This paper addresses the coupling of flows of ice, till and water, and the issue of whether such coupling provides mechanisms for meso-scale (kilometres to tens of kilometres) variability in ice-sheet flow and texture. The question of whether effective pressures at the ice-bed interface are statically or hydraulically controlled is examined in this paper. The answer is scale dependent, and has a significant effect on the relationship between ice surface and basal topography.
The consequences of these considerations on till flow, coupled ice–till flow and coupled ice, till and water flow are examined. An analysis of till-flow kinematics and shock formation is carried out. The linear stability of coupled long-wavelength ice-till flow is analysed, and regions in parameter space where this flow is unstable, with rather small rate constants are found. Upstream-moving ice surface waves are predicted. The linear stablity of coupled ice–till–water flow is examined, where water flow is modelled using a basal flow system with effective-pressure-dependent properties. Again, regions in parameter space where the system is linearly unstable are found, this time with relatively rapid rate constants. The water pressure exhibits “breather” modes.
These analyses assume that there is a substantial basal traction. A problem with models of ice streams wholly restrained at the side is identified: they seem to predict erosion rates which are unfeasibly large.
There appears to be sufficient variability in the ice–till– water system to potentially explain texture in ice-stream surfaces, variations in ice-stream thickness of tens of metres not directly relatable to topography, and waves moving upstream or downstream. Most importantly, the ice-stream–bed system is shown to exhibit meso-scale variability simply by coupling ice flow according to the shallow-ice approximation, till flow according to the hydrostatic thin-till approximation and water flow according to an effective-pressure-dependent hydraulics.