ABSTRACT
The concept of geomorphic thresholds, which involve landform change without a change in external controls, challenges the well-established basis of geomorphology—that landform change is the result of some climatic, base-level, or land-use change. The concept of geomorphic thresholds is useful in identifying those conditions at which a landform is incipiently unstable. A geomorphic threshold is a threshold of landform stability that is exceeded either by progressive change of the landform itself, that is, an intrinsic change, or by a change in an external variable. Changes of both biomass and tractive force can be the result of land-use changes and climatic fluctuations, which weaken valley-floor vegetation and increase peak discharge and tractive force. However, with external influences unchanged, the tractive force can be increased if the valley-floor slope is steepened by aggradation, an intrinsic control. Sediment was stored and then flushed as valley-floor slope thresholds were exceeded.
