Look into the new science on that one. It can make carbon sense when you flood a desert to feed a hydro dam, but if you flood a forest then the carbon math isn't so great. Microhydro seems an answer, but there too you have to calculate how much forest is being deprived of water, and what that means for carbon uptake. It's probably still better than coal, but it isn't perfect.
Generally with small-scale microhydro (low single digit acres) you get good biomass production because you've created more edge. Edges between biomes (estuaries, forest-field transitions, etc) are the most productive and diverse places in nature because you get overlapping species and the associated beneficial interactions.
Many small dams produce a lot of forest-water edge cumulatively, unlike a few huge dams.
Look into the new science on that one. It can make carbon sense when you flood a desert to feed a hydro dam, but if you flood a forest then the carbon math isn't so great. Microhydro seems an answer, but there too you have to calculate how much forest is being deprived of water, and what that means for carbon uptake. It's probably still better than coal, but it isn't perfect.