved by identifying surface elevation contours that entirely surround other contours of a lesser height. Each supraglacial catchment was then defined by the crest lines of ridges that separate the closed basins. Initially, we used 2 m contours, but these produced a large number of very small “basins”, due to the high roughness of the bouldery glacier surface. Subsequently, we used 5 m contours that yielded a set of closed basins that closely matched the location of ephemeral supraglacial ponds on the glacier surface. The extent of many basins changed between 2010 and 2015 due to ice-cliff backwasting, although all basins persisted through the period covered by the DEMs. It was not possible to delineate basins on the historical Corona or Landsat imagery because our methods depend on the availability of DEMs and cannot be applied to mono images.
Glacier surface velocities were derived using feature tracking between
synthetic aperture radar (SAR) images acquired by the TerraSAR-X satellite on
19 September 2014, 18 and 29 January 2015, and 5 January 2016. Feature
tracking was done using the method of Luckman et al. (2007),
which searches for a maximum correlation between evenly spaced subsets
(patches) of each image, giving the displacement of glacier surface features
which are converted to speed using time delay between images. Image patches
were
Plan and passage cross sections of englacial conduit NG-04. SB: sediment band; S: suture; V: voids. Dark-grey-filled and white-filled areas within the cave plan indicate the floors of the upper and lower levels, respectively. Pale grey and dashed lines are used where the lower level is occluded by higher false floors. Standing water on the cave floor is shown in blue. For location, see Fig. 3e.
To provide an overview of processes of englacial conduit formation on the glacier, we describe two sites in detail (NG-04 and NG-05) and then briefly describe and reinterpret three previously published sites (NG-01, NG-02, and NG-03; Gulley and Benn, 2007).
Conduit NG-04 (27
The main passage consisted of an upper level with a flat or gently inclined floor and a lower narrow incised canyon. The passage was highly sinuous, with a sinuosity in the surveyed reach of 5.52. Near A4 (Fig. 4), there was a tight cut-off meander loop off the main passage (Fig. 5a). The base of the abandoned loop had a flat floor and lacked the incised lower level that was present elsewhere in the system. The upper floor level could also be traced along the walls of side passages B and C, which we interpret as twin remnants of a second meander cut-off. The floor of the upper level sloped gently downward from A1 to A14, rose from there to between A18 and A19, and then descended once more. Sandy bedforms on the floor and scallops on the ice walls of this upper level indicate that water flow was from A1 towards A21.
Passage morphology on the upper level was very variable, including tubular, box-shaped, triangular, and irregular sections (Figs. 4 and 5b–d). Throughout most of the system, planar structures were visible in the ceiling or walls of the upper level, running parallel to the passage axis with variable inclination. The structures took the form of (1) “sutures” at the line of contact between opposing walls (S; Fig. 4; Fig. 5b, c), (2) intermittent narrow voids (V; Fig. 4, Fig. 5c), and (3) bands of sorted sand or gravel a few centimetres thick (SB; Fig. 4, Fig. 5d). Some of the voids increased in width inward, in some cases opening out into gaps tens of centimetres across. In some places, bands of sorted sediment could be traced laterally into open voids or sutures. At several points along the main passage, a pair of planar structures occurred on opposite walls of the passage. Side passage B had a narrow, meandering seam of dirty ice running along its ceiling, and in passage C the walls tapered upward to meet at a ceiling suture.
The floor of the incised lower level in both parts of the main passage sloped down towards side passages B and C (arrows, Fig. 4). A pair of incised channels was confluent at C1, whereas a single incised channel was present in passage B, where its lower (western) end was blocked by an accumulation of ice and debris.
Passage morphology in NG-04.
The partially debris-filled structures in the walls and ceiling of the upper
level are closely similar to many examples of canyon sutures we have observed
in cut-and-closure conduits in the Himalaya and Svalbard, marking the planes
of closure where former passage walls have been brought together by ice creep
and/or blocked by ice and debris (cf. Gulley et al., 2009a, b).
