Rivers are well oxygenated.
I'm not trying to argue with you.
And I have not done a back to back double blind study to quantify how much faster the plants grow in warmer water. It's anecdotal evidence at this point, based on my own humble experience.
But unlike some in these threads, I believe empirical evidence supports my contention in this case; warmer temperatures support higher metabolism rates in plants. Assuming they're getting sufficient water, nutrients, oxygen, etc, there's no reason to think they would not grow faster in warm water than cold.
I haven't tested this above 76F, I'm sure there's an upper limit somewhere.