ABSTRACT: Rivers are self-organized, conditionally stable systems.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

Students of rivers have long been preoccupied with studying river channel changes. In fact, the most remarkable feature of rivers is that they do not change more than they do. Consideration of river morphology leads to the conclusion that they are self-organized to minimise further change. The mechanism by which this is achieved is maximization of resistance to flow. It occurs simply because, if a river is not in a 'most stable' state, there is increased propensity for it to change further until it approaches such a state. The stability is, however, only conditional, since there is always the possibility for a flow to occur which exceeds that to which the river is adjusted. This means that the river is inherently subject to historical contingencies -- indeed, such contingencies may render certain stable states inaccessible. The particular mechanisms for channel adjustment are determined by the strength of bed and bank materials in comparison with the forces exerted by the flow. Three distinct modes of adjustment are the production of a structured sediment bed, which affects grain resistance to flow and the reluctance of grains to be entrained, the production of sedimentary bed- and bar-formswithin the channel, which affects the within-channel form resistance, and channel deformation, which influences the river energy gradient. Channels dominated by large sediment grains develop structurally reinforced grain arrangements that have great strength; channels in smaller gravels exhibit bed structure and bar-forms that reduce characteristic sediment transport rates tolow intensity. Sandy beds can frequently be entrained by the ambient flows. Bar-forms and channel deformation are important stability-promoting mechanisms, but smaller "primary" bedforms also develop at the bed surface. Channel deformation reduces the energy gradient of the flow. These considerations are summarized in an alluvial state diagram which matches the energy gradient (or shear force exerted by the flow) with channel roughness and channel form ratios. For an imposed flow and sediment throughput, equilibrium channels occupy a restricted range in the state space. Except in very steep channels, bank strength turns out to be the most important governing condition. This is an interesting outcome inasmuch as bank condition is the least studied aspect of river channels because there is no expedient means to measure bank strength. The viewpoint adopted here recognises that, contrary to conventional theory, the character of the sediments transported through a river channel changes rapidly and is most often not similar to that of the sediments forming the bed and lower banks of the channel -- the so-called bed materials. This condition further begs for a redefinition of just what an alluvial river channel (i.e., one that is self-formed in its own sediments) is.

Bibliography of Canadian Geomorphology