I was curious (and didn't have a prior), but the answer appears to be yes. I found the largest man-made reservoirs on Wikipedia: the largest 100 manmade reservoirs in the US have about 8500 sq mi of surface area. Assuming 15% efficiency you'd need something like 20,000 square miles of solar installation to power the US based on this Dept of Energy document: Link.

More interestingly, the largest 25 reservoirs in the US have just under 5000 sq mi of surface area.

Many of these reservoirs are in places that don't receive full sun of course. But I think people underestimate just how large some bodies of water are. An area the size of, say, Lake Erie would be sufficient to provide solar power for almost all of the US with full sun - less than 1% of land area in the contiguous US. The federal government owns about 40 times that much land already, much of it in places that are ideal for solar.