20 POPULAR SCIENCE MONTHLY 



by the Mississippi. These three rivers, therefore, yield over 60 per 

 cent, of the gross volume, the remainder being divided between the 

 White, Arkansas, St. Francis, Yazoo and Eed rivers, which join the 

 main stream too near the mouth to be important factors in the pro- 

 duction of severe floods. 



The Ohio is, on the whole, much the worst flood offender, partly 

 because of its normally greater volume, and partly because of the 

 conditions existing in the region it drains. The major portion of the 

 areas drained by the Missouri and the upper Mississippi are distinctly 

 less rugged than the Ohio basin, and over both areas the heaviest 

 rainfall, coming in May and June, arrives at a season when the soil can 

 take a large percentage of it. The largest tributaries of the Ohio, on 

 the contrary, in etching their valleys in the surface of the Allegheny 

 plateau, have produced the steepest and most rugged parts of the whole 

 Appalachian region. Here the heaviest rainfall comes in January, 

 February and March. Add to these factors the frequent complications 

 of melting snow and frozen ground, which sheds water like a house 

 roof, a district largely deforested, and the enormous destruction by 

 sudden rising of the Ohio is explained. It is truly fortunate that by 

 the provisions of nature the three rivers, the Ohio, the Mississippi and 

 the Missouri, have never been known, and probably never will be known, 

 to be in extreme flood at the same time. Such an unhappy coincidence 

 of high stages, if it came about, would quite certainly mean total oblit- 

 eration for everything in the lower valley. 



The flood evil is in a large way the underlying cause of most of 

 the trouble in the Mississippi. At time of flood, the erosive power of 

 the river is increased a hundredfold, caving of the banks is often exces- 

 sive, levees situated rods from the channel before the flood and appar- 

 ently safe are undermined, and the narrowing of necks between bends 

 is greatly accelerated or quickly accomplished. Much of the damage 

 from floods must be laid to the cavings of banks by which landings are 

 destroyed and whole plantations are soon swept away, while through 

 breaks in the undermined levees the raging waters sweep over the sur- 

 rounding country. This spread of the flood is fostered by the fact that 

 the river channel lies above the surrounding bottom lands. To the 

 naked eye the region appears absolutely level, but from the river the 

 broad plain slopes away at a rate varying from four to thirteen feet 

 per mile. Once outside its channel, therefore, the water finds a natural 

 course down hill into every part of the back country, carrying destruc- 

 tion wherever it goes. Unfortunately, this character of the river can 

 not be altered; on the contrary, the more the river is confined between 

 artificial levees and restricted in the area over which it is free to 

 spread, the greater will be the devastation whenever a flood does occcur, 

 so that with the extension of the levee system the occurrence of floods 

 becomes an increasingly serious problem. The recognition of this fact 



