SOIL PHYSICS 



457 



vessel or a lake is between 20 and 30 in. (see Fig. 149) . From 

 these facts it is clear that, if all of the rain which falls upon the 

 peninsula were to remain upon the surface of the earth, the 

 portion which does not run off would completely evaporate 

 during the year. But much of the water which falls as rain 

 immediately sinks into the earth and becomes GROUND-WATER. 

 This ground-water evaporates very slowly and, as only a 

 small portion of the peninsula is covered by open water, the 



FIG 280. Mean annual run-off, United States. 



total evaporation from it is much less than the amount which 

 would evaporate were it completely covered with open water. 

 535. The Origin of Lakes and Rivers. As stated, much of 

 the rain which falls upon this land sinks down into the soil. 

 Some of it evaporates from the soil, as will be shown later. 

 Much of it reaches the water table. The consequence is that 

 the ground-water becomes piled up at the center of the state, 

 for it cannot run off readily through the soil into the lakes. 

 While it tends to flow off to the lakes from the highest point, its 

 passage is constantly blocked by the soil, just as a stream is 



