[os]: What, the Ohio River was closed down? Moreover, the North dominated rail transport, so moving goods to “the interior” was no real problem.
From the Chicago Tribune of February 25, 1861:
The Two Outlets to the Ocean.
Since the Secession of the State of Louisiana fears have been freely expressed by both Eastern and Western journals that the commerce of the great valley of the Mississippi would be nearly destroyed should the mouth of that river remain in the possession of a foreign power. Without the least subtracting from the importance, in a commercial point of view, of that noble river to the great North-west, we cannot but observe that within the past ten years, a mighty change has taken place in the current of Western trade, and that the value of the Mississippi as an outlet to the ocean for the produce raised in the west has been unnecessarily magnified. Up to the year 1852, the Mississippi was the only outlet to the ocean for the greater portion of the grain, pork, and beef of Illinois, Iowa, Wisconsin, Indiana and Ohio; but since then the river has been tapped at nearly a dozen points by railroads, and the trade drawn into the great [sic] Lakes and thence to the ocean by the Erie Canal and the St. Lawrence River. The following table, however, will illustrate our position most conclusively:
FLOUR AND GRAIN RECEIVED AT LAKE PORTS IN 1860
[rb: my numbers are rounded to the nearest million bushels as best I can read them]
Chicago ... 36.5
Milwaukee ... 11.0
St. Joseph ... 0.0
Waukegan ... 0.2
Kenosha ...0.3
Racine ... 0.9
Port Washington ... 0.1
Sheboygan ... 0.2
Maniowoe(?) ... 0.1
Green Bay ... 0.3
Toledo ... 14.5
Detroit ... 6.8
Cleveland, Sandusky, and other ports ... 12.0
Total ... 82.9
GRAIN AND FLOUR EXPORTED AT NEW ORLEANS IN 1859-60
Flour, 364,811(?) bbls, equal to 1.9 million bushels
Corn ... 1.3
Wheat ... 0.0
Total ... 3.3
Here's a link that gives a figure of 30 million bushels of grain exported by Chicago and some large number of bushels of grain for individual Great Lakes ships in earlier years: [Link].
And, of course, as I've pointed out to you before, well before the firing on Fort Sumter the Confederate government passed a law providing for free navigation to and from the North via the Mississippi except for wharf fees and other minor navigation charges.
And which they could just as freely cut off at a whim, as the governor of Mississippi proved for a short period. What was to stop him from doing so again? Or Richmond, for that matter?
But if cutting off the Mississippi was no threat because of the low level of exports passing down it, why was Sumter a threat to the confederacy? After all it didn't cut off traffic into and out of Charleston. And if it did, a comparatively small percentage of confederate imports and exports passed through it. Why the justification for war?