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Bed material texture along gravel bed rivers with confluences Rice, Stephen Philip


Changes in bed material texture are examined along two confluent gravel-bed rivers. The work is distinguished from previous field studies by the scope and resolution of the sampling programme. A combination of 400-stone Wolman counts and photographic sampling provide surface grain size information at approximately 50 % of the bars along two 110 km reaches. The data reveal negligible overall fining and complex structure which can be explained only by reference to a large number of tributary inputs, non-alluvial sediment sources and the legacy of Pleistocene glaciation. A series of distinct sedimentary links are demarcated by significant lateral sediment inputs. Categorization of individual samples according to their location within particular sedimentary links significantly reduces unexplained variability. Texture tends to vary systematically within links and reflects the uninterrupted operation of sorting and abrasion processes. This implies that successful application of downstream fining models depends upon isolating significant lateral sediment sources and in turn the intervening sedimentary links. Therefore, guidelines for the a priori identification of significant tributary sources are developed. An empirical discriminant function based on relative basin area and a surrogate measure of distal tributary stream power is particularly successful, although its general applicability cannot be assessed for want of a suitable data set. A corresponding logistic model estimates the probability that a given tributary is associated with a textural discontinuity. Within sedimentary links downstream changes in D50 and D95 (mm) are adequately described by exponential functions, although power and linear functions of distance are equally valid in some cases. Diminution rate does not vary significantly between lithological groups, despite differences in relative abrasivity, and lithological composition is a minor control on diminution rate. An unequivocal statement concerning the relative importance of sorting and abrasion processes is not possible because relative abrasivity is confounded by a lithologydependent size effect. However, several lines of evidence suggest that sorting is the dominant fining mechanism. Second-order polynomial models describe the profile of individual sedimentary links such that slope is a simple linear function of distance. Channel slope and diminution rate are, in general, positively correlated. However, the prediction of diminution rate using slope is only marginally successful because slope and grain size, for reasons of history and circumstance, may be unadjusted within individual links. The study makes clear that extant models of textural change are inadequate when applied in all but rudimentary field situations. Greater predictive efficacy depends upon a better appreciation of, and ability to accommodate, the multiplicity of sediment sources which characterise real fluvial systems. This belies the need to consider both immanent and configurational attributes if geomorphological investigations and model building are to be applied at landscape scales. Models of contemporary fining processes must be situated within specific sedimentological networks which may not correspond with the hydrological network because sediment supply, unlike water supply, is spatially discontinuous in fluvial landscapes. Identification of the sedimentary network is dependent on careful examination of local geography and history and thereby provides a useful framework for reconciling process modeling and landscape contingency.

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