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Stream channel stability in the Queen Charlotte Islands : an examination of Schumm’s fluvial model Roberts, Richard Graham


Based upon investigations of ephemeral, sand-bed streams in semi-arid areas, Schumm (1977) proposed a model of the fluvial system in which long periods of relative stability are separated by brief episodes of major morphologic change (termed "dynamic metastable equilibrium"). The latter constitute "episodic behaviour" and are initiated when the system crosses a "geomorphic threshold". The resultant sediment transport-limited conditions may induce a series of progressively damped cut-and-fill cycles, which Schumm termed "complex response". In this study, the stability of perennial, gravel-bed streams in the per-humid Queen Charlotte Islands was investigated, in order to test Schumm's fluvial model in a strongly contrasting environment. Compared with semi-arid stream systems, forest streams in the Queen Charlotte Islands are less sensitive, and exhibit greater resilience, to major hydrologic events. In general, a condition of sediment supply-limitation and "dynamic equilibrium" probably has prevailed in forested watersheds throughout the past 8,000 years. Schumm's model is overly elaborate in such situations. However, the behaviour of four stream systems in the Queen Charlotte Islands, whose sensitivity was increased by streamside logging activities, approaches that of semi-arid stream systems in certain respects. The land-use change crossed an "extrinsic geomorphic threshold", lowered an "intrinsic geomorphic threshold" (stream bank stability), and generated major episodes of channel widening and the development of aggradational "sediment wedges". This is consistent with Schumm's definition of "episodic behaviour". However, the occurrence of only one cycle of aggradation and degradation at any one cross-section is not compatible with the concept of "complex response". A simple, or Mackin-type, adjustment at "grade" is proposed as a more suitable description of the heavily damped relaxation pattern of "sediment wedge" movements; this response is promoted by the propensity of gravels to "armour", and alders to revegetate these deposits, and thereby inhibit their entrainment. "Sediment wedges" represent the most notable example of fluvial disequilibrium to have occurred in the Queen Charlotte Islands during the past 8,000 years which complies with the concept of "dynamic metastable equilibrium". But it is anticipated that these creeks will return to their pre-disturbance condition of "dynamic equilibrium" due to the operation of stream channel recovery processes.

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