RIVER 



5025 



RIVER 



The basin of the Mississippi River, for instance, 

 includes that portion of the United States 

 drained by that river and its tributaries. The 

 volume of a river depends upon the area of its 

 basin and the amount of rainfall. Heights of 

 land which separate rivers and river systems 

 are known as divides. 



The Work of Rivers. The course of a river 

 is divided into three parts the upper, the mid- 

 dle and the lower course. These parts are sel- 

 dom of equal length, and each is marked by 

 distinctive characteristics. In the upper course 

 the slope of the channel is steep and the cur- 

 rent swift. The channel has been worn down 

 rapidly and the banks have a steep slope, some- 

 times being nearly perpendicular. The water 

 carries quantities of sand and gravel and some- 

 times rocks of considerable weight, which are 

 borne swiftly along by the current, constantly 

 wearing away the bed of the stream. The 

 swiftness of the current enables it to remove 

 most of the obstacles in its course, and the 

 channel is free from small curves. The chan- 

 nels of the tributaries are usually worn down 

 to the level of the main channel, forming ra- 

 vines. 



The river enters upon its middle course when 

 it leaves the mountainous or hilly region in 

 which it rises, and enters the lower lands where 

 the slope is more gentle. The current is not 

 swift enough to carry the heavy material that 

 it has brought down to this point, and this is 

 deposited on the bottom of the channel. For 

 this reason the beginning of the middle course 

 of many rivers is marked by gravel beds. Since 

 the channel is worn more slowly the slope of 

 the banks is more gentle and the valley is 

 broader. The current has lost much of its 

 velocity and it cannot remove obstacles; there- 

 fore it must flow around them, and so the 

 middle course is characterized by numerous 

 curves, some of which may take the river miles 

 out of the general direction of its flow. The 

 middle course of the Mississippi affords an ex- 

 cellent illustration of this fact. Again, obstruc- 

 tions in the middle of the channel collect de- 

 posits of silt until finally they reach the surface 

 and form islands. Islands in the upper part 

 of the stream, on the other hand, consist of 

 rocks which the current has been unable to 

 wear away. 



lower course of a river differs but little 

 from the middle course. The current is sin 

 and the continuous deposit of silt raises 

 bed of the stream. Frequent overflowing of 

 low banks forms vast flood plains which, 

 315 



when drained, make productive farms. In case 

 of the Mississippi this process of raising the 

 river bed has continued until the river is higher 

 than the surrounding country, and disastrous 

 floods follow a break in the banks. 



Cataracts and Canyons. Rivers frequently 

 flow over rocks of unequal degrees of hardness ; 

 the softer rock is worn away, leaving the harder 

 as an obstruction. These conditions are usually 

 found where the current is swift, and rapids or 

 cataracts are formed. The cataracts of the Nile 

 and the rapids in the Saint Lawrence are good 

 illustrations. When the soft rock lies under a 

 hard layer a fall like Niagara is formed. In 

 mountainous regions the swift current some- 

 times wears a deep channel with vertical walls 

 forming a canyon. The Grand Canyon of the 

 Colorado and the Grand Canyon of the Yellow- 

 stone, two of the world's most stupendous won- 

 ders, were formed in this way. 



Estuaries and Deltas. A river flowing slowly 

 into an arm of the sea, protected from great 

 waves and high tides, deposits its silt at its 

 mouth and builds up a vast alluvial plain called 

 a delta. The deltas of the Mississippi and the 

 Nile are excellent examples of such formations. 

 When the bed of the river in its lower course 

 slopes into the sea, forming a drowned valley 

 up which the tides extend without obstruction, 

 the silt is carried away and the mouth of the 

 river remains a broad estuary like the Gulf of 

 Saint Lawrence and the Rio de la Plata. These 

 estuaries form excellent harbors, and upon them 

 we find some of the greatest seaports, w J.R. 



Consult Russell's River Development as Illus- 

 trated by the Rivera of North America; Green- 

 wood's Rain and Rivera; Heliosis' River and 

 Canal Engineering. 



Related Subjects. The following topics, which 

 are closely related to this subject, are treated in 

 these volumes : 



Alluvium Flood 



Basin Flood Plain 



Canyon Rain 



Delta Spring 



Divide Valley 



Erosion Waterfall 



In addition to these general articles, these vol- 

 umes contain articles on the following rivers : 



Congo 



Gambia 



Niger 



Nile 



Orange 



Amur 



Brahmaputra 

 Euphrates 

 Ganges 



AFRICA 

 Senegal 

 Shire 



Victoria Falls 

 Zambesi 



Hoang-ho 

 Hugll 

 Indus 

 Irrawaddy 



