Sedimentation in the meandering River Endrick

Abstract

Synopsis The River Endrick carries gravel and finer sediment in a pool and riffle channel. Point bars and riffles, which alternate along most of the channel, are foci of active particle-size segregation within the general downstream size decline: they are therefore centres of facies change. This differentiation results in sediment of a particular size being systematically and repeatedly placed in similar parts of the channel: on point bars, for example, proportionately more gravel is found at the apex and more sand in the cross-over reaches of the meander. Cross-stratal dip direction in these sediments is roughly parallel to the local flow and is therefore different in sand and gravels. It also follows that sand has a higher standard deviation of cross-bed dip orientation than gravel. The bed forms of the river often have only a superficial thickness of related sediment: they were built by a lower stage of flow than most of the sediment enclosed by the form, and in this sense forms give an erroneous measure of structure. The meanders move phase downstream so that at point bars crescents of differentiated sediment accrete on the inside, as older crescents are eroded from the outside or cut bank of the channel. In this way the river builds up a facies mosaic whose interface zig-zags downstream, and the cut bank exposes a section with a lateral size decline opposite to that of the facing point bar. Sedimentary successions show more variability within than between bars and the use of this fact, together with the variation in dip of cross-strata and other criteria, facilitates the independent reconstruction of the scale and sinuosity of the river.