Channels incised at the glacier base, governed by a balance between viscous creep closure of the ice roof and melting due to turbulent heating. Such channels naturally tend to draw in surrounding water and therefore form branching arterial networks, aligned predominantly in the down-glacier direction (Hewitt (2011) J. Glaciol. 57, 202). Two types of channels are thought to exist beneath glaciers: Nye (N) channels, which are incised into the bedrock, and Röthlisberger (R) channels, which are melted upwards into the glacier ice; see J. Holden (2012), pp. 475–6.