Another effect of a dam or barrage is to change the sedimentation regime in a river. Most rivers carry some sediment downstream with them. Some, such as the Nile, carry enormous quantities of fertile material upon which a whole civilisation has depended.

When a dam is built across a river the reservoir behind it reduces the flow rate of the river and much of the sediment can precipitate onto the reservoir floor. In the worst case this will lead to the eventual filling of the reservoir with sediment. In more propitious circumstances a steady state will eventually be reached and transport of sediment downstream will become reestablished.

Whatever the situation, the amount of sediment flowing past the dam will be reduced, at least initially. This can have important consequences downstream. Erosion rates may increase in the riverbed below the dam.

More seriously, important ecostructures which rely on the sediment and its nutrients may become seriously disrupted or even destroyed.

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