Savannah's existing Fort Jackson, about three miles downriver from the city, was supplemented with two additional batteries. Defenders built fire barges. Lee first placed a battery at Causton's Bluff commanding navigable estuaries leading to the Savannah River behind Fort Pulaski. Then he added another battery situated farther upriver on Elba Island, blocking all river approach to Savannah. The Union naval commander, Admiral Samuel F. Du Pont, conducted a reconnaissance of Lee's system of defense upriver. When the commanding military general, Gen. Thomas W. Sherman, insisted on forcing Lee's riverine batteries against Du Pont's recommendation, Thomas Sherman was transferred to the western theater and replaced by General David Hunter.