Norwegian hydroelectric plants usually don't destroy a ecosystem, just some nice looking waterfall. They have so long and steep falls that ecosystem would be naturally separated by it anyway.
Most hydroelectric power in Norway are generated with the help of a reservoir, usually a natural lake that's been regulated with the help of a relatively small dam, but there are some wholly artificial ones as well, the biggest one being 84 square km.
That said, yes - the environmental impact is small. It also helps that the Norwegian population is very small.