The Mississippi proper is still more disadvantageously situated, to admit a rapid emission of its waters than the Ohio. Flowing nearly in a north and south direction, and through so great a range as nine degrees of latitude, the more southern confluents must be very much ex hausted before those towards the source arc relieved from fetters of ice. The great body of the Mississippi tide is, however, coeval with that of Ohio.
It is the accumulation of the waters of the three valleys we have surveyed, and those of the Kanses and Osage branches of the Missouri, which produce what is known as the annual Mississippi inundation. This flood, nine years in ten, reaches the highest point of its eleva tion at Natchez, between the 10th and 20th of June. Few instances occur, in a long succession of years, in which the waters at that city have not commenced their de pression by the first week of July. And yet, the heaviest mass afforded by the largest natural section, has not yet reached the Delta.
Powerful as are the natural causes which combine to prolong the discharge of the enormous body of waters contained in the three sections of the Mississippi basin we have examined, none contain such controlling impe diments to an aggregated and sudden emission, as does the Missouri valley above the Platte inclusive.
The Yellow Stone and Missouri spread their sources through seven degrees of latitude, and, assuming a ge neral course of south-east, unite their waters above N. Lat. 48°, turn to the east and south-east, and finally as sume, at the Mandan Villages, a southern course, after having flown through eleven degrees of longitude. The Platte pursues a general course from west to east, above N. Lat. 40°,and flows through fourteen degrees of longi tude. It must be obvious, from what has been shewn in this article, or by inspection of a good map, that the higher branches of Missouri must remain frozen, long after those more southward have commenced their rise, and nearly as late as the period of high tide on the Delta. The mean motion of the entire mass of water,
in any of the confluents of the Mississippi, does not much, if any, exceed one mile per hour ; therefore, be tween three and four months are necessary for the pas sage of water from the extreme sources of the Missouri to the Delta of the Mississippi ; consequently, though those waters commence their rise in May, they do not frequently reach the Delta until late in July, or early in August, consequently at a period when the main spring and summer inundation is very greatly abated, and the water retired within the banks of the Mississippi river.
Though the period of flood is well known to the in habitants of the Delta, and in common years can be cal culated within a few days, such is the inequality of the seasons over the whole basin, that no length of experi ence gives much aid in estimating the probable elevation or quantity. In 1800 and 1801, the waters of the Missis sippi at Natchez did not attain the height of the banks.
The Delta commences at the mouth of Red river, or, more correctly, at the efflux of the Atchafalaya. At this place is a gorge, through which the overwhelming mass of surplus water is confined to within three miles ; but by the channel of Atchafalaya, a very large quantity flows out to the right from the main stream, never again to re turn. One hundred and twenty miles lower, flows from the left, the Manchac or Iberville. Those two streams, the Atchafalaya and lberville, bound the Delta, and below their efflux respectively, no water which flows from the Mississippi ever returns; nor are there any bodies of arable land, except upon the immediate banks of the watercourses; all beyond, is either swamp or morass.