The dimensions of these channels are regulated by several factors: water temperature, meltwater volume, debris content in the water, ice wall closure rates (governed by the ice thickness) and squeezing of fluidized sediment.
[1] In the glaciological literature three forms of subglacial meltwater channels are commonly mentioned.
The first type of channel is the R-channel after Hans Röthlisberger who initiated work on water pressures in tubes under glaciers.
The balance between channel enlargement by viscous heating and closure by ice deformation when the channels are water-filled reflects their size and water pressure.
Such channels form where water flows at atmospheric pressure beneath thin ice and on steep downglacier bedslopes.
The final type, the N-channel (after John Nye), are those incised into bedrock, perhaps suggesting long-term channel stability under some glaciers.