Cut-and-closure conduits are typically highly sinuous and have variable
cross-sectional morphologies, ranging from simple
The tubular and box-shaped cross profiles and undulating long profile of the upper passage are consistent with fluvial erosion under pipe-full or phreatic conditions (cf. Gulley et al., 2009b). This contrasts with the canyon-like form and consistent down-flow slope of channels incised under atmospheric (vadose) conditions, typical of simple cut-and-closure conduits. The dimensions of the upper passage (typically 2 m high and 3 m wide) are consistent with high discharges. We conclude that the upper passage formed when water draining from a supraglacial pond in basin C-63 exploited the remnants of an abandoned cut-and-closure conduit (Fig. 3e).
Following formation of the upper passage, the lower level was incised under vadose (non-pipe-full) conditions when the system accessed a new local base level via side passages B and C. We infer that this occurred when a cut-off meander loop between B1 and C2 was exposed by ice-cliff retreat in basin W-6. Water flow between A1 and B2 continued in the same direction as before, but between A14 and A21 flow was reversed and discharge much reduced.
Evolution of conduit NG-04 can be summarized as follows: (1) a cut-and-closure conduit was formed by incision of a supraglacial stream; (2) this conduit was abandoned and almost completely closed, presumably after it lost all or most of its source of recharge following downwasting of the overlying glacier surface; (3) the conduit remnants were exploited and enlarged by water draining from a supraglacial pond in basin C-63; and (4) surface ablation in basin W-6 broke into the conduit, creating a new base level and initiating floor incision. This remarkable cave illustrates how relict drainage systems can be reactivated when connected to new sources of recharge and demonstrates how patterns of drainage can change dramatically within a single system in response to changing surface topography.
Plan and passage cross sections of conduit NG-05. The plan view of the upper level shows boulders and an incised channel on the conduit floor. For location see Fig. 3a.
In December 2009 a conduit portal was exposed in an ice cliff at the margin
of Spillway Lake (27
Although short, this passage is important for understanding the drainage system of Ngozumpa Glacier. The debris band and suture in the roof indicate that, like NG-04, the passage formed by a process of channel incision and roof closure. Additionally, the passage is graded to the seasonally fluctuating surface of Spillway Lake. We therefore conclude that the main drainage on the eastern side of the glacier consists of a cut-and-closure conduit graded to the hydrologic base level of the glacier. For several kilometres upglacier of the portal, the debris-covered ice surface is highly irregular and broken into numerous closed basins, implying that the conduit evolved from a surface stream that predates significant downwasting of the glacier. The significance of these conclusions will be discussed later in the paper.
NG-01, NG-02, and NG-03 (Fig. 3d) were mapped in December 2005 and described
by Gulley and Benn (2007). NG-01 had carried water southward into a large
basin on the glacier surface (basin C-25, Fig. 10a), whereas NG-02 drained
water southward out of the basin. NG-01 (27
Surface velocities derived from TerraSAR-X data:
For much of their length, all three conduits follow the trend of debris bands in the walls or roof, leading Gulley and Benn (2007) to conclude that all were structurally controlled. The debris bands were originally interpreted as debris-filled crevasse traces that had been deformed during advection downglacier. When the original work was conducted, the cut-and-closure model had not been developed, and we had yet to learn how to recognize the diverse forms such conduits can take, especially in the later stages of their development. It is now apparent that these conduits have all the hallmarks of cut-and-closure conduits. The continuity and sinuous planform of the debris bands are consistent with formation by the closure of incised canyons, rather than crevasse fills that had been deformed by ice flow. Crevasses in the upper part of the glacier ablation area tend to be short, discontinuous, and oriented transverse to flow, unlike the observed debris bands in the conduit roofs, and ice deformation is unlikely to be capable of generating the highly sinuous patterns observed within the conduit debris bands.
We therefore reinterpret NG-01–NG-03 as cut-and-closure conduits that have undergone cycles of incision, abandonment, partial closure, and later reactivation in response to fluctuating patterns of recharge on the glacier surface. The circular and elliptical cross profiles observed in NG-02 and NG-03 are consistent with phases of phreatic passage enlargement, analogous to that in NG-04. Abandoned, incompletely closed conduits create hydraulically efficient flow paths, which can be readily exploited and enlarged when surface ablation brings them into contact with new sources of recharge.
