I don’t know, putting solar panels in the drained reservoir of a former dam which might have been built for flood control seems a bit iffy. After the next really big rain storm or snow melt, the flood of the no-longer-tamed river could destroy the solar farm.
The Guardian article mentions that Edwards Dam and 3.5 MW hydropower plant in Maine was removed in 1999. https://en.wikipedia.org/wiki/Edwards_Dam
“Because the Edwards Dam did not serve any of the purposes of flood control, irrigation, or substantial hydropower production, there was much more purpose to bring it down.”
However Edwards Dam has not been replaced by 3.5 MW of solar power, so I’m not convinced that’s a good example of how “undamming rivers can offer a new source for clean energy”
The article claims “If the Conowingo Dam were removed, this would free up more than enough area
to replace the lost hydroelectric generation with power from solar parks along the former reservoir bottom”. Then it fails to support that claim, which turns out to be untrue. The Conowingo Dam provides 572 MW of power, but the article distracts us by bringing up the 392 MW Ivanpah solar power plant in California and ends up saying “Thus, about three-quarters of the river bottom would need to be in solar to match the output of Ivanpah.” If 3/4 of the river bottom could match the 392 MW output of Ivanpah, then the entire river bottom would be able to provide about 523 MW, which is not enough to entirely replace the 572 MW of Conowingo Dam. You couldn’t even use the entire river bottom anyway because there would still be a (narrower) river running down the bottom of it.
It may make good sense to remove certain dams, but I’m still not sure it’s a good idea to put a solar farm on the river bottom. The deluge from another Hurricane Agnes could wipe it out.