What are the ecological impacts to migrant fish species from building a hydro dam?

Ugly, loss of land habitat, impedes silt from reaching lowland where it is important, impededs fish migration and reproduction, causes a risk of catastophic flooding if it fails, creates a prime...

It depends on what kind of fish you are talking about. There are many different specie of fish.

it is a wall kind of thing that blocks the water from going to the other side.

A fish isn't a species. A type of fish is a species.

Possibly not the best phrased question however you could say that a hydro dam is a dam that's primary function is the generation of hydropower. I note that under the heading of dam there are links to...