[belval]

Hydro has a big upfront cost because you submerge a lot of land which usually was covered with trees. It also adds a bunch of mercury to the water. However over the lifespan of the dam this is not a real concern especially compared to gas/oil. Furthermore if you need to account for energy stockage with batteries for wind/solar as they are conditional to the weather the environmental cost analysis becomes much closer as you have to account for all the manufacturing externalities. Hydro on the other hand can increase and reduce production by drawing from its reservoir which makes it at least as green as other renewables that we have right now.