I think that mostly depends on the design of the dams. New dams that would be built where currently a river exists, should be built with a small bypass river, w

I think that mostly depends on the design of the dams. New dams that would be built where currently a river exists, should be built with a small bypass river, which is just large enough to allow the fish and other species to travel upriver still. And of course, locations have to be carefully selected. But any kind of human settlement or building alters the nature, be it by farming or building roads or houses. I think of all the possibilities, setting up a dam is so beneficial overall, that it is worth the tradeoffs. It should also be considered, that while altering the environment, it has a lot of local benefits too, as it creates a huge habitat for water-living species, full of clean and un-polluted water.