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the Mississippi river
The Mississippi River is important - even critical - but New Orleans wasn't the only port. Blockade runners managed to move in and out of Charleston throughout the war. The idea of controlling the South by controlling its economy is "the Anaconda Plan." Given that the North had to surround the eastern seaboard and also control traffic out of the Gulf of Mexico, this was never as successful as it was supposed to be.
It's not a good question. But if I could only choose one place to hinder Southern trade during the American Civil War, it would be the mouth of the Mississippi River.