The first step by the navy was undertaken early in December, when the autumn rains had caused the rivers to rise. Admiral Porter issued orders, dated November 21st, to Captain Walke to enter the Yazoo with all his gunboats, except the Benton and General Bragg left at Helena, and to destroy any batteries that he could. The object was to get possession of as much of the river as possible and keep it clear for General McClernand, who was to land and make the first attempt on Vicksburg by that way. In accordance with his orders, Walke, on arriving off the mouth of the river, sent two light-draught gunboats, the Signal and Marmora, which made a reconnoissance twenty miles up, where they fell in with a number of torpedoes, one of which exploded near them. Having received their report, Captain Walke determined, as the river was rising, to send them up again with two of the heavy boats, the Cairo and Pittsburg, to cover them while they lifted the torpedoes. The ram Queen of the West also went with them. These vessels left the main body at 8 A.M., December 12th. When the torpedoes were reached they began removing them, the two light-draughts in advance, the ram next, the two heavy boats bringing up the rear. While thus engaged the Marmora began firing musketry, and Lieutenant-Commander Selfridge, in the Cairo, pushed ahead to support her. It was found that she was firing at an object floating in the water, which turned out to be a torpedo that had already been exploded. The Marmora was then ordered to proceed slowly again, the Cairo following; but before the latter had gone her length two sharp explosions occurred in quick succession, one under the bow and one under the stern, the former so severe as to lift the guns from the deck. The ship was at once shoved into the bank, and hawsers run out to keep her from slipping off into deep water; but all was useless. She filled and sank in twelve minutes, going down in a depth of six fathoms, the tops of her chimneys alone remaining visible. The work of destroying the torpedoes was continued after the accident, in which no lives were lost. Thus, at the very beginning of operations, the flotilla was deprived of one of its best vessels, the first to go of the original seven.