196 THE WORK OF RUNNING WATER 



Lakes could be connected by navigable channels with the 

 great Mississippi system. The dredging of portions of the 

 Mississippi channel, the straightening of its course, and the 

 building of additional permanent levees must some day be 

 accomplished. Such improvements would render many cities 

 along its banks veritable inland seaports. 



Waterways such as these would relieve the great freight 

 congestions that now so frequently occur on railroads and 

 that will become more frequent with the increase of popula- 

 tion. While water transportation is slower, it has the great 

 advantage of being much less expensive. Many such 

 improvements as have been mentioned have been strongly 

 recommended by a Commission appointed by the Federal 

 Government. 



Sub-surface Water or Ground Water. The rain that 

 sinks into the ground descends slowly along the little cracks 

 or between the particles of soil until it reaches a point 

 where it can sink no further, or until it finds an opening 

 through which it can flow out to the surface at a point 

 lower than where it entered. Here it may ooze slowly out, 

 or it may be concentrated in a spring. 



If the water which comes to the spring has penetrated 

 below the surface far enough to get away from the heating 

 effect of the sun, it will be comparatively cool when it again 

 emerges, and it will form a cold spring. If, however, in 

 the region where the spring occurs the rocks are hot at the 

 depth to which the water penetrated before it found a crack 

 through which it could come to the surface of the land, 

 then it will become heated and will form a hot spring. 



As the crust of the earth is in many places composed of 

 rocks in layers, the rain often falls upon the top of a folded 



