Geomorphic Thresholds: The Concept and Its Applications

Geomorphic thresholds were defined initially as the condition at which there is a significant landform change without a change of external controls such as base level, climate and land use. Landforms evolve to a condition of incipient instability following which change or failure occurs. Subsequently, through usage, the definition has been broadened to include abrupt landform change as a result of progressive change of external controls. Therefore, it is now appropriate to recognize both intrinsic and extrinsic geomorphic thresholds. The threshold concept has practical significance. If the threshold conditions can be recognized, not only will different explanations for some landforms emerge but also the ability to identify incipiently unstable landforms and to predict their change will be of value to land managers and engineers. For example, the development of gullies and fan-head trenches can be explained by the depositional steepening of valley floors and fan-heads to threshold slope. As a consequence, as yet ungullied but potentially unstable areas can be recognized. In addition, channel pattern variations and the conversion of meandering channels to braided ones, and of braided channels to single-thalweg sinuous ones can occur naturally at pattern thresholds. Such changes can also be accomplished artificially, when it is recognized that a channel is near a pattern threshold. Sediment yield variations will be related to these periods of instability. Recognition of this will aid in the explanation of some hydrologic, sedimentologic, and stratigraphic anomalies.