CHAPTER XLVI. 



RIVER AND MARINE EROSION. 



THE rain falling upon the uneven surface of the earth, forms 

 little rivulets, which, uniting with others, form larger, and so on, 

 till the brooks and creeks are formed ; which together constitute 

 the river that receives all these little streamlets each with its 

 burden of sediment and conveys them, at length, to the great 

 and wide sea. Beside this surface water, the rivers receive the 

 water from thousands of springs, spring water, in many cases, 

 forming the greater part of the supply during the year. The re- 

 gion of country drained by a river is called the river valley, or 

 basin, and is usually considerably longer than wide. The char- 

 acter of a river will depend on the nature of the rocks of its val- 

 ley ; on the character of the climate of the valley and on the rap- 

 idity of its slope. Rivers vary in the amount of water and other 

 material they carry. Nearly all rivers have an annual flood, and 

 many are subject to change from exceptionally wet or dry seasons. 



The Mississippi river, draining a wide valley embracing a va- 

 riety of climate, is somewhat uniform in its flow, subject to its 

 annual spring flood, and sometimes an exceptionally wet season 

 will give a summer flood. 



The Nile river depends for its water supply on seasonal rains 

 in the upper part of its valley, and is very uniform in all its phe- 

 nomena from year to year. In regions subject to great irregu- 

 larities of rain-fall, the change from low water to flood is often 

 very great, as rivers in south-east Australia, or even in the south- 

 western part of the United States. 



In general a river has three parts the mountain portion, 

 where formed by mountain torrents, it rushes noisily over its steep, 

 rocky bed to the lower ground that forms its middle or valley 

 course. In this part of its course the river flows through a more 

 varied country, lower mountains, wide valleys, narrow gorges, 

 (342) 



