Tunnels driven, in rock under the glacier Bondhusbreen in Norway, and thence upward to the ice-rock interface, provide a rare opportunity to study a subglacial drainage system. Early in the melt season, suspended sediment discharges in the subglacial water channels are large, and channels apparently change position. These effects are inferred to be a consequence of increased separation of ice from the bed due to increased water pressure. Water pressures are believed to be high at this time of year because water channels, having closed by plastic deformation during the winter when discharges are low, are too small to handle the increasing spring discharges. Water thus backs up in the channel system. Calculations suggest that further changes in channel position occur as discharges increase, because the potential energy released by the water is able to melt ice in the walls of channels faster than it is replaced by inward flow of ice. Channels can thus trend diagonally across the direction of glacier flow. Although such changes may occur at any time during the melt season in response to variations in discharge, there appears to be a seasonal evolution of the drainage pattern under Bondhusbreen that is repeated each year. These shifting channel patterns make the engineering task of locating and diverting water beneath glaciers more difficult than anticipated.