A lot of the dams being taken down can be replaced by an efficient gas powered generating plant.
So you take down a clean hydro plant that uses a renewable energy source. And replace it with an emissions producing gas plant that uses a potentially finite fossil fuel? Why?
Hydroelectric power generation is by far the most efficient method of large scale electric power generation. See Comparison Chart. Energy flows are concentrated and can be controlled. The conversion process captures kinetic energy and converts it directly into electric energy. There are no inefficient intermediate thermodynamic or chemical processes and no heat losses.
The conversion efficiency of a hydroelectric power plant depends mainly on the type of water turbine employed and can be as high as 95% for large installations. Smaller plants with output powers less than 5 MW may have efficiencies between 80 and 85 %.
It is however difficult to extract power from low flow rates.
No.
There is no such thing as an “efficient” gas powered generating plant.
Natural gas is most efficient when used directly as a power source; for example, a gas cook range, a gas water heater, or a gas furnace.
Using it to generate electricity is wasteful. Nuclear is the most efficient use of creating electrical energy and hydro the next. Natural gas and coal are much less efficient.