Hot ash flows propagate from volcanic vents during some explosive volcanic eruptions. As a flow entrains and heats ambient air and simultaneously deposits pyroclasts, part of the flow may become less dense than the air and ascend from the flow. If entrainment of air is more important than sedimentation in controlling the flow behavior, then a large, buoyant ash cloud may form above the flow. Conversely, if sedimentation of particles dominates, then a much smaller fraction of the particles are able to ascend in the buoyant cloud. We have carried out a series of analog laboratory experiments to simulate the motion and buoyancy generation in dilute ash flows propagating along the ground. The first series of experiments investigated buoyancy generation through entrainment, using mixtures of methanol, ethylene glycol and water. The second series examined the role of sedimentation upon the generation of buoyancy, using particle-laden, fresh-water currents propagating in a saline ambient fluid. The experiments showed that the rate of entrainment increases while the sedimentation rate decreases as the downward slope of the terrain increases, suggesting that massive ash-laden clouds are more likely to rise from flows traveling on steeper slopes. Changes in slope angle also control the motion of ash flows. The experiments suggest that flow separation and hydraulic jumps resulting from sudden changes in slope angle can enhance entrainment or sedimentation and increase buoyancy. Observations of the interactions of ash flows with topography at Redoubt Volcano in April 1990 and at Mount St. Helens on May 18, 1980, are consistent with our laboratory observations.