In this section, we present evidence for the large-scale structure of the drainage system and patterns of water storage and release, using X-band radar and optical satellite imagery and high-resolution DEMs from 2010, 2012, and 2015.
Direct observation of the subglacial drainage system was not possible.
Instead, we use seasonal fluctuations in glacier surface velocity to infer
areas subject to variable subglacial water storage. Mean daily ice velocities
of the glacier between 29 January 2015 and 5 January 2016 are shown in
Fig. 8a. There is no detectable motion (i.e. greater than
Evidence for seasonal velocity fluctuations is shown in Fig. 8b, which shows
mean daily velocities between 29 January 2015 and 5 January 2016 (341 days)
minus mean daily velocities from 19 September 2014 to 18 January 2015
(111 days). Meteorological data from the Pyramid Weather Station, at
5050
The seasonal variations in ice velocities in the upper ablation zone are too
large to be explained by changes in ice creep rates, which would require
fluctuations in driving stress that are inconsistent with the observed
surface elevation changes on the glacier (Thompson et al., 2016). We
interpret the velocity data as evidence for variations in basal motion
(sliding and/or subglacial till deformation) in response to changing
subglacial water storage. This interpretation is supported by the spatial
distribution of areas affected by the seasonal speed-up, which coincide with,
or occur downglacier of, heavily crevassed ice. Much of the upper ablation
area of Ngozumpa Glacier consists of icefalls with surface gradients up to
30
Supraglacial stream networks are visible below the crevassed zones on all
three branches of the glacier. The most extensive network occurs on the
tongue of clean ice on the NE branch, where a set of sub-parallel channels
descends from
Distribution of crevasses on the W and NE branches of Ngozumpa
Glacier.
Surface drainage basins and lake area changes:
Extract from the 2015 DEM and selected cross profiles in 2010, 2012, and 2015, showing lateral troughs, subsidence of trough floors, and erosion of moraine slopes.
Changing pond extent in basin E-11, showing evidence of filling and drainage cycles. Pond outlines highlighted in blue.
Perennial supraglacial channels can only persist if the annual amount of channel incision exceeds the amount of surface lowering of the adjacent ice (Gulley et al., 2009b). The rate at which ice-floored channels incise is controlled by viscous heat dissipation associated with turbulent flow and increases with discharge and surface slope (Fountain and Walder, 1998; Jarosch and Gudmundsson, 2012). Because supraglacial stream discharge is a function of surface melt rate and melt area, significant channel incision requires large catchment areas. Therefore, incised surface channels tend to occur only where potential catchments are not fragmented by crevasses or hummocky surface topography (Fig. 3). At present, these conditions are met in relatively limited areas of Ngozumpa Glacier, below crevassed areas and above hummocky debris-covered areas.
Most of the lower ablation zone of the glacier (below
Of the 70 basins in the central part of the glacier, 56 (80 %) contained ponds in at least 1 of the 3 years covered by the GeoEye and WorldView imagery. Fifteen of the 42 ponds present in 2010 (36 %) had disappeared by 2012 or 2015, whereas 14 basins that were empty in 2010 contained ponds in 1 or more of the later years. Almost all of the remainder underwent partial drainage and/or refilling. In contrast, the five ponds in the terminal zone of the glacier (below Spillway Lake) clearly exhibited stability. Four showed no significant change in area between 2010 and 2015, while the other showed an increase in area.
Observations on and below the glacier surface show that drainage of perched ponds occurs when part of the floor is brought into contact with permeable structures in the ice (Benn et al., 2001; Gulley and Benn, 2007). The characteristics of NG-01–NG-05 (which all occur within the hummocky debris-covered zone) show that relict cut-and-closure conduits are the dominant cause of secondary permeability in the glacier, providing pre-existing lines of weakness along which perched ponds can drain.
The spatial extent and high temporal frequency of perched pond drainage
events on the glacier (Fig. 10a) imply a high density of active or relict
conduits within the ice. A rough estimate can be obtained by dividing the
number of complete and partial drainage events (35) by the total area of
basins in the central part of the glacier (4.62
The stability of ponds in the terminal zone probably reflects a combination of factors. These ponds are flanked by stable slopes of thick debris, which inhibit pond growth by melt or calving. Furthermore, the ponds are located at or close to the hydrologic base level of the glacier, determined by the terminal moraine that encircles the glacier terminus, inhibiting drainage via relict conduits.
