Coarse granular solids in debris flows are supported within a clay-water matrix by strength and buoyancy. The magnitude of buoyancy is determined in part by the density contrast between coarse grains and matrix, but also by the pore-pressure increase associated with transfer of the weight of coarse grains as they are supported by matrix strength. Buoyancy therefore increases with concentration of coarse grains and can greatly increase the competence of the flow. Mobility of debris flows probably is also enhanced by increased buoyancy, due to reduction of internal friction. Therefore, large buoyancy forces, due to heavy loading of the matrix by coarse solids, may be significant in producing the high competence and mobility exhibited by many natural flows.