ABSTRACTThe b-f-m diagram can be used to interpret the downstream hydraulic geometry of rivers. Six lines representing thresholds of adjustment to downstream changes in discharge and sediment load delineate twelve sets of responses. Rivers that plot within the same area of the diagram have similar directions of adjustment to increasing discharge, although the exact rates may vary considerably. The groupings of data indicate that neither general climatic setting nor the size of the drainage basin has any consistent influence on the downstream relations. Downstream hydraulic geometry does, however, seem to respond quickly and in a predictable manner to changes in discharge and sediment load. Rivers have different downstream geometries at different frequencies of flow, and therefore comparisons among rivers should be made at a common frequency of discharge. Some minimum-variance solutions for the most probable form of hydraulic adjustment can be represented as three-dimensional surfaces on the b-f-m diagram. These surfaces show the probability of different hydraulic geometries, their relative stability, and likely directions of change. The set of responses to downstream increases in discharge predicted to be the most probable is also the most commonly observed. This may indicate a tendency toward a common form of adjustment.