THE DESCENT OF THE COLUMBIA. August, 1805.--Our Indian information as to the navigation of the Columbia was of a very discouraging character. It was therefore agreed that Capt. Clarke should set off in the morning with eleven men, furnished, besides their arms, with tools for making canoes; that he should take Chaboneau and his wife to the camp of the Shoshonees, where he was to leave them to hasten the collection of horses; that he was then to lead his men down to the Columbia; and if he found it navigable, and the timber in sufficient quantity, should begin to build canoes. As soon as he should have decided on the question of proceeding, whether down the river or across the mountains, he was to send back one of the men, with information of his decision, to Capt. Lewis, who would tarry meanwhile at the Shoshonee village. Aug. 20.--Capt. Clarke set out at six o'clock. Passing through a continuation of hilly, broken country, he met several parties of Indians. An old man among them was pointed out, who was said to know more of the nature of the country north than any other person; and Capt. Clarke engaged him as a guide. The first point to ascertain was the truth of the Indian information as to the difficulty of descending the river. For this purpose, Capt. Clarke and his men set out at three o'clock in the afternoon, accompanied by his Indian guide. At the distance of four miles he crossed the river, and, eight miles from the camp, halted for the night. As Capt. Lewis was the first white man who had visited its waters, Capt. Clarke gave the stream the name of Lewis's River. Aug. 23.--Capt. Clarke set out very early; but as his route lay along the steep side of a mountain, over irregular and broken masses of rocks, which wounded the horses' feet, he was obliged to proceed slowly. At the distance of four miles, he reached the river; but the rocks here became so steep, and projected so far into the stream, that there was no mode of passing except through the water. This he did for some distance, though the current was very rapid, and so deep, that they were forced to swim their horses. After following the edge of the stream for about a mile, he reached a small meadow, below which the whole current of the river beat against the shore on which he was, and which was formed of a solid rock, perfectly inaccessible to horses. He therefore resolved to leave the horses and the greater part of the men at this place, and continue his examination of the river on foot, in order to determine if there were any possibility of descending it in canoes.