However, opponents assert that the upper San Joaquin River, which is already controlled by dozens of smaller reservoirs upstream of Millerton Lake, will not provide sufficient discharge to fill the reservoir except in very wet years. The new reservoir would flood of the San Joaquin River including whitewater runs, fishing areas and historic sites. It would also inundate two Big Creek Hydro powerhouses, causing a potential net loss of electricity generation. There would be significant evaporation losses from the reservoir, and the water required to fill it would put further pressure on already stressed water resources in the San Joaquin River basin.