The effect of the construction of Weir or Barrage on the regime of the river are given below,
that a weir is constructed across the river at a certain location
(A). The immediate effect is an increase in water levels upstream
(B). This increase is not uniform but varies from place to place.
As a consequence, flow velocities differ from place to place too. The variations in flow velocity and, hence, the variations in the capacity to transport sediment give rise to a pattern of initial erosion and sedimentation along the river . Sedimentation over a relatively long distance occurs upstream of the intervention, whereas punctuated local erosion occurs downstream. This erosion advances downstream as a rarefaction wave.
Eventually, in the long run, the river reaches a new morphological equilibrium without further trends of erosion or sedimentation . Upstream bed and water levels have become higher than at the start of the intervention. Downstream, the river resumes its original shape by sedimentation after an intermediate period of erosion.
The longitudinal profiles in the diagrams provide a simplified picture. They do not include the response of channel width, bed sediment composition or vegetation. Nonetheless, they offer the key to understanding the relation between local pressures or measures and their effects far upstream and downstream