11.5 SPECIAL METHODS FOR MITIGATION

Several specific measures may need to be taken to mitigate for the negative effects of dams.

Restoration of Flow Patterns--
One of the most serious effects of dams on vegetation, some invertebrates and fish downstream is the loss of the natural flood pattern, and the benefits that come from annual flooding. For this reason, increasing attention is being paid to releasing water from dams to create artificial floods that resemble the natural event sufficiently for floodplain vegetation to grow, seasonal invertebrates to hatch and reproduce, and for fish to breed and grow. The requirements for such releases will vary from river to river, and also according to the different species of fish, but should be at least at the minimum ecological flow. It is essential that provision be made for ecological releases when planning the construction and operation of dams.


Restoration of Special Habitats--
Two types of habitat are most affected by dams, the stony spawning substrates of riffles in or below the tailwater of the dam, and the deep pools used as refuge by large fish in the channel below the dam. Spawning substrates may have to be artificially replaced as they are washed away by the erosive capacity of the water. Deeper areas may have to be dredged in the channel where these become silted in.