Depending on location, hydroelectric power can have (mostly because of decomposing vegetation) carbon-equivalent emissions ranging from several grams of COâ‚‚ per kWh, all the way up to around two thousand grams (!) per kWh. This greatly depends on location, with shallow, large-area tropical dams being the worst offenders, and cold, high altitude Nordic dams being the most environmentally friendly.
IIUC, the vast majority of ecological impact caused by hydro is already done. It is not making things exponentially worse.
Am I missing something?