WANDERING BRAIDED RIVER CHANNEL PATTERN DEVELOPED UNDER QUASI-EQUILIBRIUM - AN EXAMPLE FROM THE HANJIANG RIVER, CHINA

This paper deals with the wandering braided channel pattern and its fo rmative mechanism, with the middle Hanjiang River as an example. The m iddle Hanjiang River is characterized by a multi-braided, shallow and wide channel with a number of unstable midchannel bars and a frequentl y shifting thalweg. However, before the construction of the Danjiangko u Reservoir it basically exhibited an equilibrium of sediment transpor t. The formation of the wandering braided channel pattern of the Middl e Hanjiang River is not caused by strong sedimentation as in the lower Yellow River, but is controlled by strong erosion of the river bank. Large quantities of sediment are supplied to the channel by bank erosi on; after exchange with the sediment carried from the upstream, the se diment from bank erosion is deposited in mid-channel bars. While some new mid-channel bars are formed, some old ones become attached to the bank. In this way, a wandering braided channel pattern with many unsta ble midchannel bars can develop under a condition of sediment transpor t equilibrium. Mineral analysis indicates that the material from bank erosion constitutes a major material source for mid-channel bars build ing. Based on a plot of channel width against bankfull discharge, a di scriminant relation has been established to distinguish between wander ing braided and the non-wandering channel patterns.