It seems unrealistic to expect this amount of power to be dissipated in such a small area without some significant collateral damage.
I would take your final statement a step futher: I am CERTAIN most people vastly underestimate just how hard it is to dissipate that much energy.
As an example, and just for starters, you run a high risk of creating your own weather from the enormous amount of spray you are certain to generate. With that much energy to dissipate, it seems highly likely the spray will be energetic enough to generate lightning.
I'll even go so far as to guess this question was never even asked when the dam was designed. I'm not a civil engineer, but it's easy to see how they may not think of it now either.
REAL WORLD EXAMPLE: Way back in ancient history (only slightly before my time), a volcano erupted in Iceland. The lava flow threatened a developed area. Some genius decided the lava could be stopped by dropping water on it.
The Air Force loaded up some aircraft with water and they took off for Iceland.
Then someone spent a few minutes calculating how much energy would be released if all that water vaporized (it would have). The energy release would have been equivalent to a nuclear bomb. Not released in a single split second, but fast enough to be devastating none the less.
The planes were called back while in flight. I learned this from one of my engineering profs while I was a cadet at the Air Force Academy. I have no idea if it ever made it into the news, but it certainly made a permanent impression on me.