SOIL PHYSICS 



459 



in 60 days, or }< mile per day. This is, without doubt, a 

 very unusually rapid motion for soil-waters, amounting as it 

 does to more than a foot per minute. Even at this rate, if the 

 ground-waters of Michigan moved evenly down the slopes to 

 the east and the west to the lakes it would take the water a 

 year to move from the watershed to the lake. That the 

 state is drained to a considerable extent by this lateral percola- 

 tion is shown by the fact that over many large areas of this 

 portion of the state there are no visible streams to carry off 

 the surplus rainfall. The drainage in such places is entirely 

 by means of percolation. 



537. Relation of the Ground -water to the River-water. 

 It is interesting and important to note the relation of the 



//V/TZ TKarrass 



FIG. 281. Ground-water and river-water. 



ground-water near a river to the water in the river bed. 

 Anyone who has followed the course of a river far is aware that, 

 in most cases, springs are common along its banks. In many 

 cases where no running water flows down the bank, the banks 

 are very moist, showing the near presence of free ground-water. 

 The cause of springs is easily seen when we know that the con- 

 ditions shown in Fig. 281 are not uncommon. On the river 

 bank to the left we have the ideal conditions for a SPRING. 



Swimmers are aware of the fact that springs of cold water are 

 often located in the very bottom of river beds. The colder 



