458 WATER SUPPLY AND SEWAGE DISPOSAL 



blocked by a dam of pebbles or of sand. It is this blocking 

 of the flow of the ground-water which causes the water table to 

 stand high under the higher portion of land. 



This portion of the state of Michigan is dotted with many 

 small lakes. A lake may now be defined as a portion of the 

 ground-water which rises above the surface of the ground. When 

 the land surface and the water table remain at about the same 

 level all the year round, certain aquatic vegetation grows in 

 abundance over the water-soaked area and we have a SWAMP 

 or MARSH. 



If the land had originally sloped off with perfect smoothness 

 from the watershed to the lake on either side, like the roof of a 

 building, it is evident that the water table would also have 

 sloped off evenly to each lake. The water table would have 

 formed the counterpart of the land surface. At times of 

 heavy rainfall and slight evaporation, the soil would have be- 

 come completely saturated. As more rain fell, it would have 

 been impossible for it to soak into the soil. This water would 

 then have flowed over the surface of the ground toward the 

 lakes. Soon furrows or channels would have been washed in 

 the soil. These channels would have grown deeper and wider 

 with the passing of time and flowing into these main channels 

 would have been many smaller branches, Many of these 

 channels would soon have been cut so deep as to lie below the 

 water table throughout the year. Such a channel is called a 

 RIVER. This is, in general, the history of river formation. 



636. Drainage through the Soil. Since the river channel is 

 cut considerably below the general level of the water table, it is 

 evident that there is a constant drainage of ground-water 

 through the soil laterally into the river channel. This lateral 

 PERCOLATION of water through the soil is usually very slow. 

 It varies much in different regions and under different circum- 

 stances. Different tests have shown the rate to vary from 5 ft. 

 to 100 ft. per day. In one instance (the valley of the Loup 

 River, Nebraska), evidence was found to show that consider- 

 able quantities of ground-water traveled a distance of 20 miles 