Elevation differences between successive DEMs indicate linear zones of
enhanced surface lowering along both margins of Ngozumpa Glacier, forming
troughs along the base of the bounding lateral moraines (Thompson et
al., 2016; Fig. 11). The inner moraine slopes consist of unvegetated,
unconsolidated till and undergo active erosion by a range of processes,
including rockfall, debris flow, and rotational landslipping (Benn et
al., 2012; Thompson et al., 2016). Although debris eroded from the moraine
slopes is transferred downslope into the troughs, the troughs underwent
surface lowering of 6–9
The lateral troughs form a series of closed basins, 12 on the west side and 22 on the east (Fig. 10b). Eight of the basins in the west trough and 17 of those in the east contained a pond in 2010, 4 (W) and 7 (E) of which had completely drained by 2012 or 2015. Four new ponds appeared in the eastern trough in 2012 or 2015, and 1 (W) and 7 (E) underwent partial drainage and/or refilling. Three basins on the western side and one on the eastern side showed no fluctuations in pond area.
Benn et al. (2001) provided detailed descriptions of pond filling and
drainage cycles in basins W-7 and W-5 (lakes 7092 and 7093, respectively, in their
terminology). In October 1998, basin W-7 contained three shallow ponds, but
by October 1999 the basin was occupied by a single large pond, and water level
had risen by
Short-term cycles of pond drainage and filling can also be demonstrated in other basins within the lateral trough systems using optical satellite imagery. Figure 12 shows a series of images of the east side of the glacier close to the junction with the E branch, where a supraglacial stream (Sect. 4.2) flows into a closed depression in basin E-11 (Fig. 10b). A pond occupying the basin expanded in area between March and May 2009 but drained between June and August. In 2015 there is little evidence of the pond in January, but a large pond is present in June.
Widespread, rapid subsidence along both margins of the glacier can be
explained by enlargement and episodic collapse of sub-marginal conduits
(Thompson et al., 2016). Potential internal ablation rates were calculated
from energy losses associated with runoff and supraglacial pond drainage, and
the resulting value of 0.12 to
The sub-marginal conduits are perennial features of the glacier drainage system and discharge water into Spillway Lake during the winter months. Winter discharge may partly reflect slow release of water from supraglacial and englacial storage, but it may also partly consist of subglacial water from the upper ablation zone (see Sect. 4.1). This hints at the possibility that the sub-marginal channels function as the downglacier continuations of the subglacial drainage system, in addition to carrying water transferred more directly from the glacier surface.
Much of the lower ablation zone appears to be bypassed by the sub-marginal conduits, as evidenced by widespread water storage in supraglacial ponds (Sect. 4.3). As noted above, water is intermittently discharged from ponds in the central part of the glacier into the lateral troughs via englacial conduits. Cycles of pond drainage and filling in lateral basins indicate intermittent connections between surface catchments and the sub-marginal meltwater channels (Fig. 10b). In some cases, drainage events can be directly attributed to exploitation of englacial conduits (Benn et al., 2001). The hourly changes in pond level recorded by Horodyskyj (2015) cannot be explained by conduit opening and blockage, and they more likely reflect short-term fluctuations in recharge from surface melt and water release from storage.
Spillway Lake, 1965–2009, showing the position of meltwater portals and upwellings and the inferred location of former englacial conduits (dashed lines). Background image: GeoEye-1 from June 2010. See text for explanation of lake evolution.
Zonation of the drainage system in
Evolution of the eastern margin of the main trunk of Ngozumpa
Glacier, 1964–2015.
