416 REPORT OF OFFICE OF EXPERIMENT STATIONS. 



to the fact that the soil is water-Uiid nuiteiial, iiotliin^ coarser than 

 river sand is encountered except the overlyinj:; hiyer of humus formed 

 by the decayin<]j ve<j;etation. A typical section of these lands would 

 show a layer of humus or muck 2 or 3 feet thick ovcrlyinj^ a grayish 

 or dral) clay subsoil com])osed of very fine particles, which when sat- 

 urated form a tough, im])ervious mass. Layers of sand of varying 

 thickness are encountered in this clay subsoil, and occasionally no 

 clay stratum intervenes between the humus and sand. The surface 

 soil is from a few inches to 5 feet or more in thickness. " 



NATURAL LEVEES. 



The embankments or natural levees along all tiic bayous and 

 streams with which this region abounds, and those along the Missis- 

 sip])i, are formed by the constant overflows. They are composed of 

 coarser sand than the clay subsoil of the prairies, as this latter was 

 deposited in the more slowly moving waters away from the main cur- 

 rents of the streams. This point, as well as the fact that the natural 

 drainage is away from the river, is well brought out by the following 

 from A Preliminary Report upon the Bluff and Mississippi Alluvial 

 Lands of Louisiana, by W. W. Clendenin, ^ written a few years since: 



With every flood the river now overflows its flood plain and deposits much of the 

 sediment from its headwaters. As with a slight increase in velocity the transporting 

 power is vastly increased, so with a slight checking of velocity, as occurs over the flood 

 plain outside of the channel, deposit takes place. As the greatest decrease in velocity 

 takes place near the channel, there the heaviest and coarsest sediment is deposited, 

 and in greatest quantity. The river banks are thus built higher by each flood and a 

 system of natural levees is produced. There is thus a niarkod difference in the "front 

 lands" and the "back lands" along the river. The former are higher and coarser tex- 

 tured than the latter, and therefore much more easily cultivated and drained. 



Drainage from the very channel margin is away from the river, and, unless forced by 

 the topography of the land, will not reach the river proper, but unite with some outlet 

 of the river produced during some extraordinary flood period and kept open by the 

 escape of water during ordinary periodic flood stages. As the feeders of the river are 

 called tributaries, these outlets have not inaptly been styled distributaries. 



The water in breaking over the banks and spreading over the 

 marshes in sheets was gradually lessened in velocity, thus gradually 

 dropping its load of sand and silt and causing a delicate gradation 

 of soil texture to the finest river silt far out in the marshes. These 

 natural levees and those that have been constructed, and the 

 improved methods of closing crevasses in the levees, have reduced 

 the danger from general overflows to a minimum; and whereas this 

 has prevented much damage which would be caused by the cutting 

 out of new channels and the destruction of much valuable property, 

 it has also checked the building up of the lowlands and their natural 



a For a discussion of these soils, see U. S. Dept. Agr., Field Operations of the Bureau 

 of Soils, 1903, p. 439. 



* Louisiana Stas. Rpt. Geology and Agriculture, Pt. IV, p. 263. 



