Toss in the Great Lakes, too, while you are at it because one thing that they have in common with the rivers you mentioned was that they lay entirely within the United States. You propose to smuggle the good to the US through the US? Neat trick.
That about covers water, but what about 2200 miles of borders?
Sure, what are you going to do? Hide them in a semi and take them down a back road? Come on, Pea! Goods in the volume we are talking about either moved by water or rail or they didn't move at all. Paved roads didn't exist outside of cities. So that means we are back to the Mississippi and the few rail lines that connected North with South. That is the only way that 99% of the imports could have moved. Watching them would have been a piece of cake.
I will rely upon those that were there. A newspaper in March 1861:
It does not require extraordinary sagacity to perceive that trade is perhaps the controlling motive operating to prevent the return of the seceding States to the Union. Alleged grievances in regard to slavery were originally the causes for the separation of the cotton States; but it is apparent that the people of the principal seceding states are now for commercial independence. They dream that the centres of traffic can be changed from Northern to Southern ports. The merchants of New Orleans, Charleston, and Savannah are possessed with the idea that New York, Boston and Philadelphia may be shorn, in the future, of their mercantile greatness, by a revenue system verging upon free trade. If the Southern Confederation is allowed to carry out a policy by which only a nominal duty is laid upon imports, no doubt the business of the chief Northern cities will be seriously injured thereby.
The difference is so great between the tariff of the Union and that of the Confederated States, that the entire Northwest must find it to their advantage to purchase their imported goods at New Orleans rather than at New York. In addition to this, the manufacturing interest of the country will suffer from the increased importations resulting from low duties.
But in case you need some facts to help you along, enjoy:
If the Confederacy wanted an army the size of the Union, in 1860 it would have cost $16 million. If they wanted a navy to match the Union, in 1860 it would have cost another $11 million. Operation of a government less than 25% the size of the Union, $4 million.
Grand total of $31 million.
Value of total Southern exports in 1859 was $208 million, bringing in a like valued import amount at an 8% tariff, and what do you have? Left over $16 million? About $3 a person of personal taxes? Check my math.
They would just charge those yankee traders a port tax.