Abstract

Current orientation patterns in the sand bed of the braided-to-meandering transition zone of the Red River vary with discharge and flow character, and do not correspond with overall river trend. Patterns produced by flood and falling discharge have high azimuthal dispersions in channels with wedge-shaped cross sections and areas of low topographic relief, and low dispersion in areas where the channel is constricted or where a chute is formed during the flood. Low discharge patterns have high dispersion in tabular channels where bed relief is high.

Three types of flow which combine to form a sedimentation cycle were discerned during the study. They are: (1) channel flow/low discharge; (2) sheet flow/flood discharge; and (3) post-sheet flow/falling discharge. These types of flow are differentiated on the basis of their characteristic sedimentation load, flow pattern, channel morphology, and bed form occurrence.

Overall current directions measured during low discharge, when channel flow was dominant, show a 21.78° mean deviation from the river trend, and an azimuthal spread of 285°, Current vectors measured after a flood, when sheet flow and post-sheet flow conditions prevailed, show a 25.08° mean deviation from the river trend, and a 332.5° azimuthal spread. Variations in river bed relief, flow behavior, sedimentation, and channel bank stability resulted in variable orientation patterns which do not have a downstream trend in azimuthal character.

Paleocurrent studies of deposits which formed under conditions similar to those described in this study must take into account the great variability in flow patterns as well as the local river bed topography which determine the type and extent of sediment depositon.