"Nothing could be done to stop the flooding above the dams, but much could have been done to alleviate any flooding below them. Let's not let it happen again!"
You got that right. You only need to look at the job done by FE Walter Dam on the Lehigh River to know that is true. It held back 30 billion gallons of floodwater and never topped the spillway. It had 22,000 cfs flowing in and it released only 250 cfs during the height of the flooding. It survived and offered downstream protection from a rain event like Hurrican Diane which it was modeled after when it was built. It saved downriver communities from much more serious flooding.
Once a dam has water going over the spillway the river is totally out of control. If you have a reservoir at 100 capacity and get a significant rain event the dam is absolutely useless, in fact it may be detrimental to upstream flooding.
Look at what East Sidney Dam did to Franklin NY. It backed the floodwaters all the way up into town, and at the height of the flood released all of the water that was flowing in contributing greatly to the flooding downstream on the Susquehanna. I'm not convinced an over filled Cannonsville didn't do the same to Walton, NY. When Cannonsville is at full pool the river is backed up to just a few miles below town. A 300 foot wide swath of water can't flow out of a 100 foot wide spillway as fast as it flows in.
If the water can't run downstream where else is it going to go?