Contrary to a widely held view, specification of debris-flow rheology does not lead to accurate interpretation and modeling of debris-flow behavior. Mixtures of solids and fluids in debris flows exhibit behavior too complex to be represented by rheological equations that uniquely relate stress and strain rate. Instead, debris behavior can vary from nearly rigid to highly fluid as a consequence of temporal and spatial changes in mixture agitation and pore pressure. Behavior can also vary if debris composition changes as a result of grain-size segregation or gain or loss of solid and fluid constituents.

Debris-flow behavior that cannot be mimicked by any specific rheology is evident in the field and is readily measured in controlled, field-scale experiments at the USGS debris-flow flume. Results of these experiments illustrate the importance of interactions between mixture deformation, pore-pressure change, and grain-size segregation. Almost universally, coarse-grained snouts that form at the fronts of advancing debris-flow surges have lower pore pressure and greater internal and basal friction than does finer grained debris behind these fronts. The presence of high-friction snouts pushed from behind by more-fluid debris controls debris-flow runout distance, levee formation, and deposit morphology. The most realistic numerical models account for evolving interactions of high-friction snouts with trailing debris as well as for mass and momentum conservation as debris flows move unsteadily (from initiation to deposition) across three-dimensional terrain. Effects of progressive erosion or sedimentation by debris flows have not yet been modeled in a satisfactory manner.