Process of River Reformation and Impact on Navigation Channel Condition n Near-dam Reach in Different Types of Rivers Downstream the Junction

The reservoir operation changes water and sediment factors at the downstream reach.And the change of runoff process is the key factor which initiated river bed adjustment in downstream of the dam.It changes the hydrodynamic condition,the bed building ability and sediment discharge of different rate of flow.Based on the observed data of the near-dam reach downstream the Three Gorges and Danjiangkou river,this paper analyzes the characteristics of erosion and deposition adjustment after impoundment in different river patterns,and the impact on navigation condition.The result shows that the erosion presents as incision when the riverbed is relative narrow and deep;To the low-water branching reach with strong boundary conditions where the riverbed is wide and shallow,the scour and deposit variation characteristics mainly presents as erosion and deposition adjustment of river island,height deviation of thalweg in the inlets and the adjustment of split ratio after impoundment;To the low-water branching reach where the riverbed is wide and shallow and the lateral boundary conditions are weak,the scour and deposit variation characteristics is not restricted to the adjustment of split ratio,and the pattern of the braided reach may even be changed.It will bring new channel problems at all adjustments.