From Cape Girardeau southward the highlands fall back to a much greater distance from the Mississippi and the river flows through an alluvial plain of its own making. Only at two points on the west—New Madrid, Mo., and Helena, Ark.—and at several points on the east—Columbus and Hickman, Ky., the Chickasaw bluffs and various localities between Vicksburg and Baton Rouge —do the hills approach the river banks. These places are impor tant for they offer the chief sites for towns and cities. Elsewhere the river banks are unstable, for the river itself is constantly shifting—islands becoming peninsulas, peninsulas becoming islands, slight bends becoming in time long meanders., and these in turn becoming old river channels with crescent-shaped lakes as the river again cuts across its peninsula. From Cape Girardeau to the Gulf in a straight line is about 600 m., hut by the windings of the Mississippi it is almost 1,700 m. From the Ohio to the head of the delta the river steadily diminishes in width from r,500 to about Soo yd., but increases in depth from an average of so which are relied upon to protect the country from floods. These serve for the lower floods, but there are few high floods where the bank is not cut and a crevasse opened at some weak spot through which the waters pour over the lowland plantations.
The levees cease with the arable lands, beyond which, in the delta, the land lies too low for cultivation. The delta, built up of the enormous amount of sediment brought down by the river, extends out into the Gulf in the pattern of a goose foot. Over this great mud plain, the river spreads, breaking up into branches or "passes," as they are locally known, each of which in turn extends far out into the water where it has built up its own smaller delta. On the farthest banks nothing grows except the tall reeds which give a little cohesion to the mud. There are five chief openings, South-west pass, South pass, South-east pass, North-east pass and Pass a l'Outre. At different times shifting conditions made steamers use first one pass and then another, but after 1878 engi neering works made South pass the principal entrance.
The volume of water carried by the Mississippi varies greatly with the seasons. Usually the river is lowest in early autumn and again in early winter. A minor rise in November is generally checked by the freezing of the upper tributaries. In January a rise again commences, as a consequence of early rains from the Gulf which sweep over the upper Ohio valley before the frost leaves the ground, and carry along with them the melting snows of the mountains. Unless the Ohio river contributes its general spring flood, it is rare that a large Mississippi flood follows. The Upper Mississippi has its heaviest rainfall in May and its waters and those of the Missouri do not reach Cairo at the mouth of the Ohio until the crest of the Ohio flood has passed. It is when the waters of the upper Mississippi are unusually early, or rainfall precipitates a second flood in the Ohio valley, so that the two crests meet, or one precedes but slightly and holds back the other, that a great flood occurs. The comparative height of floods is
more accurately measured at Cairo than at lower points where the levels may be artificially affected by crevasse breaks. A height of 5o ft. or more above low water at Cairo generally results in a major flood which the levees are not capable of withstanding. Before the great flood of 1927 the record height was 54-69 ft. in 1913. The flood of 1927, the highest on record, registered 56.4 ft. at Cairo.
This great flood lasting more than six weeks with 47 recorded levee breaks, inundated about 28,00o sq.m. and submerged the homes of 750,000 people. Over 600,000 people were rendered destitute and were temporarily dependent upon the American Red Cross and other relief agencies for shelter, food and medical attention. Property and other flood losses were finally fixed at $355,147,000. Expenditures of the Red Cross were more than $14,000,000 and those of the Federal Government during the six weeks were estimated at $5,000,000.
The disastrous flood of 1937 in which about 900 lost their lives was most severe in the Allegheny and Ohio river valleys. Ample advance warning permitted cities along the Mississippi to strengthen levees and open floodways to relieve the pressure of the water and thus minimize the damage. (X.) Engineering work on the Mississippi river has two purposes, viz., improvement of navigation and control of floods. Work for improvement of the river for navigation and flood control is ex ecuted by the Corps of Engineers, U.S. Army, under the direc tion of the Secretary of War and supervision of the Chief of En gineers. Flood control and improvement for navigation are insep arable operations and works for either purpose are co-ordinated so as to aid the development of the other purpose. For example, channel dredging and contraction works primarily for improve ment of the navigable channel are planned so that increased chan nel capacities may result during floods. Similarly, revetments and improvement dredging primarily for flood control are planned so as to stabilize and give better alignment to the navigable channel.