... looked at from its two fundamental distinctions; 'discrete' and 'distributed'. In the former, water is confined to a few channels or conduits, whereas distributed systems transport water over the whole of, or a large proportion of the bed. Subglacial water was first considered to form a film or sheet at the sole of the glacier (Weertman, 1972). Water was thought to cover around 80% of the bed, originating from basal melt rather than surface percolation. Weertman described a thin film which formed on bedrock irregularities through the process of regelation. This formed the basis of his theory of glacial sliding. Nevertheless, this was soon considered an over-simplified synopsis of subglacial processes, and was followed by theories of channelised flow. Discrete drainage systems are "efficient transporters of meltwater, allowing rapid flow through well connected channel systems" (Benn and Evans 1998). In contrast, distributed systems are "inefficient, and meltwater follows more ...

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