6 BULLETIN" 652, U. S. DEPARTMENT OF AGRICULTURE. 



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. 



Since practically all land in the Delta region now is protected from overflow 

 by levees along the Mississippi River, all building up of the low marshlands has 

 been checked. However, at the mouth of the Mississippi River deposition of 

 material is continually taking place. 



Even before the construction of the artificial levee system, there was no 

 raising of the general level of the marshes during periods of normal flow and 

 probably little sedimentation of the river bed excepting at its mouth, the most 

 of the material which was carried in suspension to the lower portion of the 

 river being carried out and deposited in the Gulf. As the river rose, however, 

 the waters constantly sought additional outlets through the various bayous of 

 the Delta country. At times of extreme high water there was a general break- 

 ing over the banks of the river and its outlets. It is probable that the most of 

 the building up of the lands above sea level has been done at such times. 1 



The above statements show that while the Mississippi River and its various 

 distributaries are extending themselves continually through deposition at their 

 mouths, it was only at times of overflow that the ridges along the channels 

 were raised or widened. The peculiar branched nature of the Delta, with 

 bodies of land extending fingerlike into the Gulf, with open spaces of water 

 between, is also thus accounted for. As these ridges gradually widened they 

 approached each other, thus forming lakes and bayous. Tidal action usually 

 kept these ridges from inclosing the open water between them, and heavy and 

 prevailing winds no doubt often would change their character and direction. 

 It is reasonably certain that the large inland lakes, such as Lake Des Allemands 

 and Lake Salvador, were inclosed in this manner. 



The fact that the silt-bearing capacity of water is directly dependent upon 

 the velocity is clearly demonstrated by observing the natural embankments 

 formed by streams of various sizes. In the case of smaller streams, when the 

 water overflows its force is soon spent and the silt is quickly deposited near 

 the stream, forming narrow ridges with steep side slopes, while those formed 

 by large streams are broad with slight slopes. Three typical examples showing 

 this difference and the manner in which the land surface has been raised on 

 the marshes are given in figure 2, A, B, and C. 



The sections were taken as follows : 



A — From the right bank of the Mississippi River across the Willswood- 

 plantation, about 10 miles above New Orleans. This section is about 2 miles 

 .long, and a part of the lands crossed have been under cultivation for a great 

 many years, while those farthest from the river were reclaimed only 12 or 

 15 .years ago. The lowering of the surface of the cultivated and drained 

 fields due to the shrinkage of humus soils is here well illustrated. There are 

 many examples of highlands having been built up for much greater distances 

 from the river than this, but as such accretions are indirect, on account of 

 being formed by a number of small bayous or temporarily contracted areas of 

 overflow which assisted in maintaining the velocity, these have not been 

 considered as being typical. 



B — The right bank of Bayou Lafourche at Lockport, extending back through 

 the village of Lockport and beyond to Lake Fields. Until 1903 Bayou La- 

 fourche served as an overflow outlet for the Mississippi River, the opening 

 at Donaldsonville not having been permanently closed until that year. 



O — This is a very small bayou extending to about 4 miles west of Lockport. 

 The abrupt rise of the ridge from the surrounding marshes is especially no- 

 ticeable and is characteristic of smaller bayous. 



1 Manuscript report of .A. M. Shaw. 



