Abstract
Planform change has occurred in three contiguous alluvial reaches of the Brazos River, Texas, from the 1930s to 1988. The reaches encompassed 260 km and more than 125 bends and were selected based on confluences with two tributaries. Maps of the channel were produced by digitizing the channel centerline from aerial photographs and rectifying the images on a GIS. The nature of bend planform change was described in terms of fourteen morphological variables representing bend shape and other variables characterizing the fluvial environment. A migratory activity index (MAI) was developed which provides an objective means of assessing changes in channel stability. The MAI indicated that the river's rate of migration has decreased substantially since 1939. This change in behavior results from diminished discharges and sediment loads brought about by flow regulation. Bend-planform size was found to be controlled by the presence or absence of resistant rock and sediment layers interspersed among the alluvial sediments in a small number of bends, rather than by discharge. Although planform controls on migration rates were found, the interaction of variables resulted in poor correlations with migration. Of greatest significance was that 37 percent of the bends on the river experienced negative migration, i.e., migration toward the baseline connecting the inflection points which define a bend. This phenomenon, which is not a form of meander cutoff, has not been described before, and its significance lies in the fact that it precludes the development of predictive models of migration and planform evolution. Also, of significance is- the fact that its occurrence contradicts standard theoretical models of fluid flow and erosion in channel bends which have been developed to describe conditions in freely migrating rivers. Given that most rivers are not freely migrating, the results from this dissertation suggest limitations to the applicability of models based on studies of freely migrating rivers.
Gillespie, Ben Marcus (1992). The nature of channel planform change : Brazos River, Texas. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1394914.