Incorporation of bed texture into a channel evolution model

Channel evolution models (CEM) have been developed to qualitatively describe the morphological adjustments of channels undergoing incision, but the grain size of the river bed material has not been addressed in existing evolution models. Herein, bed material grain size is incorporated into an existing CEM to describe the sequence of grain size changes. The analysis is based on the data from bed material that were available from a 1986 sampling program in northwestern Mississippi. Samples were taken along three sand-and-gravel-bed channels at 300-m intervals. To provide a comparable data set, sampling was repeated in 1996. Observed longitudinal grain size distributions were highly variable in space and time. Overall downstream fining trends were absent. Bed texture in incising channels is as dynamic as channel morphology, with composition shifting from a mixture dominated by sand to one dominated by gravel, or vice versa, within a decade or less. The modified CEM predicted direction of changes in grain size in a meandering incising channel, but not within two straightened, incising channels, most likely due to the complex influence of upstream and lateral sediment sources (bed and bank erosion). We suggest that over the temporal (10 years) and spatial scales (~10 to 20 km) of this study, sediment sources are the dominant factor in the development of longitudinal grain size distributions.