Ch. 12] DESCRIPTION OF ENGINEERING STRUCTURES 219 



most cutoff, and Thompson Creek, Louisiana, is the southernmost 

 point at which gravels are dumped into the Mississippi River. Below 

 Baton Rouge. Louisiana, meanders and point bars form very slowly, 

 whereas south of Donaldsonville, Louisiana, the channel is almost sta- 

 tionary. Here the sediments are composed almost entirely of clay 

 and fine silt which, owing to their cohesiveness, are extremely diffi- 

 cult for the river to erode. Meanders in the modern delta are almost 

 non-existent, with the few bends present showing very little change 

 from the earliest surveys to the most recent. 



In meander-belt areas, the variety of sediments already discussed 

 exert many material influences on river behavior where they are cut 

 into by encroaching channels. The effects are generally directly 

 related to grain size and plasticity of sediments and extent of in- 

 dividual deposits. The following basic subdivisions of the alluvial fill 

 materials may be made. 



Coarse-grained soils. Sands and gravels. These materials wear 

 away easily through the process of surface sloughing. Little resistance 

 is offered to river migration, and channels are wider and shallower 

 than those in finer grained soils. 



Fine-grained soils. Silts (semi-plastic to non-plastic). These soils 

 are more cohesive than sand and offer more resistance to meandering. 

 The river is restrained to some extent by finer, more cohesive silts, so 

 that some banks become comparatively stable. 



Clays. Of all the flood-plain sediments, clays offer the maximum 

 resistance to river erosion. Banks tend to be quite stable, and channels 

 cutting into such deposits tend to be narrow, as evidenced by the 

 Mississippi River in the southern portion of the valley. Since clay 

 plugs exist in considerable numbers in the upper reaches of the valley 

 and are of considerable individual size, they are often of great im- 

 portance in halting river migration at various points. Integration of 

 levee systems with existing clay plugs may serve to increase the use- 

 fulness of the former. 



The knowledge of relative stability of river channels as determined 

 from adjacent sediments is of valuable assistance in engineering plan- 

 ning. 



ENGINEERING PROBLEMS 



Description of Engineering Structures 



Vast areas of land in the Alluvial Valley of the Mississippi River 

 would be inundated were it not for the levees along the river and along 

 some of its tributaries. When it is realized that the Mississippi River 



