The downstream diminution in sediment size in a braided reach of the proglacial Sunwapta River, Alberta, Canada, was examined statistically to identify the sources of the observed variation about an expected exponential relationship between clast size and distance. Major deviations from this hypothetical relationship, such as a relative increase in grainsize, may be attributed to the effects of tributary sediment inputs and downstream changes in channel behaviour, whilst local variation is associated with complex patterns of sediment deposition observed at a bar scale. A comparison of diminution coefficients, calculated for separate lithologies and for subreaches along the river, with those obtained from previous studies, is used as an indicator of river behaviour and sediment transport processes. It is shown that rates of diminution vary within the reach in response to differing rates of aggradation and to the backwater effects created by tributary alluvial fans. The relatively high values for the calculated diminution coefficients indicate that processes of differential transport are the main cause of the grain size decrease.
Facies analysis of Severn Main terrace sediments at Eardington, Shropshire is instructive in determining aspects of the sedimentary environment at a 'proximal' location in the terrace. Evaluation of the structural and directional properties of the gravels indicates that the terrace is a composite feature comprising two units. The lower unit is notable for the widespread Occurrence of large sandy facies, a near absence of massive gravel facies, and the presence of large, locally derived, lithoclasts in excess of 1 m in diameter. Facies associations observed indicate the existence of in-channel bars and large channels with bedforms at the dune-plane bed transition. The directional properties of the unit indicate flows parallel to the main valley axis and although the unit is unlike previously reported examples it is interpreted as being the product of a proximal low sinuosity environment. The upper unit is composed mainly of multistorey, coarse, massive gravel units with limited interbedded sand facies. Fabric analysis indicates progradation of the unit from the west. The facies sequence is similar to those encompassed by facies models proposed for proximal alluvial fan deposits and therefore, on structural and directional grounds, the upper unit is interpreted as an alluvial fan deposit which prograded into the main Severn valley from the adjacent Mor Brook tributary. The implications of the composite nature of the terrace at this point are considered both in terms of terrace correlation and palaeohyrological estimation.
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