They couldn't hold New Orleans...
The River Defence Fleet was various armed merchantmen, none of them heavier than a frigate, up against 10 or 12 real purpose-build warships, all with well trained crews. The river forts, Jackson and St. Phillip, were the only real defence the city had. That and the iron chain across the river, which was blown up by engineers, and several ironclads that were never completed. Read James L. Nelson's historical fiction series about the Confederate Navy and you'll understand just how terribly outgunned the C. S. N. was from the beginning, and the divide just got worse and worse.