Large-scale bed configurations (macroforms), Platte River Basin, Colorado and Nebraska: Primary structures and formative processes

Abstract

Large-scale, current bed forms, defined here as bed configurations having dimensions commensurate with the size of the turbulent boundary layer, are considered by many workers to be hydrodynamically equivalent to the regime bed forms. These bed forms were investigated at formative flows (bankfull flows) on the South Platte and Platte Rivers in Colorado and Nebraska to determine structures and formative processes. Results of this study suggest that large-scale bed forms, here termed macroforms, are not hydrodynamically equivalent to the regime bed forms but constitute a unique hierarchical class of bed configurations produced by turbulent vortices that involve the entire boundary layer. Macroforms are oriented obliquely to the mean flow direction and are attached to one or both channel banks. Three members of a continuum of geometries are recognized in the channels of the Platte River Basin. The internal stratification for each of the three macroform types recognized in the basin is similar and in its simplest form consists of the coarsening-upward sequence apron laminae-foreset laminae-topset laminae. Although it may not be possible to differentiate the three macroform types solely on the basis of internal stratification, the presence of a distinctive coarsening-upward sequence in coarse-grained fluvial systems offers a potentially powerful tool for identifying these environments in the geologic record.