Dude, it's totally fucked. Like removing the sea from seaside towns. i'm sure if you're a mediterranean fisherman you would not be happy having the sea forcibly moved away from your home... The environmental impact is probably immeasurable.
If the engineering effort fails, an incalculable amount of life and property would be destroyed. Even if it succeeds, a huge amount of costly adjustment would be required by a very, very large number of people.
There would be some pretty vast ecological repercussions. Fish habitats and coastal ecosystems, would be immediately affected. Then, connecting rivers will cut deeper, unforeseeably altering each and every environment they affect.