Like all rivers of great length, the Mississippi is subject to an annual rise and fall. The periods of those floods are tolerably regular, hut the quantity and eleva tion of the water differs continually, and often to many feet in two succeeding seasons. The river commences its swell generally in March, or the beginning of April. The increase of the water, at first rapid, gradually be comes more slow, until the supply beginning to exhaust, the volume commences its fall or decrease, which, at Natchez, is in a long series of years, from the 15th to the 20th of June, and, at New Orleans, in the first week of July. There is, however, in most years, a lesser and earlier flood, which varies in time, much more than that of the spring and summer. In October, November, or December, this lesser flood intervenes generally, in the last days of December, and first of January. This lesser flood seldom rises so high as to overflow the banks, and it has been observed, that when the earlier flood is more than commonly high, that the later flood is in proportion, lower than common. The efflux of Plaquemine, is the common scale in the Delta of the excess and period of both floods; which article see.
The velocity of the current of the Mississippi has been greatly mistaken. Taking the motion of its swells, the only safe criterion, it is found that, below Ohio, the en tire mass does not move as much as one mile per hour. The line of upper current moves more rapidly than the mass, but even the former does not move by any means with the rapidity usually supposed. This subject will be more amply explained under the head of Rivers, which see.
The depth of water in the Mississippi varies of course with the increase and decrease of its floods, but this dif ference of elevation is more perceptible above, than in the Delta. At Natchez it is from 30 to 40 feet, at La fourche about 23 feet, and at New Orleans about 9 feet. There is usually about 12 feet water on the bars of the North-east and South-west Passes. In the South and West Passes 9 feet, and in those of the North and Pass a la Loutre 8 feet. Above the passes, the river deepens to upwards of 100 feet. At New Orleans it is upwards of 120, and at the Lafourche 153 feet at high water. There is no place below the mouth of Ohio, where the channel is ever less than 12 feet in depth.
Valley of the Mississippi proper ; above the mouth of Missouri.—In point of area, the valley of the Mississippi proper is not so extensive as that of Ohio, the former only extending over a surface of 180,000 square miles. Its greatest length is from the sources of the Mississippi river to the junction of that stream with the Missouri, 650 miles, and its greatest breadth from the sources of the Ouisconsin to those of Lemoine river, 350 miles.
Before proceeding farther, however, with the descrip tive geographical detail, it may not be considered irrele vant, to review one of those great natural features of North America; without attention to which, no correct conception of its peculiar topography can be obtained. This feature is the great natural meadows, known by the name of Prairie. In a state of nature, with but very partial exceptions, a dense forest covered all those parts of the continent of 'North America contained in the At lantic slope ; the lower part of the basin of St.'Law
rence, below the head of Lake Eric, and two-thirds of that basin to its extreme north-western point; north of the St. Lawrence basin to the 55th degree of N. Lat. ; four-fifths of the valley of Ohio; the residue of the con tinent east of the Mississippi river to the Gulf of Mexico ; and west of that stream from 50 to 100 miles. This enormous forest, one of the largest on the globe, remains yet nineteen parts in twenty ;—the efforts of man having made but partial inroads on either its mass or extent. This great forest is bounded on its western limit by another region of much greater area, but with a very different character. The second may be strictly call ed the grassy section of the continent of North America, which, from all that is correctly known, stretches from the forest region indefinitely westward, and from the Gulf of Mexico, to the farthest Arctic limits of the con tinent. The two regions are not divided by a determi nate limit. In passing from one to the other, the fea tures respectively are so blended, that the change is in most places imperceptible ; though in some instances extremely abrupt.
In general, the prairie region is less hilly, mountain ous, or rocky, than that of the forest ; but exceptions in both cases are frequent. Plains of great extent exist in the latter, and mountains of no trifling elevation and mass chequer the former section. We are now to approach the survey of regions in which prairie forms no small part of the entice surface, and will in the sequel reach others, where those seas of grass extend over almost the whole superficies.
The extreme northern source of the Mississippi, strange as it may appear, continues unknown. In Me fish's map, it is laid down at Turtle lake, 47° 46' N. bat., whilst in Schooleraft's Map, lately published, the sources a that great river are extended to the Lakes Labeish and Turtle, both extending above N. Let. 49°: a discrepancy between these two authorities of course exists of 1° 14' of Lat. The nature of the country, indeed, renders precision on this point difficult, without extreme expense of labour and time. We will assume N. Lat. 48° as the source of this great river ; its junc tion with Missouri being at N. bat. 38° 56', it flows con sequently through nine degrees of latitude, within a trifling fraction. The length of the Mississippi above the mouth of Missouri, is still less accurately known than the position of its source, every authority differing from each other on the former subject. That nearly all esti mates of its length are overrated, there are many weighty reasons to believe. Compared with the Ohio, on most maps, no essential difference appears, when the Allegany is added to the latter. The actual length of the Ohio, by its meanders, we have shown to be 948 miles, that of the Allegany is about 250 ; the two distances united amount to 1198, say 1200 miles. For a long period, the Ohio itself was estimated at 1188 : more correct informa tion corrected the error. On all maps, and from every information we have seen, the sinuosities of the Ohio are at least as great as that of the Mississippi ; we may, therefore, without much risk of material error, give 1327 miles as the maximum length of the latter stream.