Less water flowing into it, which can change the mineral and salt composition of the water, cause changes in flow and temperature, and these in turn can dramatically change the types of plant and animal life in the area.
Of course, this is a moot point because virtually all major rivers in the world have been dammed up already.
The damming of the river upstream, may result in the estuary mouth closing
How might the damming of an river affect an estuary at the river's mouth
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