The sediment comprising a deformable bed beneath an ice mass is typically modeled as a homogeneous fluid without internal structure, although clearly, deformation must result from the spatial rearrangement of the particles that make up the sediment. Laboratory deformation experiments were used to simulate the effects of active deformation on subglacial sediments. These experiments indicate that deformation results in the rearrangement of particles and changes in sediment structure. Scanning electron and optical microscopy of postdeformation sediments revealed both (1) particle alignment parallel or subparallel to the direction of principal strain and (2) dilation where active deformation had occurred. Both of these effects will alter the permeability and hence hydraulic conductivity of the sediment body, increasing the rate of water flow through the bed parallel to deformation. Microscopic examination of these sediments allows estimation of postdeformation permeabilities. These permeabilities may be anisotropic. Furthermore, water throughflow by Darcian processes, enhanced by the effects of deformation, may increase the proportion of basal water that can be evacuated by throughflow. Water will tend to be retained in the high-permeability actively deforming layer and may destabilize this layer. ¿ American Geophysical Union 1992