In the US the environmentalists are call for dams to be torn out because they interfere with salmon migration. Even though all the dams that I know of have Salmon ladders. Hydro is a big deal in Vermont, Oregon and Washington. Its the reason those three states (if you do consider it a renewable) have been more than 50% renewable for decades. MT, SD, TN, NY have respectable amount of hydro as well, but we pretty much have maxed out capacity. Many of the rivers in the rest of the US don't have enough vertical drop to make hydro worthwhile. There are a few, such as the Platte that may work. and others in northern California that would work if the environmentalists didn't oppose them.
As it so happens my SO works on salmon and dams. A few points:
Fish ladders don't work nearly as effectively as once though, and dams (even with decent fish ladders) are sizable barriers to salmon migration. As big a problem as they are to upstream migration, there's an additional problem with downstream migration; fish that errantly choose the turbine path get pummeled to bits, ones that go over the spillway often die (falling from great heights and whatnot), which leaves downstream fish-ladders... which many dams don't have and the ones that are only semi-effective.
Also, this isn't just environmentalism for environmentalists sake. Salmon are important both for the ecosystem and for fisheries. In California (until very recently) the Salmon fishery was valued at over $4B. Now it's frequently closed for the entire year. In Puget Sound and BC it's a much bigger (and currently sustainable) fishery, in part because dams there were better constructed and because we use methods other than fish ladders to bypass dams
such as active transport (capturing and trucking them above dams and natural barriers). There's an entire scientific conference every year that's dedicated to nothing but studying and improving fish passage, because its so important and because its still far from optimal. Ecologically, salmon (and their carcasses) are pretty vital to streams. Those environments tend to be nutrient-poor, and salmon (+ alewife and others) are a huge transfer of ocean productivity into terrestrial environments. For example, trees which grow near the river benefit from these nutrients, and you can actually see a "salmon signature" in the wood of trees. No salmon, less tree growth.
The elephant in the room here though is cost. In order to be economically competitive, dams need to be very large. Currently Canada has two major hydro projects, one in Quebec's Romaine river, and another one in BC on the Peace River. Current costs are $13B and $8.3B, respectively. That dwarfs the cost of medium-sized LNG plants, coal plants, wind farms, etc. Only nuclear plants cost more (and there have been no completely new ones built for a generation due to cost, complexity and regulatory burdens. Dam construction takes longer than any other electricity project; the Rivière Romaine damn will take 15 years assuming it doesn't run over. For a region it also "puts all your eggs in one basket" - a bit risky, especially given that droughts, sediment runoff, natural disasters and additional regulatory burdens (e.g. What you must do if your fish ladder does not operate as planned - a common problem)
I'm not saying dams are bad. I'm glad that most of our power here in Quebec comes from hydro, though its far from a 'perfect' technology. There's just a lot more issues at play than people often realize. Hope this primer was a bit useful.
ETA: Final note: we are moving increasingly to a less centralized system of power generation. This has positives and negatives, but things like solar and wind turbines and much smaller (yet efficient) LNG plants are allowing small municipalities to generate much of their power. Hydrodams, particularly the very large ones needed to be economical, move in the opposite direction. This presents challenges with power distribution (new corridors of transmission lines are highly controversial, as they have a large overall footprint and get into NIMBY politics).