Each of the larger streams, as well as a large proportion of the smaller ones, is accompanied by a belt of bottom land, of greater or less width, lying low as regards the stream, and liable to overflow at times of high water. These flood plains form collectively what is known as the alluvial region, which extends in a broad belt down the Mississippi, from the mouth of the Ohio to the Gulf of Mexico, and up the Ouachita and its branches and the Red river to and beyond the limits of the state. Its breadth along the Mississippi within Louisiana ranges from 10 to 50 or 60 m., and that along the Red river and the Ouachita has an average breadth of 10 m. Through its great flood-plain the Mississippi river winds upon the summit of a ridge formed by its own deposits. In each direction the country falls away in a succession of minor undulations, the summits of the ridges being occupied by the streams and bayous. Nearly all of this vast flood-plain lies below the level of high water in the Mississippi, and, but for the protection afforded by the levees, every considerable rise of its waters would inundate vast areas of fertile and cultivated land. The low regions of Louisiana, including the alluvial lands and the coast swamps, comprise about 20,000 sq. m., or nearly one-half the area of the state. The remainder consists of the uplands of prairie and forest. The alluvial region of the state in 1909 was mainly protected against overflow from the Mississippi river by 754 m. of levee on the Mississippi river within the state, and 84 m. on the Mississippi river, Cypress and Amos Bayou in Arkansas, forming part of the general system which extends through other states, 1000 m. up to the highlands about the junction of the Ohio river. The state and the national government co-operate in the construction and maintenance of this system, but the Federal government did not give material aid (the only exception being the grant of swamp lands in 1850) until the exceptionally disastrous flood in 1882. For about a century and a half before that time, levee building had been undertaken in a more or less spasmodic and tentative way, first by riparian proprietors, then by local combinations of public and private interests, and finally by the state, acting through levee districts, advised by a Board of Engineers. The Federal government, after its participation in the work, acted through a Board of Engineers, known as the "Mississippi River Commission." The system of 754 m. of Mississippi river levees, within the state, was built almost entirely after 1866, and represents an expenditure of about $43,000,000 for primary construction alone; of this sum, the national government contributed probably a third (the state expended about $24,000,000 on levees before the Civil War). Some of the levees, especially those in swampy regions where outlet bayous