A GENERAL THEORY OF RIVERBANK INSTABILITY
In log-log magnitude/frequency graphs of the observed mountain river systems, the measured slope of the power-law function (the tau-exponent, t) of spatial failure frequencies in alluvial reaches of these systems (from 1.07 to 1.44) is less than that for all reaches (alluvial, colluvial, and bedrock) combined, suggesting that alluvial reaches are more susceptible to large riverbank instabilities. Changes in the t-exponent are related to differences in average channel slope and in near-channel bank material. If in fact riverbank systems are at a critical state rather than an equilibrium state, then long-term local stability is an unlikely or even impossible engineering or restoration goal. The probable existence of criticality in natural stream settings suggests that local human alterations designed to increase channel stability, while lowering the local frequency of small failures, will only encourage an increase in the magnitude of system-wide, low-frequency large failures. A local stabilization or restoration effort will not eliminate the bank instability, but will instead transfer that instability to adjacent riverbank areas.