The fast ice flow characteristic of the low-gradient Lake Michigan Lobe of the southern Laurentide Ice Sheet during the last glaciation likely resulted from some combination of subglacial sediment deformation and sliding at the ice-bed interface. Both mechanisms require high basal water pressure relative to overlying ice pressure. To evaluate the basal water pressures of the Lake Michigan Lobe, we used the groundwater model MODFLOW to simulate groundwater flow along a 1040-km-long flow line corresponding to the flow path taken by the ice lobe. Our simulations show that groundwater flow directions and velocities were substantially different than modern conditions. Simulations also indicate that subglacial aquifers were not capable of evacuating the estimated basal meltwater, but that excess water draining through a basal drainage system such as one that may underlie Ice Stream B, West Antarctic Ice Sheet, would prevent basal water pressure from exceeding the ice-overburden pressure. The buried Mahomet bedrock-valley system near the distal end of the Lake Michigan Lobe could have drained enough subglacial water to substantially lower basal water pressure and thus affect the dynamics of the lobe. By draining basal meltwater away from the Lake Michigan Lobe, the buried Mahomet bedrock-valley system may have attenuated the effect of permafrost on basal water pressure. During the Last Glacial Maximum, groundwater-flow patterns and velocities were altered or reversed relative to present conditions.