Forms and processes of two highly contrasting rivers in arid central Australia,and the implications for channel-pattern discrimination and prediction

摘要

Description and quantification of the diverse controls on alluvial-river styles have long been key themes in the fluvial literature,but many uncertainties and contentions remain.On the Northern Plains in arid central Australia,ephemeral rivers are commonly low sinuosity (P<1.15) but vary from single thread to anabranching,thus providing an opportunity to examine concepts of channel-pattern discrimination and prediction.Comparison of the closely adjacent middle reaches of the Plenty and Marshall Rivers demonstrates that although channel-bed gradient,discharge,and bank strength are essentially similar,bed-material caliber and the pattern of tributary drainage are markedly different.These differences result in strong contrasts in channel cross-sectional geometry and planform.The Plenty River,which transports medium to coarse sand and is joined by few tributaries,remains predominantly single thread but is variably wide (approx100-1200 m) and in places appears transitional to braiding.By contrast,the Marshall River,which transports coarse sand to granules and is joined by several minor tributaries,has numerous narrow (usually<60 m wide),anabranching channels that divide around vegetated ridges and broader islands.It is important to note that whereas channel-pattern discrimination diagrams predict that hydraulic,sedimentary,and energy conditions are conducive to the development of meandering or braiding on these rivers,both the Plenty and the individual Marshall anabranches remain essentially single-thread,planar-bed,low-sinuosity ("straight") channels.This fact appears to be due primarily to the high degree of bank strength provided by riparian vegetation and secondarily to certain flow and sediment-transport conditions characteristic of these ephemeral rivers.In particular,field observations and theoretical model results suggest that (1) a wide,single-thread channel relatively free of in-channel trees is sufficient to convey a sandy bed-material load (the Plenty),but (2) where tributaries provide small supplies of water that encourage the growth of obstructing in-channel trees or introduce some additional coarse sediment,then the formation of narrow anabranches minimizes flow roughness and maintains sediment throughput (the Marshall).Hence,although major changes in fluvial processes can be produced by subtle differences in water and sediment supply,the resulting channel patterns cannot be predicted by traditional discrimination diagrams that are based solely on simple combinations of morphometric,hydraulic,and/or sedimentary variables.