Subglacial Hydrology and Water Pockets
The base of a glacier is one of the least accessible environments on Earth. Hundreds of metres of ice separate the bed from the surface, temperatures are near freezing, and pressures are immense. Yet this hidden interface is where some of the most consequential glaciological processes occur. Water accumulates beneath glaciers from three sources — geothermal heat melting ice from below, frictional heat generated by the sliding of ice over the bed, and surface meltwater descending through the glacier via crevasses and moulins — and the way this water moves determines much of the glacier's dynamic behaviour.
Subglacial hydrology is the study of this water: where it comes from, how it flows, how it distributes pressure, and how it modulates the sliding speed of the glacier. Advances in understanding subglacial hydrology over the past three decades have transformed glaciology's ability to explain the often puzzling speed variations observed in outlet glaciers, and have improved projections of ice sheet contributions to sea level rise. The glacier map provides context for the locations of glaciers where subglacial processes play key roles in global ice dynamics.
How Water Gets Under a Glacier
Surface meltwater is the dominant input to the subglacial drainage system in most temperate and polythermal glaciers. During the summer melt season, water produced on the glacier surface collects in streams that flow across the ice until they encounter a moulin — a near-vertical shaft drilled through the glacier by the rotating motion of a surface stream. Moulins can extend hundreds of metres from the glacier surface to the bed, delivering large volumes of water rapidly to the subglacial system.
Where moulins are absent, water may also enter through crevasses, pooling at depth until pressure drives it toward the bed. On ice sheets with very thick ice, surface meltwater may refreeze before reaching the bed, but on tidewater glaciers and outlet glaciers with thinner ice or warmer basal temperatures, the hydraulic connection between surface and bed is often direct and responsive.
Basal melting driven by geothermal heat is a constant background process. Earth's interior heat flux varies geographically but is sufficient to melt a few millimetres of ice per year at the base of most glaciers. This is small relative to surface melt inputs but continuous, and it means that even cold-based glaciers in polar regions can maintain thin films of water at their beds if insulated from surface cold by sufficiently thick ice. Beneath active volcanic terrains — Vatnajökull in Iceland sits above multiple volcanoes — geothermal heat flux can be dramatically elevated, generating large volumes of subglacial meltwater.
Distributed Versus Channelised Drainage
Subglacial water can move through two fundamentally different types of drainage systems, and the transition between them has profound consequences for glacier sliding speed.
A distributed drainage system consists of a network of thin films of water, interconnected cavities on the lee side of bedrock bumps, and water saturating porous subglacial sediment. In a distributed system, water moves slowly through many parallel pathways, and water pressure throughout the system remains high — close to the ice overburden pressure. High water pressure reduces the effective normal stress on the glacier bed, which is the force holding the ice against the bed minus the water pressure. When effective stress is low, the ice can slide more easily, and the glacier moves faster.
A channelised drainage system develops when a few conduits — either Röthlisberger channels incised upward into the ice or Nye channels eroded into the bedrock — grow large enough to capture most of the drainage. Channels are self-reinforcing: faster-flowing water dissipates more heat, which melts the channel walls and enlarges the conduit, which draws more water. A mature channelised system drains efficiently, carries water at relatively low pressure, and removes the water pressure that would otherwise facilitate fast sliding. Channelised systems develop during the melt season as water inputs increase.
The seasonal transition from distributed to channelised drainage is detectable at the glacier surface as a deceleration of flow velocity. Early in the melt season, the first large meltwater inputs overwhelm the as-yet undeveloped drainage system, water pressure rises dramatically, and the glacier surges forward at speeds that can be several times the winter baseline. As the drainage system evolves over weeks, channels develop and drain the excess water pressure, and the glacier slows again. This spring speed-up phenomenon has been observed on glaciers in Greenland, Alaska, the Alps, and Antarctica.
Subglacial Lakes
Some subglacial water does not drain downstream but instead collects in topographic depressions beneath the ice, forming subglacial lakes. These lakes are maintained in a liquid state by the combination of pressure — which depresses the melting point of ice — and geothermal heat. The ice surface above a subglacial lake is characteristically flat, because the lake effectively lubricates the base uniformly and the ice above flows at a consistent speed without the surface deformation features associated with rough bedrock.
Lake Vostok in East Antarctica is the largest known subglacial lake, covering roughly 15,000 square kilometres and located beneath approximately 4 kilometres of ice. It has been isolated from the surface for at least 15 million years and possibly much longer, and the microbial communities it may harbour have attracted intense scientific interest. Russian researchers drilled to the lake surface in 2012, and subsequent analysis of refrozen lake water has found evidence of microbial life.
Beneath Vatnajökull in Iceland, the subglacial lake Grímsvötn periodically fills and drains in dramatic jökulhlaup events. The lake is fed by geothermal heat above the Grímsvötn volcanic system, and water accumulates until the ice dam holding it in place is lifted by buoyancy, releasing the stored water in a flood that can exceed the discharge of the Amazon River for brief periods.
Water Pockets and Englacial Storage
Not all glacially stored water resides at the bed. Water can also be stored within the glacier body itself — in englacial channels, in water-filled fractures, and in the porous firn layer near the surface in cold-climate glaciers. The term water pocket is sometimes used informally to describe discrete, bounded reservoirs of liquid water within or beneath the ice that drain suddenly rather than continuously.
Such pockets form where ice geometry creates a hydraulic seal — a local depression in the subglacial or englacial drainage network where water cannot escape until the overlying ice pressure builds to the point where the drainage path opens. When a water pocket drains, the sudden release of stored water can cause the glacier surface to subside by metres over a period of hours or days, and the outflow can temporarily dominate the proglacial stream discharge.
Relevance to Ice Sheet Stability
Subglacial hydrology is not merely an academic concern. The behaviour of the Greenland Ice Sheet and the West Antarctic Ice Sheet — the two ice bodies whose future mass loss will dominate 21st-century sea level rise — is strongly conditioned by subglacial water. The fast-moving outlet glaciers that drain the bulk of the ice sheet's volume sit on beds where liquid water modulates sliding speed. Better understanding of subglacial drainage evolution, and how it responds to increasing surface meltwater production as the climate warms, is central to reducing uncertainty in sea level projections.
The accelerations of outlet glaciers like Jakobshavn Isbrae in Greenland and the Pine Island Glacier in West Antarctica are driven in part by changes at the bed, some of which involve hydrological changes. Monitoring subglacial hydrology remotely — through satellite-based ice surface velocity measurements, passive seismic monitoring of subglacial water movement, and airborne radar sounding of bed topography and water presence — is an active area of research with direct implications for climate adaptation planning worldwide.