I have a soft spot in my heart for the Cannon River. It's the first place I ever tipped a canoe, not on purpose. I've done a number of trips down it, including one that ended with a journey from the confluence with the Miss, down to Red Wing. It's a lovely little river, and has a nice bike trail along it all the way up to Cannon Falls.
Its impact on the Mississippi was negligible, almost unnoticeable.