The USCG has radio sites indeed along the entire Mississippi River, and up along some of its tributaries, many of which are shared with the USA-COE. There are also beacons at every bridge crossing, dam, and lock on the Big Muddy. You often in Spring, and Fall, and before strong storm fronts get some pretty good VHF High Tropo Ducting down there in the South. It is quite normal to get VHF High Ducting openings of 100 to 200 miles for short periods. In times of very big Troposhperic Ducting, you can get widows for VHF High up to 500 or more miles, VHF Low up to 3000 miles, UHF 200/300 miles, and 700/800/900 MHz 100/200 miles. I have personally monitored California Highway Patrol from Charleston, SC on VHF Low, St. Bernard Parish back when they were on UHF from Jackson, and McComb MS, and FD's/PD's on VHF high from as far as 5 states away when I lived in Union Church, MS.