NASA image of a supraglacial MWC draining into a moulin
MWC types 1. Supraglacial 2. Marginal 3. Englacial 4. Sub-marginal and 5. Subglacial meltwater channels
Meltwater channel types
Glacial meltwater channels are most frequently classified by the position of the channels in relation to the former glacier. Such classification is often difficult, however, as different types of channel frequently occur within a single area and large, rock-cut channels may be re-occupied during successive phases of glaciation and so have complex origins.
Sub-glacial and englacial channels
Thesechannels reflect the primary control of active ice movement on meltwater flow beneath relatively thick ice. The channels flow parallel to the direction of ice flow. Channels may be incised into rocks spurs and cols. The capacity of meltwater to flow uphill to cross cols is provided by the water in the glacier tunnels moving under hydrostatic pressure.
In former glaciated areas, the meltwater channels may be intermittent and disappear in depressions. Meltwater in the glacier was following ice tunnels and it is only where the meltwater streams ran on a bedrock floor that a trace of the flow is retained. In East Lothian, Sissons (1961) estimated that the ice was about 100 m thick when this type of channel was formed, sufficient to allow both hydrostatic flow and for the meltwater to reach the bedrock floor.
Beneath modern glaciers two types of channel occur:
Subglacial channels often form complex branching systems. The topography exerts a strong control over flow, with meltwater moving through cols and pre-existing valleys. In narrow cols, only one channel is usually found; in broad cols multiple channels may occur.
Marginal and sub-marginal channels
These channels are related to meltwater at the glacier margin flowing under gravity. Marginal channels often runalong slopes at shallow angle to contours. The channels may have one wall in rock and the other in ice. The channels may turn abruptly to plunge down crevasses or mouilns. Where these streams are in contact with rock or sediment, chute channels are formed. Occasionally, the bowls of plunge pools on the former floor of the glacier can be seen as enclosed, water-filled hollows. Fluvioglacial deposits may occur at the mouth of these channels.
Meltwater flowing away from the glacier front is often heavily charged with debris and moving at high velocities. Like normal rivers but with high and fluctuating discharges, proglacial channels are capable of rapid deepening of channels, even in bedrock. Proglacial channels are often choked by coarse fluvioglacial debris, giving braided and highly dynamic river channel systems.
Glacier advance often dams side valleys and water may be ponded there. The filling of the lakes may eventually overtop a col, leading to the cutting of an overflow channel. Prior to the 1960s many channels in the UK were interpreted as overflow channels before it was fully understood that meltwater could cut channels beneath glaciers.