FLOOD 



2210 



FLOOD PLAIN 



of the Nile, which deposits on the plains of 

 Egypt fertile soil from regions farther south, 

 has enriched the country and has built the 

 world's greatest oasis, and there are many lesser 

 instances of a similar nature (see NILE). Sud- 

 den and violent floods bring huge losses, and 

 the study of means of prevention is a task 

 of growing importance. River floods are more 

 frequent than sea floods, though not more 

 serious. 



River Floods. The Mississippi and its 

 tributaries and the Hoang-ho, or Yellow River, 

 in China, are noted for their tendency to 

 overflow their banks. The Hoang-ho flows 

 through a channel raised above the level of 

 the surrounding plain partly by nature, partly 

 by man-made dykes. When it breaks its banks 

 it sometimes devastates an area as large as 

 Great Britain. After each flood the river takes 

 a new course; in the last 2,500 years its mouth 

 has shifted over 200 miles, or nearly a mile 

 every twelve years. Floods in the Mississippi 

 basin are caused by excessive rainfall and sud- 

 den melting of snow and ice. The Upper 

 Mississippi is not subject to them, but its 

 lower course and its branches, the Ohio and 

 the Missouri, frequently overflow. 



The greatest flood in American history oc- 

 curred in the states of Ohio and Indiana in 

 April, 1913, in valleys tributary to the Ohio, 

 principally along the Miami. It is commonly 

 thought that such floods are due to the partial 

 blocking of the river by bridge piers, filled 

 land, sandbars and other obstructions, but 

 engineers say that these are but minor sources 

 of the trouble; the real difficulty lies in the 

 fact that the natural channel of the Miami will 

 hold only one-tenth as much water as it is 

 sometimes given to carry; there are many 

 other rivers even less capable of the work they 

 may be called upon to do. 



River lands become more liable to flood as 

 the higher land surrounding them is stripped 

 of trees and other vegetation, for then a 

 heavy rainfall runs off rapidly and a river 

 must do in a day the work of weeks in ordi- 

 nary times. So tree planting is recommended 

 as the first step in flood prevention. Also, 

 reservoirs may be built which will hold back 

 the water until the river can carry it off. The 

 danger here is that a second great flood may 

 immediately follow the first, overflowing, per- 

 haps bursting, the reservoir. What would hap- 

 pen then is instanced by the very disastrous 

 Johnstown (Pa.) flood in 1889. Levee building 

 is a more common step. Much .of the Missis- 



sippi below Cairo, 111., is bordered by levees, 

 but these frequently break and allow the 

 stream to spread out, in some places to a 

 width of seventy miles. On such occasions the 

 damage to property is often millions of dollars. 

 Seacoast Floods. Floods from the sea are 

 most frequently due to the piling up of the 

 waters by high winds. Holland, which is partly 

 below sea level, protects itself from them by 

 its famous dykes, which in modern times sel- 

 dom fail. In other regions such tidal waves 

 come unexpectedly, and there is no provision 

 against them. After its great flood of 1900, 

 Galveston built a sea wall, but in the storms 

 of 1915 this proved insufficient to protect the 

 city completely. Earthquakes and volcanoes 

 also produce waves, sometimes a hundred feet 

 high. There have been notable instances of 

 them at Lisbon, Messina and in Japan; at the 

 eruption of Krakatoa in 1883 a large vessel was 

 swept inland a mile and a half and stranded 

 thirty-nine feet above the sea. R.D.M. 



Related Subjects. The reader is referred to 

 the following articles in these volumes : 

 Earthquake Lisbon 



Hoang-ho Mississippi 



Johnstown, Pa. Netherlands 



Levee Nile 



FLOOD PLAIN, a term used in physical 

 geography to describe a plain formed of sedi- 

 ment deposited by a river. Floods in high 

 regions carry off quantities of earth and other 

 matter, which are deposited lower down in 

 the plains by the flooding of the river channel. 

 The overflowed waters lie practically still on 

 the surface of the land and a natural deposit 

 occurs. This is clearly illustrated along the 

 banks of the Mississippi River, where flood 

 plains covering an area of 50,000 square miles 

 have been formed by the deposits from the 

 often-flooded river. In the case of the Missis- 

 sippi flood plains, the deposits are so light 

 that the river is constantly washing out new 

 channels. 



The nature of the deposit varies greatly, and 

 is governed by the violence of the flood and 

 the nature of the country through which the 

 stream passes. A raging torrent will carry 

 gravel, stones and even small rocks, while 

 flood of less violent proportions might ca 

 only light gravel; sluggish waters would con- 

 tain only fine silt, slowly carried along. A 

 flood plain may be caused by a bank or bar 

 across the river mouth, which compels the 

 river to drop its load, which gradually ac- 

 cumulates until sometimes it forces the river 



