When the South seceded and closed off the rivers in the west, it became obvious to many that war was at hand because of the strategic importance of these transportation routes. Before the war, the rivers served as transportation and shipping routes. During the war, the Union and Confederate armies used them to transport troops and all of the materiel necessary to wage war. During the summer of 1861, the South followed a defensive strategy by attempting to control the rivers through the construction of a series of forts. The needs of the North differed. The Union needed to invade, occupy, and control the vast area claimed by the Confederacy, and in order to launch such a large-scale invasion, the Federals sought to control all forms of mass transportation. For these reasons, rivers (and railroads) were of vital military importance to both armies.
In 1861, the Union War Department purchased several boats for conversion to gunboats and decided to build seven new ironclad gunboats. These gunboats formed the basis of the Union's western flotilla. The War Department awarded James B. Eads, a civil engineer with riverboat experience, the contract to build the new ironclad boats. Named for cities along western rivers, the flotilla became known as the City Series. Eads built the Mound City, Cincinnati, and Cairo at Mound City, Illinois, and the St. Louis, Carondelet, Louisville, and Pittsburg at Carondelet, Missouri. The gunboats looked so much alike that builders painted a different color band around each boat's chimney. The Cairo received a gray band; the Carondelet, red; the Cincinnati, blue; the Louisville, green; the Mound City, orange; the Pittsburg, brown; and the St. Louis, yellow.
The steam-powered boats used coal for fuel and each measured 175 feet long and 51 feet, 2 inches wide. Seventeen officers, twenty-seven petty officers, and 131 sailors crewed each gunboat. The boats had flat bottoms and Eads designed them especially for river travel; each needed only a six-foot draft. The boats' design protected the paddlewheel from enemy fire and placed the boilers below the water line to shield them from damage. Two-and-one-half inches of iron protected the 24-inch-thick oak sides of the boats. Each boat carried thirteen heavy cannons. In most major battles fought along the western rivers, the Union Navy used these gunboats. The weight and slow-moving speed of the ironclads made them obsolete after the Navy opened a river. Lighter, faster boats made better patrol boats.
Five of the ironclad gunboats survived the war; the War Department sold the vessels to recover some of the war debt. The other two boats did not survive the war. The St. Louis (renamed Baron De Kalb September 8, 1862) was sunk by a torpedo one mile below Yazoo City, Mississippi, on July 13, 1863. The Cairo was sunk in the Yazoo River near Vicksburg, Mississippi, during the war. It has been raised and is on display at Vicksburg National Military Park.