In 2010, the area of the Spillway Lake surface catchment was
0.8
The evidence presented above demonstrates that the drainage system of
Ngozumpa Glacier comprises six linked elements: (1) a seasonal subglacial
drainage system below the upper ablation zone; (2) supraglacial channels,
allowing efficient meltwater transport across parts of the upper ablation
zone; (3) sub-marginal channels, allowing long-distance transport of
meltwater; (4) perched ponds, which intermittently store meltwater prior to
evacuation via the englacial drainage system; (5) englacial cut-and-closure
conduits, which may undergo repeated cycles of abandonment and reactivation;
and (6) a base-level lake system (Spillway Lake) dammed behind the terminal
moraine. These elements have a distinct spatial distribution (Fig. 14a).
Evidence for seasonal subglacial water storage is restricted to active parts
of the glacier downglacier of crevasse fields, where surface water can be
routed to the bed. Supraglacial channels occur where surface catchments and
discharge are large enough to allow channel incision rates to outpace surface
ablation rates. Thus, perennial channels only occur where the glacier surface
is not broken up by crevasse fields or into small, closed basins. Perched
ponds occur where the glacier surface is broken up into closed basins, where
the overall gradient of the glacier is
In this section, we present evidence for changes in drainage system structure through time, including features visible in Corona images from 1964 and 1965, speleological observations, and repeat surveys of Spillway Lake since 1999.
In 1964, a connected supraglacial drainage stream network was present on the
eastern side of the main trunk above the junction with the E branch
(10–8
We hypothesize that the supraglacial channels became deeply incised and transitioned into cut-and-closure conduits, which continue to evacuate meltwater below the glacier margins despite fragmentation of the surface topography. Channel incision may have been encouraged by thickening debris cover (from melt-out of englacial debris) that would have reduced glacier surface lowering rates.
At the distal end of the eastern lateral trough, conduit NG-05 (Fig. 6) emerges into Spillway Lake. Passage morphology indicates that at this point the conduit formed by cut and closure (Sect. 3.2). Thus, there is evidence for a cut-and-closure origin of subsurface conduits at both ends of the eastern lateral trough. We therefore infer that the sub-marginal conduits originated as supraglacial streams that became incised below the surface. Such a scenario would require a continuous slope along both glacier margins. We conclude that supraglacial streams occurred along both margins before development of the current irregular topography, but transition to cut-and-closure conduits allowed these drainage routes to persist after break-up of the glacier surface.
Transition of drainages from supraglacial channels to cut-and-closure conduits appears to have been a widespread process on the glacier. The presence of sutures, planar voids, and bands of sorted sediments in the ceilings and walls of conduits NG-01–NG-05 record former episodes of channel incision. As was the case for the lateral channels, we infer that systems of supraglacial channels existed in the central part of the lower tongue before the glacier surface was broken up into small closed basins (Fig. 14c).
Differential surface ablation can eventually cause fragmentation and abandonment of cut-and-closure conduits, cutting off downstream reaches from former water sources. In abandoned reaches, processes of passage closure dominate over those of enlargement, and systems gradually shut down. Because cut-and-closure conduits are generally located close to the glacier surface, shut-down is commonly incomplete. Zones of narrow voids or sutures with infills of unfrozen sediment may persist, forming meandering lines of high permeability through otherwise impermeable glacier ice.
Reactivation of abandoned conduits will occur if a new water source becomes available, and a conduit remnant connects this source with a region of lower hydraulic potential. These conditions are met on stagnant, low-gradient glacier surfaces. Supraglacial ponds in closed basins provide both reservoirs of water and regions of elevated hydraulic potential. Drainage is highly episodic, and water may be stored in supraglacial ponds for years before passing farther down the system.
On the debris-covered part of Ngozumpa Glacier, cut and closure is the dominant primary process of conduit formation, and active and relict cut-and-closure conduits create a secondary permeability that can be exploited by water from supraglacial ponds. Debris-filled crevasse traces may provide additional lines of weakness in some cases, although this is likely a minor process. We have not observed hydrofracture-type conduits in the debris-covered area of Ngozumpa Glacier, although it is possible that they may form under compressive flow conditions as described on Khumbu Glacier by Benn et al. (2009). Hydrofracturing likely plays a dominant role in surface-to-bed drainage in the crevasse fields of the upper ablation zone.
