I think some really large and powerful big water rapids on the Gunnison River were lost when they built Morrow Point Reservoir, just downstream from Blue Mesa.
My dad is a big train buff (a foamer for those of you in the know) and he's got some movies that have old footage from the 1940's and 50's of trains rolling through the upper Black Canyon (some of them in color) and some of them were during high water, and you can see some large, powerful big water rapids in the river below the train. Not unrunnable cascades like in the Big Black, but rapids like you would see in Gore (but less blast rock).
The section from the current Blue Mesa Dam to the Cimarron confluence isn't as steep as the big Black downstream, and it was shot before the era of any dams, so the Gunny in flood at that point could have been pushing 4k to 7k, or more. Pretty incredible to see! I've got those movies on my hard drive at home, next time I cruise home for a visit I will try to upload some of them. The rapids look big and powerful but certainly runnable, and if the dams didn't exist it would be a run that flowed year round. Definitely a big loss.
"Don't f$&@ing eddy out, just run it! Whaddya doin??" -LMyers