Glacier Hydrological Systems

Glacial meltwater finds its way through the glacier from its point of origin along a variety of different flow paths (Figure 4.2). Meltwater derived from surface melting and from basal melting will tend to follow different paths through a glacier, although both will invariably involve channel flow either within or on the glacier. The type of channel system within a particular glacier depends primarily on its thermal regime. On glaciers consisting of cold ice, meltwater is unable to penetrate without freezing and tends to be confined to surface and ice-marginal channels. On glaciers consisting of warm ice, meltwater is able to penetrate without freezing and water flow can occur in supraglacial, englacial and subglacial channels. In glaciers with mixed thermal regimes, more complex flow patterns are likely and water can flow through supra-glacial, englacial and subglacial channels. It can also be stored in small subglacial lakes at the junction between ice of different temperatures (see Section 4.3).

4.5.1 Supraglacial and Englacial Water Flow

Supraglacial channels tend to be less than a few metres wide and may exploit structural weaknesses within the ice (Figure 4.1A). In plan they may adopt either meandering or straight courses. Velocities within these channels are usually high because their smooth sides offer little frictional resistance. On temperate glaciers, supraglacial channels are usually short and are interrupted by crevasses or vertical shafts known as moulins (Figure 4.1B), which divert the water into the glacier where they may become englacial (Figure 4.1C).

The internal geometry of moulins and englacial drainage routes has been studied in detail on Storglaciaren in Sweden and on debris-covered Himalayan glaciers. Both sets of drainage routes develop by exploiting crevasses and other pre-existing structures within the ice. Detailed three-dimensional mapping of englacial drainage conduits on the Himalayan glaciers shows that their location is determined by pre-existing lines of high hydraulic conductivity and that channels are graded to local base level. Grading is accomplished by headward retreat and downcutting, producing canyon-like passages within the ice. The Storglaciaren moulins are also structurally controlled. When a crevasse opens on the glacier surface it often intersects a supraglacial meltwater stream. The crevasse will then fill with water, until it opens and deepens sufficiently to intersect englacial drainage passages, at which point the water drains away. When glacier flow moves the crevasse into an area of compression the crevasse may close, but the heat carried into it by the meltwater may keep the drainage channel open and thereby form a moulin. The Storglaciaren moulins consist of near-vertical shafts, 30-40 m deep, which feed englacial tunnels that descend from the base of the moulin at angles of between 0° and 45°. The orientation of the englacial tunnels is often controlled by the orientation of the original crevasse from which the moulin formed.

4.5.2 Subglacial Water Flow

Subglacial drainage systems can take one of four main configurations.

1. Configuration 1. Water can flow in a thin film with a thickness of millimetres between the glacier and its bed, often known as a Weertman film. In this configuration, the meltwater is spread widely across the bed. A Weertman film is most likely to develop where meltwater is derived primarily from basal melting and in situations where there are restricted inputs of surface meltwater. Mathematical analyses suggest that a water film is unstable because the film tends to rapidly organise itself into networks of small channels. The presence of a thin film of meltwater is central to theories of glacier sliding because it acts as a lubricant between the ice and its substrate (see Section 3.3.2).

2. Configuration 2. Conduit or tunnel networks discharge meltwater through a small number of large channels (Figure 4.1D). Tunnel networks cover a limited area of the glacier bed and are efficient in transferring meltwater through the glacier. These channels can be within the ice (englacial), or at the ice-bed interface (subglacial). Where channels at the glacier bed cut upwards into the ice they are termed Rothlisberger channels or R-channels. In plan view, this type of drainage network is often dendritic. The size of the tunnels is determined by the balance between the processes that act to enlarge them (i.e. ice melt of tunnel walls from flowing water) and processes that act to close them (i.e. ice deformation). The natural shape for a tunnel or conduit, well away from the bed, is near-circular due to these two opposing factors (Figure 4.1D). More importantly, the size of a conduit will vary with changes in discharge over a matter of weeks. As melt-water discharge increases, so the conduits grow in size.

3. Configuration 3. Linked-cavity systems are found where water collects in low-pressure areas within the cavities that are created as ice slides across bedrock obstacles. These cavities cover much of the glacier bed, and are connected by a tortuous network of small links to create a more-or-less continuous drainage network (Figure 4.4). The links can be cut down into bedrock (Nye or N-channels; Figure 4.5), or can be cut upwards into ice as small R-channels. In this drainage configuration, meltwater is spread across a wide area of the bed and, because the channel geometry is relatively inefficient, meltwater transit times are slow.

Figure 4.4 Network of linked basal cavities in plan and cross-section. Each cavity is linked by N- or R-channels. [Reproduced with permission from: Hooke (1989) Arctic and Alpine Research,

Figure 4.4 Network of linked basal cavities in plan and cross-section. Each cavity is linked by N- or R-channels. [Reproduced with permission from: Hooke (1989) Arctic and Alpine Research,

Figure 4.5 A Nye channel on Isle of Mull, western Scotland. [Photograph: N. F. Glasser]

4. Configuration 4. Meltwater can also flow within subglacial sediments if sufficient soft, potentially deformable sediments exist beneath the glacier. To be effective, this style of drainage requires the presence of a layer of sediment at the base of the ice, unlike the other three drainage styles, which relate to 'hard' rock beds. Subglacial sediments are usually permeable, so that they deform easily when saturated and subjected to stresses transmitted to the bed by the overlying glacier. The precise mechanism by which the water moves through the sediment is unclear. Suggested mechanisms include: advection the water within the sediment layer is carried forward as the sediment deforms; Darcian flow, whereby water moves through the pore spaces of the sediment from areas of high water pressure to low water pressure under the influence of the hydraulic potential gradient; within pipes and small channels in the subglacial sediment itself; or as a thin film or sheet of meltwater at the upper surface of the sediment.

An important distinction in subglacial hydrology is between distributed and discrete drainage systems. Configuration 1 (Weertman film) is often referred to as a distributed drainage system because the meltwater is widely distributed across the glacier bed. Configurations 2 and 3 are often referred to as discrete drainage systems because the meltwater is confined to discrete channels and tunnels at the glacier bed. Configuration 4 (meltwater flow within subglacial sediments) can be regarded as either a distributed or discrete drainage system, depending on whether the water flows through the sediment as Darcian flow (distributed), or in a sheet (distributed) or in tunnels and pipes in the sediment (discrete).

It is important to remember that the drainage system in many glaciers is constantly undergoing both spatial and temporal changes. Consequently, the drainage system beneath an individual glacier can vary in space; for example if the glacier bed is 'patchy' and composed of a discontinuous sediment cover of variable thickness over bedrock. It may also vary through time; for example seasonally, where there is a well-documented transition from an early melt-season within an inefficient drainage system to a more efficient drainage system later in the melt season. Rapid transitions between different subglacial drainage configurations are also possible where water flow is non-steady-state, for example beneath surge-type glaciers (see Section 3.5).

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