| Abstract: | A mathematical model based on data for the period 1957-1988 representing changes in space and time in the bed morphology of the 623 km long alluvial reach of the Brahmaputra River in Assam State, India has been developed. This model reveals that in the upper reaches the bed level decreases downstream exponentially with distance from a reference point, while in the lower reaches the exponential decrease is at a smaller rate probably due to differences in tectonic setting. The lower reach is marked by sinusoidal oscillations with a time period of about 9 yr. High runoff in 1977 seems to have caused a significant change as the maximum, average, and minimum bed level were decreasing before 1977 and these increased just after 1977 and then started decreasing at a lower rate than before. Smaller changes in shorter reaches governed by input of discharge and sediment from tributaries are superimposed on the overall degradation/aggradation character. The variation in degradation/aggradation with time is sinusoidal with a wavelength of 4,450 km before 1977 that reduces to about 1,038 km after 1977. |
