Red river enters the Mississippi at N. Lat. 31° 01', and W. Long. from Washington City 14° 40'.
The next confluent of the Mississippi, in point of mag nitude, which enters from the right bank below the mouth of Ohio, is White river. This latter river, though of greatly inferior length or volume to the two former, is of great importance from the extent of excellent land it drains. White river rises in the angle between saw and Osage rivers, and has its sources mingled with those of both the latter, also with those of Merrimac and St. Francis. White river, after a comparative course of about 40o miles, falls into the Mississippi, a few miles above the mouth of the Arkansaw.
St. Francis and Merrimack would neither deserve par ticular notice amid the description of as we have been surveying, except as flowing from one of the most interesting metallic regions of this globe.
St. Francis rises in the counties of Washington and St. Genevieve, in the state of :NI issoori, and flowing south by comparative course 250 miles, enters the Mississippi about one hundred miles by water above the White river.
The Merrimack rises in the highlands between the sources of St. Francis and those of the Gasconade, a branch of Missouri, flows east by comparative course one hundred miles, enters the Mississippi eighteen miles below St. Louis.
It is unnecessary in this to notice the soil, climate, or productions of the lower Mississippi valley, as these subjects will come more appropriately under the re spective heads of the states of Mississippi and Missouri, which see. A reference is also made to the article Mis sissippi state, as respects the confluents of the Mississippi river, from its left bank, below the mouth of Ohio.
We may here remark, that including the basin of Co lumbia, the Trans-Mississippian Territory of the United States amounts to 1,144,843 square miles, equal to 732,699,520 United States' acres.
To close the survey of the Mississippi Basin, it only remains to examine the general laws by which the an nual floods of the Mississippi are regulated and deter mined. In order to elucidate this subject, it is necessary to combine under one view the entire surface of the ba sin. This investigation embraces one of the most im portant questions in physical geography ; it is to ex amine one of the most stupendous operations of nature, performed on a scale commensurate with the magnitude of the tfrect.
In our review of the valley of Ohio, 320.8 feet was given as the elevation of the waters at the junction of Ohio and .Mississippi rivers. From want of correct data as to the real length or general fall of its waters, no satisfactory calculation can be made as to the absolute elevation of the sources of Missouri. We have assumed in this article 3000 feet as the probable height, above the respective oceans, of the sources of the Missouri. In an analysis similar to that on which we are engaged, relative elevation is one of the elements most necessary to a correct result. We may remark in this place, with sincere regret, that during all the time in which our citi zens have visited the sources of Missouri, that the eleva tion of not one peak or pass has been determined and published. We have, however, some facts which enable us, by analogy, to approach an accurate estimate of the height of the sources of the Missouri. From the ex treme cold, and from the late continuance of snow at N. Lat. 45° or 46°, we cannot suppose the base of the Chip pewan mountains to be in that region less than 2800 or 3000 feet. At the first glance, when it is known the great length of the stream, from its discharge into the gulf of Mexico to its remotest sources, so moderate an elevation of the latter may appear too limited ; but 3000 feet would demand very nearly a foot per mile, an enor mous descent, much more than double that ordinarily found in rivers. In the foregoing estimate, the absolute height of peaks, or even ridges, are not taken into ac count.
It has often excited astonishment in those who knew the extent, without duly attending to the structure of the Mississippi basin, that the entire Delta is not annually submerged. We now proceed to investigate the causes of the long continuance, and in common years, the mode rate elevation of the Mississippi floods. Recurrence to a good map will render intelligible what is to follow, and render obvious, that the peculiar structure and relative position of the respective valleys which compose this great basin, are the true causes which prolong the du ration, and mitigate the height of the annual inundation.