Classifications of flowing sediment-water mixtures have, in the past, been based primarily on relative, qualitative differences in the style and rate of movement as well as on morphology and sedimentology of deposits. A more quantitative and physically relevant classification is presented here, based on thresholds in rheologic behavior. The classification is constructed on a two-dimensional matrix in which flows are located according to deformation rate (mean velocity) and sediment concentration, with composition of the mixture constant. Three major rheologic boundaries are crossed as sediment concentration increases from 0 (clear water) to 100 percent (dry sediment): (1) the acquisition of a yield strength—the transition from liquid “normal streamflow” to plastic “hyperconcentrated streamflow”; (2) an abrupt increase in yield strength coinciding with the onset of liquefaction behavior—the transition to “slurry flow”; and (3) the loss of the ability to liquefy—the transition of “granular flow.” These three rheologic boundaries shift according to particle-size distribution and composition of the mixture.

Processes controlling flow behavior depend on deformation rate (velocity). Rate-independent frictional and viscous forces dominate at lower velocities and in finer grained mixtures; rate-dependent inertial forces dominate at higher velocities and in coarser grained mixtures. As velocity increases, grain-support mechanisms change from low-energy varieties (buoyancy, cohesion, structural support) to progressively higher energy mechanisms (turbulence, dispersive stress, fluidization).

Existing nomenclatures of geologic flow phenomena can fit within this rheologic classification. The morphology and sedimentology of flow deposits commonly can be used to deduce rheologic behavior, but caution needs to be exercised in inferring processes from deposits.