The recent history of Spillway Lake was discussed in detail by Thompson et
al. (2012, 2016) and is briefly reviewed here. The present spillway through
the SW side of the terminal moraine has been in existence since at least
1965, when water emerged from the glacier and entered a small pond behind the
lateral moraine (1; Fig. 13). In the following decades, the Spillway Lake
system expanded upglacier from this point. On the Survey of Nepal (1996) map
based on aerial photographs taken in 1992, the lake has a ribbon-like form,
extending NE for
The predominantly linear patterns of lake expansion, and the location of meltwater portals and upwellings, indicate that evolution of the Spillway Lake system was strongly preconditioned by the locations of shallow englacial conduits (a, b; Fig. 13). Conduit NG-05 (Sect. 3.4 and Fig. 6) and other examples exposed around the lake margins show that the drainage system consists of cut-and-closure conduits graded to lake level. This near-surface englacial conduit system provided pre-existing lines of weakness in the ice which, when opened up to the surface by internal ablation and collapse, were exploited by ice-cliff melting and calving processes.
Spillway Lake was thus established on a template provided by two englacial conduits (a, b, Fig. 13), which were confluent prior to 1992. As it expanded upglacier, Spillway Lake encroached on areas formerly occupied by perched ponds and incorporated former supraglacial basins. A recent example is basin C-33, which forms an inlier within the Spillway Lake catchment (Figs. 10a and 13). This basin contained a perched pond in 2009 and 2010, but this drained prior to December 2012 and has not reformed. It is likely that this basin will become entirely subsumed within the Spillway Lake catchment in the near future, as a consequence of ice-cliff backwasting.
Comparison of the drainage system structure in 2010 with evidence in Corona imagery from 1964 shows an upglacier expansion of the area occupied by closed depressions and perched ponds, and the formation and upglacier expansion of the base-level Spillway Lake (Fig. 14b). The widespread occurrence of cut-and-closure conduits provides evidence of an even earlier stage in drainage evolution, when supraglacial channels extended along most of the glacier tongue and closed basins were absent or rare (Fig. 14c). The upglacier limit of supraglacial channels was similar in 1964 and 2010, due to the persistent location of crevasse fields in the upper ablation zone. The channels are likely to have had similar upglacier limits in earlier times, because of the strong topographic control of the crevasse fields. Figure 14c shows a hypothetical distribution of supraglacial channels on the glacier during the Little Ice Age and early 20th century.
Ngozumpa Glacier has thus responded to a prolonged period of negative mass balance with a systematic reordering of its drainage system, characterized by less efficient evacuation of meltwater and greater amounts of storage. More recent elements of the drainage system retain a memory of older elements, and processes and patterns of ablation on the glacier continue to be influenced by active and relict channels and conduits. Former supraglacial channels preconditioned the location and density of cut-and-closure conduits, which in turn precondition the formation and drainage of perched ponds and provide templates for the expansion of Spillway Lake.
Observations on other debris-covered glaciers in the Himalaya indicate that their drainage systems share many of the characteristics described in this paper. Seasonal velocity fluctuations have been documented on other large glaciers in the Mount Everest region and on Lirung Glacier, Nepal (Benn et al., 2012; Kraaijenbrink et al., 2016), indicating surface-to-bed drainage and variations in subglacial water storage. Perennial supraglacial channels occur in the upper ablation zones of many glaciers, in places where catchments are not fragmented by crevasse fields or irregular surface topography (Gulley et al., 2009b; Benn et al., 2012). Continuity between a supraglacial channel and an englacial cut-and-closure conduit has been observed on Khumbu Glacier, clearly demonstrating the genetic relationship between the two features (Gulley et al., 2009b). Perched ponds are widespread on Himalayan debris-covered glaciers, and evidence for repeated filling and drainage (Watson et al., 2016; Miles et al., 2017) suggests that englacial conduits may play an important role in their life cycles. However, englacial conduits have only been explored in a few glaciers (Gulley and Benn, 2007; Gulley et al., 2009b; Benn et al., 2009), and much research remains to be done. Similarly, very little is known about possible sub-marginal channels in Himalayan glaciers, and our few attempts to enter these highly dynamic environments have been repulsed.
