The disruption of Grassy Creek: implications concerning catastrophic events and thresholds

Two catastrophic events, occurring simultaneously in the valley of Little Grassy Creek, IL, allow for an examination of the threshold concept in geomorphology. Movement of debris associated with failure and sliding of valley-side material, caused damming and avulsion of Little Grassy Creek. Slope and river disruptions, both severe in character, were linked because the effect of one event (slope failure) was the cause of the second event (fluvial avulsion). The slope failure represents a true threshold-crossing event because the results are irreversible on a graded-time scale. In contrast, the fluvial disruption was not a threshold crossing, although the event was catastrophic and short-term instability occurred. In the fluvial case, a new channel developed, and the re-establishment of equilibrium, as estimated by channel characteristics, occurred within 10 years. The river system functions as it did before the slope failure/avulsion, though the channel reach is now in a different location. Criteria needed to employ thresholds to explain geomorphic events are suggested, and a definition of thresholds as time-dependent phenomena is presented as a means of reducing confusion over the use of the threshold concept.