244 The Realm of Nature CHAP. 



calculates that of 29,350 cubic miles of rain falling on the 

 land every year, only 6520 cubic miles reach the sea as the 

 discharge from rivers, the remainder being re-evaporated or 

 absorbed in the Earth's crust. 



319. River Systems. The connected streams which 

 unite to form a river constitute a river system. The 

 series of convergent slopes down which a river system flows 

 in other words the land which it drains is called its basin, 

 and is separated by a watershed or water-parting from the 

 basins of neighbouring river systems. A watershed is 

 always the meeting-place of the highest part of two diverg- 

 ing slopes. This is sometimes a mountain range, but often 

 only the crest of a gently rising ground, on which the line of 

 water-parting is difficult to trace ( 360, 362). It is usual to 

 name a river system after the river into which the water is 

 collected from the whole basin, the other streams being 

 called tributaries or affluents. The basins of all river 

 systems draining into one ocean are known collectively 

 as the drainage area of that ocean. The beginning of a 

 river is called its source, and must necessarily be the 

 highest part of its course. When a large river flows from 

 a lake it is often difficult to decide which of the short 

 streams entering the lake is to be viewed as the ultimate 

 source. The name of the main river in a great system, such 

 as that of the Amazon or the Mississippi, is given by some 

 geographers to the tributary which has the most direct 

 course, by others to that of greatest length or to that with 

 the highest source. This diversity of opinion accounts to 

 some extent for the great difference in length assigned to 

 rivers by different authorities. The area of the basins or 

 the volume of discharge is a better measure of the size of a 

 river. It is interesting to notice in the following table of 

 the five greatest rivers that although the Nile basin receives 

 one-third more rain than the Mississippi, its discharge is only 

 one-fifth, on account of the great evaporation in crossing 

 the desert. The Yang-tse-Kiang, Yenesei, Amur, afid Mac- 

 kenzie are intermediate in length between the Amazon and 

 Congo, and the Yang-tse-Kiang and Orinoco have a dis- 

 charge equal to the Mississippi. 



