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Longitudinal profiles of most Japanese rivers which are today not at grade can be described by either exponential or power functions. The differences in fluvial processes should reflect the differences in best fit function. In order to clarify the downstream changes in fluvial processes of alluvial river, the downstream changes in the relationships between characteristics of grain size distribution and tractive force, and between tractive force and channel slope were ex amined for the five rivers in the Kanto plain, central Japan. The composition of channel sediment was separated into several log-normally distributed populations at each sampling point of the river beds. The A-population which is the coarsest size in the separated populations is interpreted to be tractional load, and its size depends on the tractive force which is strongly affected by the channel slope. For the river expressed by an exponential function, the A-population having the grain size of -7 to -6 phi disappears abruptly with the decrease in channel slope in the middle reach . In contrast, for the river expressed by a power function, the A-population having the grain size of -7 to -6 phi is distributed down to near the river mouth , because the decreases both in the curvature and slope of longitudinal profile are small. The downstream limit of the depositional area of the A-population having the grain size of -7 to -6 phi corresponds to the position where the tractive force markedly decreases associated with the decrease in slope along the course of a river. At these positions, the values of the first derivative of the functions best fitting the profiles show about 1/000 for five rivers. These positions migrate downstream when the best fit function type changes from exponential to power due to the difference in their mathematical characteristics. The distributions of grain size along the middle reach of an alluvial river show the characteristics peculiar to the best fit function type of river profile governing the downstream changes in hydraulic conditions. This implies that both the fluvial processes and the development of the fluvial landforms can be evaluated from the morpholog ical properties of the longitudinal profiles; the best fit function type and the curvature .

Downstream Change in Grain Size of River Bed Sediments and Its Geomorphological Implications in the Kanto Plain, Central Japan