And that reservoir can emit very significant biogenic methane. Large reservoirs for pumped storage are likely not a scalable option for climate change.
One thing to note. While biogenic and thus "green", the methane is about 30 times as potent greenhouse gas (over 100 years), so we can not consider pure "carbon footprint" here. There are similar problems with restoring wetlands and undraining peatlands, which used to be carbon sinks, where biogenic methane production goes up for a while.
There are also problems with N2O emissions on the reservoirs, specially near the agricultural regions. And this is all without even talking about reducing biodiversity and destroying spawning areas for the fish by filling those with mud. So yes, efficiency is not always friendly.
