Report of Admiral Porter.
United States Mississippi Squadron. Flag-ship Black Hawk, off Vicksburgh, July 18, 1863.sir: I have the honor to inform you that the expedition I sent into the Red River region proved very successful. Ascending the Black and Tensas Rivers, (running parallel with the Mississippi,) Lieutenant Commanding Selfridge made the head of navigation — Tensas Lake and Bayou Macon, thirty miles above Vicksburgh, and within five or six miles of the Mississippi River. The enemy were taken completely by surprise, not expecting such a force in such a quarter. The rebels that have ascended to that region will be obliged to move further back from the river, if not to go away altogether. Lieutenant Commanding Selfridge divided his force on finding that the transports, which had been carrying stores to Walker's army, had escaped up some of the narrow streams. He sent the Mainton and Rattler up the Little Red River, (a small tributary of the Black,) and the Forest Rose and Petrel up the Tensas. The night was dark, and it was raining very hard, and the Mainton and Rattler succeeded in capturing the rebel steamer Louisville, one of the largest and perhaps the best steamer now in the Western waters. Up the Tensas, or one of its tributaries, the Forest Rose and Petrel captured the steamer Elmira, loaded with stores, sugar, and rum for the rebel army. Finding that the steamers which had conveyed General Walker's army had returned up the Washita, the expedition started up that river, and came suddenly upon two rebel steamers; but the rebels set them on fire, and they were consumed so rapidly that their names could not be ascertained. One steamer, loaded with ammunition, escaped above the fort at Harrisonburgh, which is a very strong work, and unassailable with wooden gunboats. It is on an elevation over one hundred feet high, which elevation covers what water-batteries of heavy guns there are. Lieutenant Commander Selfridge was fortunate enough, however, to hear of a large quantity of ammunition that had lately been hauled from Natchez, and deposited near Trinity, (nearly due west of Natchez,) and from whence stores, pro. visions, cattle, suns, and ammunition are transported. He captured fifteen thousand rounds of smooth-bore ammunition, one thousand rounds of Enfield rifle, and two hundred rounds of fixed ammunition for guns, a rifle thirty-pounder Parrott gun-carriage, fifty-two hogsheads of sugar, ten puncheons of rum, nine barrels flour fifty barrels salt, all belonging to the confederate government.  At the same time he heard of a large amount of ammunition that had started from Natchez for Trinity, and was lying in wagons on the road half way across. He despatched a boat around to inform me of it, but General Ransom, who had landed a few days before at Natchez, hearing of it, also sent a detachment of cavalry and captured the whole. Thus Walker's army is left almost without ammunition. The officers and men have shown great energy on this expedition, and have met with no mishaps. They procured a good deal of information by which future movements will be regulated. The people in the whole of that section are very hostile to the Government — rank rebels. I have the honor to be, etc.,