The effect of pressurized sub-glacial water on the sliding process is quantified by calculating a “bed separation index”. The water pressure distribution i s calculated assuming the existence of a Rothlisberger channel at the bed. Kamb's formulation is used to describe the variation of normal stress over periodic bed undulations. The hypothesis is that as either basal shear stress or water pressure is increased the extent of ice-bedrock separation (on the down-glacier side of undulations) increases and enhanced sliding occurs

Data from three glaciers of widely varying size are used to test this hypothesis. For Columbia Glacier and “Ice Stream B” the importance of including the effects of water pressure in any “sliding law” are pronounced. More complete data from the third test case, Variegated Glacier, are used to compare a number of possible formulations of sliding law which encompass the above hypothesis. A modified Weertmantype law appears to be most preferable while while some possibilities, including Budd's lubrication factor hypothesis, are tentatively rejected.

Consideration of the temporal variations of the “bed separation index” reemphasize that, especially in the short time scale, variations of water pressure can dominate the sliding process. An order of magnitude increase in water discharge causes a hundred fold transient increase in the water pressure.

This paper has been accepted for publication in the Journal of Glaciology