People have been using the energy of flowing or falling water for centuries before electricity was developed into a practical means of transporting energy. The first hydroelectric plant (at least in the U.S.) was built at Niagara Falls in 1895. Many areas of the world still get the majority of their power from hydro facilities.
Hydro is also “conventional” in the sense that it can be controlled in more or less the same way as thermal plants — its output can be adjusted over a wide range to match demand. Wind and solar, on the other hand, only produce when the wind blows or the sun shines. Of course, hydro can only produce as long as there’s rain or snowmelt to supply the reservoirs, but that’s on a much longer time scale; over hours or days, hydro capacity is very predictable.
