In the 1840's how did Louisiana Plantation Owner's use the Mississippi to export goods?

+ Firstly the plantations were agricultural so the products (cotton being the largest) would go both north and out to sea to the markets which needed the materials. England, New York, Boston, all were manufacturing based so they needed the materials for finished product production. So the plantation owners sent the products to the ships on the Mississippi for transport to the markets via ship brokers and received money for it. I don't quiet get your question it seems pretty self evident. just look at a map.