There are always constraints. But technology for building roads adequate for transporting heavy machinery, and for building hilltop reservoirs using it, is extr

There are always constraints. But technology for building roads adequate for transporting heavy machinery, and for building hilltop reservoirs using it, is extremely mature. Dozens were built in California's Sierra Nevada range in the 1920s using pulley-operated equipment. Those were actually hydro power reservoirs feeding penstocks that have since had pumps attached. Power loss from flow in penstocks is typically negligible.