A bio engineer once told me that dams are giant bio fermenters. Their reservoirs collect huge mounts of bio matter which then collects in the basin and starts to rot. The rotting process liberates CO2 and methane, much of which remains in the silt and the mud. Until, something changes. This something is often water pressure and that’s exactly what is happening when a reservoir is used as a giant battery for wind and solar power. When you pump water up, surface levels rise and pressure builds up. When the reservoir gives water back to the power plant to make energy from the water, the pressure drops. The constant pressure changes make sure that huge amounts of CO2 and methane get into the atmosphere.
For many, hydroelectric dams seem as if they should be environmentally benign, delivering large quantities of carbon-free electricity. The water is renewable, there are no fossil fuels involved and you end up with lovely lakes.