West Antarctic ice streams move rapidly over beds that are thawed by heat generated by the motion. Intervening zones of slow-moving ice are probably freezing at their bases, where heat generation is absent. Heat is also generated in the shear zone between the slow and fast moving ice. Through modeling of coupled heat and mass flow, we show that the position of the boundary between freezing and melting at the bed is unstable for an otherwise morphologically uniform bed. Above a threshold speed in the ice stream, heat generation in the shear zone allows melting beneath the slow ice and possible widening of the stream. Below this threshold, heat generation at the edge of the thawed zone is insufficient to maintain melting, leading to freezing and narrowing of the stream. The threshold speed appears to be a function of the temperature at the base of slow ice (relative to the melting point) and is also highly sensitive to the inflow of colder ice from the slow zone. For environmental conditions typical of West Antarctica, the threshold speed is about 100 m yr-1 in the absence of inflow and increases to typical ice stream speeds (500 m yr-1) for inflow speeds of a few meters per year. Thus the ice streams can be in a delicate thermal balance with the potential of melting outward or freezing inward. Rapidly moving ice streams could expand unless blocked by bed properties that would prevent fast motion even when there is melting at the bed. ¿ 1998 American Geophysical Union