The subglacial hydrology is considered for a temperate ice sheet resting upon a deformable aquitard (till) with a hydraulic conductivity several orders of magnitude lower than that of an underlying aquifer. The evacuation of basal meltwater by the aquifer-channel-bed hydraulic system is studied, particularly as it affects aquifer pressurization, till deformation, and channel failure. Sensitive parameters are aquifer hydraulic conductivity, ice sheet length and elevation, and basal meltwater rate, A criterion is developed to determine channel length; channels need not extend all the way to the divide. The model predicts that, in general, there is a region adjoining the terminus where the bed pore water pressure is equal to the overburden pressure and drainage along the bed is possible, Outside this region the bed pore water pressure is below the overburden pressure, and ice penetrates the soil to prevent drainage along the bed. An important result is that deformable substrates beneath temperate-based ice sheets may be unstable, particularly under conditions of a diminishing profile. ¿American Geophysical Union 1987