A systematic series of experiments on the instantaneous release of two-dimensional, heavy, particle-driven gravity currents has been conducted. High-concentration currents propagated in a qualitatively different way than low concentration currents. In particular, beyond a critical initial volume fraction of particles, the resulting dense current came to an abrupt halt at some point down the channel, depositing the bulk of its initial sediment load as a relativly thick layer of fairly constant thickness, characterized by a pronounced, steep snout. A very much thinner layer of sediment extended for some distance beyond the arrest point. This layer was deposited from the subsequent propagation of a slower moving, low concentration residual cloud. ©1998 American Institute of Physics.