We passed in succession the mouths of the Nishnebottona and the Little Nemahaw,[165] and arrived on the 7th at the Grand Pass. Here the Nishnebottona, a beautiful river about sixty yards wide, approaches within one hundred and fifty yards of the Missouri, being separated from it by a sandy prairie, rising scarcely twenty feet above the surface of the water. After pursuing for a short distance a parallel course, the two rivers diverge, and the Nishnebottona meanders along the side of the Missouri valley, about sixty miles to its confluence with the latter river.[166] From this point is a pleasing view of the hills called the Baldpated Prairie, stretching along the north-eastern side of the Nishnebottona, and diminished to the size of anthills in the distant perspective.[167] Here the navigation is much obstructed by sand-bars, and the ordinary current of the Missouri, according to the statement of Lewis and Clarke, corroborated by our observation, is something more than one fathom per second.[168] In many places the Missouri hurries across concealed sand bars and other obstructions, with the velocity of seven, eight, or even twelve feet in a second.[169] Between these obstructions, the channel becomes deeper, and the current more moderate; consequently the aggregate velocity at times of low water may be reckoned something less than six feet to the second. As the volume of water is increased by the heavy rains, and the melting of the snows within the Rocky Mountains, the current is proportionably accelerated, and becomes more equable, running for many miles in succession, not less than seven hundred and twenty feet per minute. At the time of our ascent the summer floods had not entirely subsided, and in contending against the current, we found occasion {134} in a few instances to make use of the towing rope.