It seems likely that they simply designed it for a max expected water temp and they're exceeding that max. If you need more water to cool the same amount, you're gonna need some bigger pipes.

It's not an issue of being unable to cool the plant, it's an issue of the plant being prohibited from heating up the river too much. Apparently this is done for ecological reasons, though seeing as it results in more fossil fuel emissions it might be better to hear the river.