Abstract
The Complex Response model originated by Schumm and Parker (1973) claims that multiple paired stream terraces can form as a result of “complex” mutual feedbacks between the downstream and upstream reaches of the river system in response to a single drop in base level whereby the river initially becomes ungraded and then recovers a graded state. To critically test this model, a series of analog flume experiments was conducted, during each of which it was observed how the alluvial lower reaches of a river responded to base level fall under the conditions of constant sediment discharge, constant upstream water discharge to convey the sediment downslope, and a uniform flume slope. The model river was graded using a downstream weir; sudden removal of the weir created a discrete drop in base level. The results of the experiment suggest that (1) even in a graded system, there can form “incipient” stream terraces owing to autogenic or self-organizing geomorphic process of the river, (2) base level fall functions simply to amplify the relief of pre-existing incipient terraces and is not necessarily responsible for the formation of stream terraces, thus, (3) the complex response of river systems is regarded as a manifestation of the alluvial autogenic responses, and (4) the autogenic responses observed in the runs are in the form of fluvial-diffusive processes. The upscaling of a model alluvial river implies that in a prototype, small-scale, high-gradient, gravel-dominated natural system, the diffusive alluvial response will completely regain its graded state in a very short time after becoming ungraded by a discrete drop in base level.
