Here we examine the motion of large, catastrophic turbidity flows such as the Grand Banks event. A brief review of the general theory for such phenomena is given. Two special cases of the general theory are applied to the Grand Banks and Orleansville flows. Both cases are time dependent, thus permitting calculation of the origin of the flow. The models differ fundamentally in the parameterization of the hydrodynamic drag moving through the water at the sea floor. Nevertheless, application of both models to the Grand Banks turbidity flow yields good agreement on the origin of this event. The new origin is much lower on the slope than others have assumed and is consistent with plots of earthquake epicenters. Given a sufficiently dramatic initiating event, such as an earthquake, our results suggest that turbidity currents can be generated on slopes of less than 1 degrees and accelerate within minutes to a maximum velocity of between 20 and 25 m/sec.