A group is working on starting a set of distance races on the upper Mississippi.  They don't have registration open yet, and perhaps are still working out some details.  You can follow the below page on Facebook, and I'll try to remember to post the website link on this thread once it becomes available.

Two Paddles Canoe and Kayak Races
@TwoPaddlesMN

Has there been any interest in this race from the St. Louis area? I'd really like to try the 140 out as the views from Minnesota nd the Mississippi River will be terrific. Plus, good training for the 340.