FLOOD-PLAIN DEPOSITS. 



25 



figuration of the surface, b b the successive levels of deposit, e the level 

 of the river at low water, and i i the level of flood- water. 



A 



Fig. 16.— Ideal Section of a River subject to Floods. 



The extent of such river-swamp deposits is sometimes very great. 

 The river-swamp of the Nile constitutes the whole fertile land of Egypt 

 above the delta. The river-swamp of the Mississippi Eiver, or its 

 flood-plain exclusive of the delta, extends from fifty miles above the 

 mouth of the Ohio to the head of the delta, a distance of about five 

 hundred miles ; its average width is over thirty miles, and it includes 

 an area of 16,000 square miles. It is bounded on either side by high 

 bluffs belonging to a previous geological period. The depth of this 

 deposit at the head of the delta is assumed by Lyell to be 264 feet.* 

 But Hilgard has shown that but a small portion of this is alluvium or 

 river deposit of the present epoch. 



Natural Levies. — It is seen by the cross-section (Fig. 16) that the 

 level of the river-swamp slopes gently from the river outward, so that 

 the river is bounded on each side by a higher ridge, d d. The material 

 of this ridge is coarser than that of the swamp farther back. Such 

 natural levees are found along all rivers subject to regular overflows. 

 They are formed as follows : In times of flood the whole flood-plain is 

 covered with water moving slowly seaward. Through the midst of 

 this wide expanse of water runs the rapid current of the river. Now, 

 on either side, just where the rapid current of the river comes in con- 

 tact with the comparatively still water of the flood-plain, and is checked 

 by it, a line of abundant sediment is determined, which forms the natu- 

 ral levee. Except in very high freshets, these natural ridges are not 

 entirely covered, so that the river in ordinary floods is often divided 

 into three streams, viz., the river proper and the river-swamp water on 

 either side. They can not, however, confine the river within its bank 

 and prevent overflows, since the river-bed is also constantly rising by 

 deposit. Thus the river-bed, the natural levee, and the river-swamp, 

 all rise together, maintaining a certain constant relation to one an- 

 other. 



Artificial Levies. — This constant relation is interfered with by the 

 construction of artificial levees. These are constructed for the purpose 

 of confining the river within its banks, and thus reclaiming the fertile 



Lyell, Principles of Geology, vol. i, p. 462. 



