Page:The American Cyclopædia (1879) Volume XI.djvu/678

660 MISSISSIPPI RIVER BASIN. Area, iquare miles. Annual downfall, in cubic feet. Annual drainage, in cubic feet. Ratio. Ohio river 214,000 20,700,000,000,000 5,000,000,000,000 0--J4 518000 25,200,000,000,000 8,780,01 K),000,000 0'15 16 000 13,800.000,000,00*) 8,300,000,000,000 0-24 Small tributaries 82400 8,600,000,000,000 8,240,000,000,000 0'90 189000 13,000,000,000,000 2 000,000,000 000 0-15 97000 8,800,000,000,000 1,800,000,000,000 0-20 13850 1,500,000,000,000 1,850,000,000,000 0-90 St Francis river .... 10500 1,100,000,000,000 9,990,000,000,000 0-90 Entire Mississippi exclusive of Red river 1,147,000 78,900,000,000,000 18,900,000,000,000 0-25 Below the mouth of Red river, the Mississippi is divided into numerous arms or passes, each of which pursues an independent course to the gulf. The highest of these is the Atchafalaya on the W. side of the river. Below its point of separation from the Mississippi the region of swampy lands, of bayous and creeks, is known as the delta. Above this the alluvial plain of the river extends to the Chains, 30 m. above the mouth of the Ohio, and to Cape Gi- rardeau in Missouri, where precipitous rocky banks are first met with. These are the lower secondary limestone strata lying in nearly hori- zontal beds. The total length of the plain from the mouth of the Ohio to the gulf is esti- mated at 500 m. Its breadth at the upper extremity varies from 30 to 50 m. ; at Mem- phis it is. about 30 m., and at the mouth of White river 80 m. The extreme width of the delta is rated at 150 m., its average width is probably 90 m., and its area 12,300 sq. m. The 'elevation of the bottom lands at Cairo above the sea level is about 310 ft., while the slope of the high- water surface from that place to the gulf is from 322 to 0. These bottom lands are subject to inundation, and conse- quent annual enrichment. Under the system of slave labor large plantations were opened in the dense forests which cover them, but vast tracts of unsurpassed fertility are yet cov- ered with canebrakes and cypress. The allu- vial plain, extending from above Cairo to the gulf, is terminated on the east and the west by a line of bluffs of irregular height and direc- tion, composed of strata of the eocene and later tertiary formations. Down this plain the river flows in a serpentine course, fre- quently washing the base of the hills on the E. side, as at Columbus, Randolph, Memphis, Vicksburg, Grand Gulf, Natchez, and Baton Rouge, and once passing to the opposite side at Helena. The actual length of the river from the mouth of the Ohio to the gulf is 1,097 m., increasing the distance in a straight line by about 600 m., and by its flexures also reducing the rate of its descent to less than half the inclination of the plain down which it flows. The range between high and low water at Cairo, near the head of the plain, is 51 ft., and at New Orleans it is 14-4 ft. The river flows in a trough about 4,470 ft. wide at the head, and 3,000 ft. at the foot. The immense curves of the stream in its course through the alluvial plain sweep around in half circles, and the river sometimes, after traversing 25 or 30 m., is brought within a mile or less of the place it had before passed. In heavy floods the water occasionally bursts through the tongue of land, and form what is called a "cut-off," which may become a new and permanent channel. The height of the banks and the great depth of the river bed check the frequent formation of these cut-offs, and attempts to produce them artificially have often failed, especially when the soil is a tough blue clay, which is not readily worn away by flowing water. This was the case at Bayou Sara, where in 1845 an excavation intended to turn the river was made, by which a circuit of 25 m. would have been reduced to a cut of one mile ; and also at Vicksburg in 1862-'3, where the Union army endeavored to make a cut-off out of range of the confederate guns. Semicircular lakes, which are deserted river bends, are scattered over the alluvial tract. These are inhabited by alligators, wild fowl, and gar fish, which the steamboats have nearly driven away from the main river. At high water the river overflows into these lakes. The low country around is then entirely sub- merged, and extensive seas spread out on either side, the river itself being marked by the clear broad band of water in the midst of the forests that appear above it. The great freshets usually occur in the spring, and are often attended with very serious consequences. Crevasses are formed in the banks and increase with the flow, which becomes so violent that boats are occasionally carried with their crews into the intricacies of the bayous which lead the waters to the streams at the foot of the bluffs. The lower portion of the alluvial plain, called the delta, rises from a few inches to 10 ft. only above the level of the sea, and is formed of sands and clays in horizontal layers. The delta protrudes into the gulf of Mexico far beyond the general coast line, and is slow- ly but imperceptibly advancing into the gulf by the shoaling caused by the deposition of the sediment brought down the river. This is mostly dispersed by the waves and currents, and distributed over the bottom of the gulf. Although the banks of the passes are some- times observed to have advanced in the course of a few years sensibly into the gulf, these are but narrow strips of land, which may be swept away by the rush of the gulf waters driven up by storms, leaving the long coast of the delta but slightly changed. The old French maps of the early part of the last century still very cor-