Subglacial stream

Subglacial streams are conduits of glacial meltwater that flow at the base of glaciers and ice caps. Meltwater from the glacial surface travels downward throughout the glacier, forming an englacial drainage system consisting of a network of passages that eventually reach the bedrock below, where they form subglacial streams. Subglacial streams form a system of tunnels and interlinked cavities and conduits, with water flowing under extreme pressures from the ice above; as a result, flow direction is determined by the pressure gradient from the ice and the topography of the bed rather than gravity. Subglacial streams form a dynamic system that is responsive to changing conditions, and the system can change significantly in response to seasonal variation in meltwater and temperature. Water from subglacial streams is routed towards the glacial terminus, where it exits the glacier. Discharge from subglacial streams can have a significant impact on local, and in some cases global, environmental and geological conditions. Sediments, nutrients, and organic matter contained in the meltwater can all influence downstream and marine conditions. Climate change may have a significant impact on subglacial stream systems, increasing the volume of meltwater entering subglacial drainage systems and influencing their hydrology.

Formation Subglacial streams derive their water from two sources: meltwater transported from the top of the glacier and meltwater from the glacial bed. When temperatures are high enough to induce melting on the surface of the glacier, typically during summer, water flows down into the glacier. Surface meltwater flows downward through millimeter-sized channels that join in a network of tributaries, growing in size until reaching the bedrock. Additionally, some water is transported to the surface by moulins (large, vertical shafts up to ten meters wide that range from the surface to a lower elevation, sometimes all the way to the glacial bed). Fractures, crevasses, and cavities between glaciers and valley walls can also provide pathways for water to reach the bed. While surface meltwater can be seasonally dependent, the beds of temperate glaciers are maintained at the pressure melting point (the combination of temperature and pressure at which ice melts). This liquid water at the bed—present in temperate but not polar glaciers—provides a constant input of water to subglacial stream systems. Water from these two sources meets and is concentrated at the bedrock base of the glacier, where pressure from the ice above forces it to move towards the glacial terminus, creating a network of passageways as it works its way out of the glacier.

Hydrology

Direction of Streams Water in subglacial streams is subject to large amounts of pressure from the mass of ice above; as a result, the direction of water flow cannot be explained in the same way as typical surface streams. Subglacial water flow is, to a large extent, determined by pressure gradients created by the weight and movement of the glacier. As a result, instead of following the slope of the bed, streams can flow up and across slopes. This behavior can be described by viewing the pressure inside glaciers as equipotential surfaces; as the water is pushed from areas of high pressure to areas of low pressure, it travels in a direction normal to these surfaces.

Stream Systems Subglacial stream systems can be placed in two categories based upon the arrangement and type of passages that make up the system: channelized and distributed.

Channelized Channelized drainage systems are characterized by water flowing predominantly through tunnels along the bed of the glacier that take meltwater rapidly and directly to the glacial terminus. These tunnels are arranged in a network of tributaries, joining and growing in size as they near the terminus. Water is fast-moving in these systems, and pressure inside the channels is relatively low compared to pressure in the ice around them. Turbulence in the rapid flow produces heat, which is able to melt the ice walls of the tunnels. While the total water added to the system by this process is insignificant compared to water from the surface and from basal melting, the melting of the channel walls allows the channel to remain open even when the ice pressures surrounding it are much greater than the pressure of the water inside. The constant erosion of the tunnel walls is able to offset the narrowing of the tunnel caused by deformation of the ice. Depending on the water supply and the characteristics of the bed, the tunnels can take different forms, including semicircular tunnels cutting into the ice, broad and low tunnels, and tunnels that cut into the bed rather than the ice. Broad and low tunnels form in channels with variable amounts of meltwater, as melting is concentrated on the tunnel walls rather than the ceiling when the tunnel is not completely full of water. Channels that maintain long-term stability in water flow and location can erode the bedrock over time, resulting in tunnels that cut into the bed rather than the ice above.

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