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GENERAL SCIENCE 



and the waste of the land from higher to lower ground. 

 Usually rivers flow to the sea. The principal stream and 

 all the branches that flow into it constitute a river system 

 (Figure 258). The term " river " is usually applied to the 

 main stream or tb the larger branches of a river system, 



FIG. 258. The Mississippi River System. 



while the term " stream " is a general term and is used 

 quite indiscriminately and with little reference to size. 



Lakes and Inland Seas. A great deal of the surface 

 water in some regions drains into, depressions. When a 

 depression is filled, the water runs over the lowest part 

 of its rim and continues its course to the sea. If these 

 bodies of water are small, we call them ponds ; if they are 

 large, we call them lakes. Lakes are constantly being 

 filled by the land waste that is carried by the inflowing 

 rivers. If the lake has an outlet, the water in it is usually 

 very clear, due to the fact that all the sediment has been 

 deposited on the bottom. The outlet of the lake slowly 



