Since the middle of the 20th century, humans have significantly altered the hydrological and hydraulic system of European rivers, with (hydropower) dams, dredging, rectifications, channelling, etc. One of the most damaging effects of these activities results from constructing crossing works over rivers (dams, waterwheels, bridge foundations, etc), which frequently impede or limit the free movement of fish fauna.
The solution to the migratory problem caused by such obstacles is to either demolish the obstacle or to construct an additional structure which facilitates the upstream/downstream movement: a fish pass (commonly known as a “fish ladder”).
Note 7 Fish-passes: fish ladders and other pass systemsdownload
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