322 



FIRST YEAR SCIENCE 



153. Run-off. The rain that falls upon the land and 

 neither evaporates nor sinks into the surface runs off as 

 fast as it can toward the sea. It is joined sooner or later 

 by the water from the springs and by the rest of the 

 underground drainage. Sometimes the journey is long 

 and there are many stops and delays in lakes and pools ; 

 sometimes the course is quite direct and quickly traveled. 

 The run-off most profoundly affects the earth's surface. 

 Gullies and valleys are cut, depressions are filled ; in fact, 

 running water is the chief tool which has carved the 

 features of the earth. It has had a long time to act and 

 it has kept unremittingly busy, so that the results of its 

 action appear now in our varied landscape. 



154. Pools and Lakes. The water which runs off the 

 surface first fills the depressions. As soon as these are 



filled, it runs over the 

 lowest part of their rims 

 and starts again on its 

 course to the greatest of 

 all depressions, the sea. 

 If depressions of con- 

 siderable size become 

 filled with water, we 

 call them lakes. As 

 with mountains, the 

 term lake gives no defi- 

 nite idea as to size. In some localities a water surface 

 of a few acres is called a lake, while in other localities, 

 the area must be several square miles to merit this name. 

 As a rule, when the area covered by water is small, it 

 is called a pool or a pond. 



The streams that flow into lakes are continually bring- 

 ing down the sand and mud they have gathered in their 

 course, and are thus filling up the lakes. Lake Geneva in 



AN UNDRAINED UPLAND. 



