INTERACTIONS BETWEEN CHANNEL-BOTTOM TOPOGRAPHY THAT PRODUCE SETS OF DUNE CROSS-STRATA IN A SANDY BRAIDED RIVER

It has long been recognized that the deposition of sets of dune cross-strata require a systematic transfer of sediment out of the migrating bed forms and into the shallow subsurface. This exchange of sediment requires a reduction in the dune size as these topographic features migrate downstream. In order to place quantitative estimates on the depositional properties associated with preserved cross-beds it is therefore necessary to understand the processes that control this rate of dune reduction. I explore these processes using a combined set of low-altitude aerial photos and direct measurements of the sediment transport field, the flow field, and bed topography. These data from the North Loup River, NE, capture spatial adjustments within trains of dunes moving over bars that drive local aggradation of the river bottom. Two scales of interactions are observed in the river: 1) interactions between the larger bars and the trains of dunes covering their surfaces; and 2) interactions between adjacent dune forms that affect change in their size, shape and migration rate. Understanding the interplay between bar forms and trains of dunes, as well as between individual dune-forms are both necessary to accurately predict sediment transfer into the bed and production of dune sets. Measured rates of bed aggradation were as high as 10mm/hour on the stoss side of a bar and 55mm/hour on a lee surface. These deposition rates were very low in comparison to migration rates for the dunes transferring sediment into the bars. The average migration rate for dunes on bars was 2.5m/hour. Rates of dune migrations were relatively constant as these bed forms moved up onto and across bar tops, with one important exception, the zones where new dunes were being created. On these sections of the bed, the migration rates were as much as 3 times greater than those measured both immediately upstream and downstream from these zones of high activity. Local heights of dunes, including trough scour, were observed varying at rates up to 100mm/hour. All of these data will be used to evaluate how the feedbacks between bar-and-dune and dune-and-dune topography govern the generation of sets of dune cross strata.