188 GEOLOGY. 



Flood-plains due to obstfuctions.^Again, any obstacle in a stream^s 

 course is likely to cause deposition above. Thus dams built across 

 rivers entail the deposition of sediment above. Where a stream flows 

 over the outcropping edges of strata of different strength, the more 

 resistant serve, in some sense, as dams. Above them the stream cuts 

 its bed to a low gradient, and, becoming sluggish, drops more or less of 

 the detritus brought down from above. Obstacles of any sort across 

 a stream's channel, therefore, favor the development of alluvial plains. 



Fig. 178. — The levees of the Mississippi in cross-section, 4 miles north of Donaldson- 

 ville, La. Vertical scale X50. The horizontal line in the diagram represents 

 sea-level. The bottom of the channel at this point is far below sea-level. 



Levees. — As the stream in flood escapes its channel and overspreads 

 its plain, its immediate banks are the site of active deposition, for it 

 is here that the velocity of the overflowing water is first notably checked. 

 On the banks of the channel, therefore, low alluvial ridges, called 

 natural levees, are built up (Fig. 178, and PL XV). They may be nar- 

 row, or hundreds of feet in width, and are often several feet above 

 the plains behind them, giving the latter a slope away from the channel 

 of the stream. They are sometimes high enough t control the courses 

 of tributary streams, as shown by numerous tributaries to the Mississippi 

 below the Ohio. The Yazoo, for example, flows some 200 miles on 

 the flood-plain of the Mississippi before it joins that river near Vicks- 

 burg. The levees even become divides, directing drainage away from 

 the streams they guard (PI. XV). Streams sometimes build levees 

 faster than their tributaries aggrade their channels. The latter are 

 then ponded, giving rise to lakes. The lakes on the lower courses 

 of the tributaries to the Red River of Louisiana are examples.^ They 

 are sometimes built up above their natural level and kept in repair 

 by human agency so as to confine the streams in time of flood. This 

 is a source of danger unless they be steadily maintained, for the break- 

 ing of such levees often occasions great destruction. A case in point 

 is the breaking of the levees of the Mississippi near New Orleans 

 in 1890. The water broke through the levees at the Nita and 

 Martinez crevasses (Fig. 187) and flowed eastward (from the former) 

 with a current of 15 miles per hour, spreading destruction in its 



1 Davis. Science, Vol. X, p. 142, 1887. 



