•9'-l STEVEXSOX— FORMATION OF COAL BEDS. 539 



crevasses are formed — at times half a mile wide — through which a 

 stream pours with amazing velocity. The conditions are materially 

 different from those prior to settlement of the region, when the 

 floodwaters spread over an area of 100,000 or more square miles ; 

 the energy of the flood stream, when it bursts through a crevasse, is 

 much greater than when there were no levees. This, however, is 

 unimportant, for if the later floods are incompetent to inflict serious 

 injury upon lands protected by vegetable cover, the incompetence 

 must have been more marked when the natural conditions existed. 



The protection afl'orded by levees is shown by constant decrease 

 in extent of the flooded area; the flood of 1887 overflowed almost 

 30,000 square miles below the mouth of the Ohio; that of 1897 cov- 

 ered somewhat more than 13.000, while the area was reduced in 1903 

 to somewhat less than 7,000 — and in this year the extent would have 

 been much less if the new levees at critical localities had been com- 

 pleted so as to resist the very high water. Rivers carrying much 

 detritus and subject to flood build low levees in their passage through 

 the lowlands. The Mississippi constructed such ridges for long dis- 

 tances, thus preventing return of the floodwater, much of which is 

 ponded in swamps and gradually finds its way to the river farther 

 down. This secondary drainage complicates the problem of recla- 

 mation. 



The ^Mississippi floods, imlike those of the Xile, are very complex, 

 for below the mouth of the Ohio the river receives great tributaries 

 from the east and the west, whose floods rarely coincide ; while the 

 upper Alississippi, receiving the iMissouri and other rivers, has its 

 own periods of flood. The source of floodwaters is in the conti- 

 nental storms, arising in the west or southwest and moving toward 

 the east-northeast. The eft'ects are felt first in the lower Missis- 

 sippi, which is filled by streams entering from the west ; the storm 

 advances to the western ridges of the Appalachian where rise streams 

 forming the Ohio, Cumberland and Tennessee rivers. The heavier 

 rains on the Appalachians pour out chiefly through the Ohio but the 

 other streams contribute a great mass. Important floods in the 

 eastern tributaries occur in the spring months, when heavy rains are 

 reinforced by melting snow. The upper Mississippi is not an impor- 



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