After the storm was over, they proceeded to a fountain, perhaps the largest in America. It is situated in a pleasant, level plain, and about twenty-five yards from the river, into which it falls over some steep, irregular rocks.
In this part of the country a remarkable phenomenon was noticed. A loud report, precisely resembling the sound of a cannon, was repeatedly heard from the mountains, at different times, both of the day and night; sometimes in one stroke; at others, in five or six successive discharges. This report was occasionally heard when the air was perfectly still and without a cloud; and it was supposed to be occasioned by the bursting of rocks.
The party, continuing indefatigable in their exertions, dragged the canoes, or pushed them along with poles, up the current of the Missouri. This they did, day after day, until the 27th of June, when they arrived at the _Three forks of the river_; that is, at the point at which three rivers, each of considerable size, flow together, and form the great stream. As it was difficult to determine the largest of the three, Captains Lewis and Clarke decided on discontinuing here the appellation of Missouri; and named the streams, respectively, Jefferson's, Madison's, and Gallatin's river. As the first of these flowed from the west, they ascended it in preference to the others; but they continued to experience great difficulty with the canoes, in consequence of the rapidity of the current.
They were now approaching the termination of the first great division of their journey. The river continued to lessen as they proceeded: its width, in the part at which they arrived on the 8th of July, was not more than forty yards; and, on the 11th, it was diminished to twelve, so as to admit of being waded over without hazard. They had now proceeded, by computation, three thousand miles from the mouth of the Missouri; and they, not long afterwards, reached its extreme navigable point, in latitude 43 degrees 30 minutes, and nearly in longitude 112 degrees west from Greenwich.
Here they laid up their canoes, until they should return from the Pacific ocean; and, proceeding by land, had the gratification of tracing the current to its _fountain head_, in the midst of the Rocky Mountains.