The upglacier expansion of the area occupied by closed depressions and perched ponds on Ngozumpa Glacier (Fig. 14) also appears to have occurred on other glaciers in the Everest region during the current period of negative mass balance. Iwata et al. (2000) noted an increase in the area occupied by high-relief hummocky topography on Khumbu Glacier from 1978 to 1995. The presence of cut-and-closure conduits below hummocky terrain on that glacier shows that these areas formerly supported supraglacial streams (Gulley et al., 2009b).
There is strong evidence on many glaciers that growth of base-level lakes is preconditioned by englacial conduits. For example, upglacier expansion of the proglacial lake at Tasman Glacier, New Zealand, has repeatedly followed the location of former chains of sink holes on the glacier surface (Kirkbride, 1993; Quincey and Glasser, 2009). Recently formed chains of ponds in the lower ablation zone of Khumbu Glacier strongly suggest that the same process is underway on that glacier (Watson et al., 2016). The integrated picture of drainage system structure and evolution presented in this paper provides a framework for predicting what the future may have in store for other debris-covered glaciers in the region.
This paper has provided the first
synoptic interpretation of the drainage system of a Himalayan debris-covered
glacier, including the spatial distribution of system components, their
evolution through time, and their influence on processes and patterns of
ablation. Our specific conclusions are as follows.
In the upper ablation zone, seasonal variations in ice velocity indicate
routing of surface meltwater to the bed via crevasses and fluctuations in
subglacial water storage. Systems of supraglacial channels occur where the glacier surface is
uninterrupted by crevasses or closed depressions, allowing efficient
evacuation of surface melt. Active sub-marginal channels are evidenced by linear zones of subsidence
along both margins of the glacier and by fluctuations in surface water storage
and release. These channels likely formed from supraglacial channels by a
process of cut and closure, and they permit long-distance transport of meltwater
through the ablation zone. Transport of sediment via the lateral channels
destabilizes inner moraine flanks and delivers debris to the terminal zone,
where it modulates ablation processes. In the lower ablation zone (below Enlargement of englacial conduits removes ice mass that is not captured
by surface observations until conduit collapse occurs, with the implication
that observations of sudden surface lowering need not reflect sudden glacier
mass loss over the same time period. Subsurface processes play a governing
role in creating, maintaining, and shutting down exposures of ice at the
glacier surface, with a major impact on spatial patterns and rates of surface
mass loss. A large lake system (Spillway Lake) is dammed behind the terminal
moraine, which forms the hydrologic base level for the glacier. Since the
early 1990s, Spillway Lake has expanded upglacier, exploiting weaknesses
formed by englacial conduits. As part of the glacier response to the present ongoing period of negative
mass balance, the structure of the drainage system has changed through time,
characterized by decreasing efficiency and greater volumes of storage.
Processes and patterns of ablation on the glacier are strongly influenced by
active and relict elements of the drainage system. Former supraglacial
channels evolved into cut-and-closure conduits, which in turn precondition
the formation and drainage of perched ponds and provide templates for the
expansion of Spillway Lake. Thus drainage elements that initially formed
during earlier active phases of the glacier's history continue to influence
its evolution during stagnation.
Supplementary data are available at
The authors declare that they have no conflict of interest.
Funding for Sarah Thompson was provided by the European Commission FP7-MC-IEF grant PIEF-GA-2012-330805, and for Lindsey Nicholson by the Austrian Science Fund (FWF) Elise Richter Grant (V309-N26). Financial support for fieldwork in 2009 was provided by the University Centre in Svalbard and a Royal Geographical Society fieldwork grant to Sarah Thompson. Field assistance was given by Annelie Bergström and Alison Banwell. TerraSAR-X data were kindly provided by the German Aerospace Center (DLR) under project HYD0178. The meteorological data were collected within the Ev-K2-CNR SHARE Project, funded by contributions from the Italian National Research Council and the Italian Ministry of Foreign Affairs, and we thank Patrick Wagnon of the Institut de Recherche pour le Développement, France, for collecting and releasing the 2014–2015 data used in this paper. Careful and constructive reviews by Akiko Sakai and Duncan Quincey are gratefully acknowledged. Edited by: Andreas Vieli Reviewed by: Duncan Quincey and Akiko Sakai