Bed material texture along gravel bed rivers with confluences

Stephen Philip Rice
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
more » ... s, 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 [...]
doi:10.14288/1.0087248 fatcat:nefophwfpvhfpcbxv45k5jbo5a