Dams, weirs and other obstacles may seriously
affect all fish species: migratory fish which need
to move between their freshwater habitats and the
sea in order to reproduce and non amphibiotic fish
which need circulating along the river system. River
bypasses, fish ladders or other types of passes may
maintain or improve passing at one point; they
however do not address the degradation of habitat
because of the still water mass created just upstream
of the obstacle.