Incorporation of bed texture into a channel evolution model

Channel evolution models (CEM) have been developed to qualitatively describ e the morphological adjustments of channels undergoing incision, but the gr ain size of the river bed material has not been addressed in existing evolu tion models. Herein, bed material grain size is incorporated into an existi ng CEM to describe the sequence of grain size changes. The analysis is base d on the data from bed material that were available from a 1986 sampling pr ogram in northwestern Mississippi. Samples were taken along three sand-and- gravel-bed channels at 300-m intervals. To provide a comparable data set, s ampling was repeated in 1996. Observed longitudinal grain size distribution s were highly variable in space and time. Overall downstream fining trends were absent. Bed texture in incising channels is as dynamic as channel morp hology, with composition shifting from a mixture dominated by sand to one d ominated by gravel, or vice versa, within a decade or less. The modified CE M predicted direction of changes in grain size in a meandering incising cha nnel, but not within two straightened, incising channels, most likely due t o the complex influence of upstream and lateral sediment sources (bed and b ank erosion). We suggest that over the temporal(10 years) and spatial scale s (similar to 10 to 20 km) of this study, sediment sources are the dominant factor in the development of longitudinal grain size distributions. (C) 20 00 Elsevier Science B.V. All rights reserved.