THE RIVER CHANNEL PATTERN CHANGE AS INFLUENCED BY THE FLOODPLAIN GEOECOSYSTEM - AN EXAMPLE FROM THE HONGSHAN RESERVOIR

The floodplain geoecosystem can be defined as a system composed of fac tors such as floodplain morphology, alluvial materials, soils, groundw ater and vegetation etc. A strong coupling relation between the floodp lain geoecosystem and the channel system is established by the matter and energy fluxes through their interface. Before and after the constr uction of the Hongshan Reservoir, the succession of the floodplain geo ecosystem in the wide-valley river reach upstream of the reservoir can be generalized as three stages, namely, the semi-desert geoecosystem, the marsh and swamp geoecosystem and the agricultural ecosystem. Afte r the upstream-reservoir floodplain ecosystem turned from a semi-deser t to a marsh and swamp, the dynamics of matter exchange between the fl oodplain and the channel has been changed significantly, leading to a strong reformation of the composition of floodplain material. Before t he growth of the aquatic and marsh weeds, the clay content in the bed material was very low. After dense weeds appeared, the water flaw velo city declined greatly, a factor favorable to fine sediment deposition. So the clay content of bed material increased rapidly reaching an ave rage value of 45%. With the narrowing of the channel due to the strong sedimentation induced by aquatic weeds growth, the width-depth ratio of the channel tends to be much smaller, associated with a lower and l ower degree of braiding. Finally the channel becomes a single-thread o ne. In the meantime, the channel sinuosity increases continuously, fro m 1.06 in 1960 to 1.27 in 1985, and the channel is tending to a meande ring one, from the wandering braided pattern in the predam period.