ABSTRACT
Previous work on event frequency in fluvial systems has tended to focus on the frequency of the most effective event in terms of total sediment transport, or on the frequency of the event, often bankfull discharge, which expresses overall morphological adjustments. This paper attempts to identify process thresholds within fluvial systems and to consider the role of their frequencies in adjustments within dynamic equilibrium. Two main groups of event magnitude are identified: major events controlling overall morphology and moderate events causing adjustment within the overall forms produced by the major events. The discussion is based on field observations in three areas in N.W. England. The areas represent fluvial environments ranging from steep headwaters to braided and meandering river channels. Within these systems the major events occur from two to four times per year to once every two years. Moderate events occur from 14 to 30 times per year. The major events, although occurring with roughly the same frequency as is often quoted for bankfull discharge, do not equate with bankfull conditions. In this context bankfull discharge is not a very useful concept.
The period of field investigation coincided with the abnormally dry years of the early 1970’s, during which time actual event frequencies were markedly less than expected. Although there were morphological adjustments to the changed relative frequencies of major and moderate events, the systems remained within dynamic equilibrium. There was no evidence of any approach to a major system threshold, nor evidence of any permanent change in the equilibrium.
