DRAINAGE OF WET LANDS OP SOUTHERN LOUISIANA. 7 



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

 by levees along the Mississippi River, and as overflow is now very rare, 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 continually extending themselves 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 would no doubt often change their character and direction. It is a notice- 

 able fact that the trend of the majority of the waterways in this section is 

 toward the southeast. As the prevailing winds are from the southeast, and as 

 the usual Gulf currents flow from that direction, most of the sediment was 

 deposited on the western side of the channel. As a result the deeper water 

 always remained to the eastward, and the deposition on the western shore 

 continually forced the channel to the eastward. 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 over- 

 flows, 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 over- 

 flow 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 Lafourche 

 served as an overflow outlet for the Mississippi River, the opening at Donaldson- 

 ville not having been permanently closed until that year. 



C — 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 notice- 

 able and is characteristic of smaller bayous. 



i Manuscript report of A. M. Shaw. 



