My first thought was if this was remotely possible on this scale, how many things would be disrupted and changed from the water movement alone. The Panama canal has to have locks because of the ocean differences, but no way would you have locks spanning a few hundred miles across. This thing would have tides back and forth.
Panama canal has to have locks because of the ocean differences
It's actually mostly due to the landscape of Panama, including the lake it uses to traverse and the mountains. The Pacific and Atlantic oceans don't different that much, maybe a few feet. And mostly due to tidal differences.
Just made the entire river underground! A big underground river spanning thousands of miles. It'd require a hell of a lot more work but it wouldn't disrupt things on the surface as much.