Steve, there are two battles I can think of where Union gunboats supported the infantry with their cannons. At Shiloh the boats in the Tennessee River played a role in checking the Confederate advance on the first day and two gunboats were largely credited for driving away the Confederates when they attacked Milliken's Bend, Louisiana, in June 1863 (in that case, I think, an officer stood on the river bank and directed the cannon fire with hand signals). I might be wrong, but I seem to remember there also was a substantial number of infantry on Porter's troop transports so my personal opinion is the navy could have given Banks quite a bit of support if he had stayed close to the river.