My assumption is that rising CO2 levels in the atmosphere pose a much greater existential threat to humanity than localized ecosystem damage resulting from river blockages. But maybe I'm wrong.

Speaking of hydro, I do think pumped hydro-storage ought to be looked at a lot more for energy storage (esp. versus giant lithium-ion battery banks), especially as we transition to inconsistent renewable sources like solar and wind. I'd assume that creating new, isolated bodies of water wouldn't incur as much ecological damage as blocking off existing rivers or greatly increasing our mining of rare earth minerals)