A simple “laboratory” flow is described which exhibits the bistable behavior observed in more complicated geophysical flows. The behavior is present in a blocking experiment involving a shallow, homogeneous flow over an obstacle. It is demonstrated that within a certain parameter space, the blocking tendency depends not only on the heightbOof the obstacle but on the history of the flow as well. The implied hysteresis confirms behavior predicted by Baines and Davies (1980, p. 239). It is further shown that the hysteresis is associated with a nonuniqueness in steady solutions which is, in turn, attributed to the existence of stationary blocking bores upstream of the obstacle.