Re the Wiki on the Mississippi River there is this language in the Treaty of Paris.
So if citizen of the UK begins in Canada can they sail the Mississippi River to the Gulf without being hassled by US immigration or border guards?
From your cite: “Flowing entirely in the United States, this river rises in western Minnesota…” He won’t be able to start in Canada.
More specifically, the Mississippi starts in Lake Itasca in Minnesota. At the time the Treaty of Paris was drawn up, it was believed that the origin was much further north, and that clause made sense.
This misunderstanding is also the reason for the Northwest Angle:
At the time of the treaty of 1783, the west bank of the Mississippi River was Spanish territory – and later, briefly, around 1803 it was French territory. So, perhaps, the Louisiana Purchase of 1803 may have had some effects on how the treaty of 1783 is implemented by the United States.
Yes. Once your friend legally enters the US there should be no problem putting that inner tube in the water at Lake Itasca and go tubing all the way to the Gulf. Can’t say that other boaters and an occasional Coast Guard boat might take notice, though …
If your friend goes that way, I’d suggest that your friend take that inner tube out of the river and walk around the various locks and dams on the Mississippi River between Cairo IL and Minneapolis MN. I don’t think the lock-keepers will want him taking the inner tube through their locks, and the associated weirs would be far too dangerous for him to float through in an inner tube.
So far I haven’t been able to find anything more specific, but the U.S. State Department publication Treaties in force (PDF file) notes that “Only article 1 is in force” for the Treaty of Paris (page 295, which is page 305 of the PDF document as there is a separately numbered foreword & table of contents).
How would they determine that you were a UK citizen without “hassling” you?
But once you show your passport, sure, you can travel the Mississippi. In fact, according to Article III of the later Jay Treaty, we can’t even impose duties on your peltries. Do you have any peltries?
As yabob has noted, the Treaty of Pariswas a bit muddled in places when it came to setting out the boundary between what is now Canada and the USA. With respect to the area just north of the headwaters of the Mississippi, the 1842 Webster–Ashburton Treaty tried to address this, and among other things set out that “It being understood that all the water-communications, and all the usual portages along the line from Lake Superior to the Lake of the Woods; and also Grand Portage, from the shore of Lake Superior to the Pigeon river, as now actually used, shall be free and open to the use of the citizens and subjects of both countries.” This still the case. Since you are not a citizen or subject of either country, you would not be able to legally do this.
The Webster-Ashburton Treaty still was not as clear as it should have been, which is why on the east coast my Uncle Gordie spends Christmas (and a lot of the rest of the year) as one of Canada’s very few remaining lighthouse keepers (we went to automated lighthouses some time back). He and his fellow lighthouse keepers staff the lighthouse on Machias Seal Islandin the Bay of Fundy, and are paid by the Department of Foreign Affairs rather than the Fisheries and Oceans because their purpose is to assert sovereignty. He does this by treating the very rare visitors – including an American who boats over to the island each year to assert American sovereignty (based on the Treaty of Paris) – to a fish fry. Since you are neither a Canadian nor an American, you would not be able to legally do this, however, I doubt very much if Uncle Gordie would mind at all, for he is a very sociable person.
What you could do, despite not being a Canadian, would be to purchase your very own Canadian lighthouse. We have approximately 480 active lighthouses and approximately 490 inactive lighthouses that are up for sale. Enjoy!
Someone please mind my parasol while I thrash an impertinent Yankee Doodle.
And bring DEET. Lots and lots of DEET.
[never mind]
Given the grievance involved in flying into the US through regular customs this might be worth a try 
Logistics of asking the pilot to take a detour and bailing out over the Caribbean are nothing compared to USCIS at O’Hare
Seriously though could I (as a UK citizen) actually avoid regular customs if I decide to navigate the Mississippi from the gulf of Mexico ? Maybe I want to enter the country at Waterproof LA as it sounds cool ? Surely lockkeepers are also bound by these treaties ?
Not really: treaties don’t become really effective until they are enacted into domestic law (in this case being federal law of the United States). So (for example) you could cite this treaty to the lock-keeper at Lock and Dam No 1 in Minneapolis as he tried to slow your progress down the Mississippi, but he might respond that he’s only bound by the relevant sections of the United States Code as enacted by the United States Congress. Your only recourse might be to try to get Her Majesty’s ambassador in Washington DC to intervene, but good luck with that 
Earlier this week I had to remove myself from a Canadian case because my American client refused to believe that an international Convention applied to his matter. He would have made a good lock keeper.
I should note that the lock-keepers are not opposed to small craft going through their locks.
Many people travel the entire length of the Mississippi by canoe every year. While not exactly popular, you probably couldn’t write a book about it, because so many stories are out there already of people who have done it.
For something really unusual; you can go North up the Mississippi, make a portage into the Red River Basin in North Dakota, then all the way to Hudson’s bay.
Eric Sevareid of TV news did it when he was 19 in 1935 (at least the Red river to Hudson bay part)
I’ve also met a guy back in the 90’s who kayaked east to west. Statue of Liberty to the mouth of the Columbia River. It was almost entirely a water passage except for the portage across the great divide. I still have his Trip Notes…Now THAT was a great trip and interesting read.
Of course, even with the inner tube, when you start out from Lake Itasca, you will scrape bottom, encounter logs, culverts and braided channels. You’ll have to pick it up and wade or portage for a bit early on:
http://www.bucktrack.com/Canoeing_Down_the_Mississippi.html
Lot’s of people have canoed it, yeah, but I wonder - has anybody inner-tubed it?
Nope–not even under the Treaty of Paris. “Free and open to navigation” deosn’t mean “no customs”. It merely meant that the US couldn’t enact an outright ban on Britons/Canadians travelling and trans-shipping goods down the Mississippi. It was understood that a later commerical agreement would stipulate terms of trade.
That later commerical agreement was the 1795 Jay Treaty, which repeated the language on open navigation:
. . . but then added with respect to customs:
This is an early version of “most favored nation” status–tariffs are permissible, but must be non-discriminatory with respect to other imports.
Except for that all-important exception for peltries:
So bring your peltries, and trade away.
Was that William Least Heat Moon? He wrote a book, River Horse, about such a voyage. Jonathan Raban, a Briton as it happens, wrote a book (Old Glory: An American Voyage) about taking a small craft down the Mississippi. No word on whether he brought fur.
No it wasn’t William Least Heat Moon. I think he used a powerboat from St. Louis. (Willliam LHM and I used to live in the same city and I’ve met him a couple of times). This guy did it at the same time and actually asked me if I was Least Heat Moon when he met me on the Missouri.
Google searching leads to a few hits of people traveling across the continent by water..but not the one I met. Someday, I may find him again but not yet.
As to the Jay Treaty, the State Department is of the opinion that “Only article 3 so far as it relates to the right of Indians to pass across the border, and articles 9 and 10 appear to remain in force. But see Akins v. U.S., 551 F.2d 1222 (1977).” (Same PDF file as linked to above).
So, alas, it appears the duty-free status of Peltries may have been superseded. Unless Akins v. U.S had something to say about the matter.