216 TURNBULL et al. RIVER ALLUVIUM PROBLEMS [Ch. 12 



tion when the river overtops its banks during flood. The sudden loss of 

 velocity, and consequent carrying power, of river water upon leaving 

 its channel during flood results in deposition of thick, coarse silty 

 materials nearest the river and successively finer grained and thinner 

 deposits at a greater distance. The natural levees of the Mississippi 

 River are approximately 10 feet in height, but near New Orleans and 

 out onto the delta they are lower and extend to varying depths below 

 the flood-plain surface, often more so below than above, as the load 

 of these natural levee deposits has caused depression of soft underlying 

 materials. Levee widths are extremely variable, from % mile or so to 

 as much as 4 miles. Normally they are widest behind concave banks 

 and are also likely to widen downstream from a bend. Surrounding 

 False River (Fig. 3), distinct natural levees can be distinguished by 

 their utilization for agricultural purposes. These form the firm land 

 in this area and have the added advantage of being slightly above 

 normal flood level. 



Crevasses. In areas directly behind natural levees there are fre- 

 quent topographic irregularities caused by crevasses developed during 

 floods. These features are almost like river distributaries, but they are 

 smaller and much more temporary, serving only during very high 

 water. Deposits laid down in this manner are generally thin but have 

 many irregularities resulting from all sorts of braided-channel pat- 

 terns. Silts of crevasse origin may form thin veneers over troublesome 

 plastic clays and prove misleading in engineering work unless very 

 careful borings are made. Scouring action by sudden crevassing some- 

 times produces limited basins known as "blue holes," which are filled 

 with clays and fine silts after their channel has been abandoned. 

 Crevasses are recognizable in the natural levee south of the point bar 

 of Fig. 3. Here the channels can be seen to have developed small 

 scour holes and to terminate with distributary courses. The crevasse 

 channels in this area have become wooded, and their adjacent silty 

 deposits have been utilized for farming. 



Channel-fill de-posits. With abandonment of meander loops by de- 

 velopment of natural cutoffs, the familiar oxbow lakes are formed. 

 These, however, are only temporary features, as tributary creeks and 

 river floods tend to fill them slowly. In time these crescent-shaped 

 scars become filled chiefly with fine clayey sediments, so that they 

 have earned the name "clay plugs." Generally, in the basal part of 

 a section, the clay is heavy and plastic. Above this, the material is 

 more silty and less plastic, and, near the ends of the crescent, the mate- 

 rial may be sandy or composed of coarse silts. The upper arm of a 

 crescent generally accumulates the coarsest sediment, whereas the lower 



