CHAPTER IV 

 LAND WATERS STREAMS 



The average amount of precipitation on the land is estimated 

 at about 40 inches per year. A part of this water sinks beneath 

 the surface, a part forms pools or lakes on the surface, a part runs 

 off over the surface at once, and a part of it is evaporated. The 

 proportion of the rainfall in any given place which follows each 

 of these courses depends on several conditions, among which are 

 (1) the topography of the surface, (2) the rate of rainfall (or the 

 rate at which the snow melts), (3) the porosity of the soil or rock, 

 (4) the amount of water which the soil contains when the rain falls 

 or the snow melts, (5) the amount of vegetation on the surface, 

 and (6) the dryness of the atmosphere. The steeper the slopes, 

 the more rapid the rainfall, the less porous the soil, the wetter it is, 

 and the less the vegetation, the more water will run off without 

 sinking beneath the surface. 



The water which sinks into the ground becomes ground-water,, 

 while that which flows off over the surface without sinking is the 

 immediate run-off. Much of the ground-water ultimately reaches 

 the surface again, and some of it joins the immediate run-off in 

 the streams. All the water which the streams carry, whether it 

 has been beneath the surface or not, is the run-off. Whether the 

 water which falls as rain stands on the land in lakes, runs off as 

 rivers, or sinks beneath the surface, it is more or less active, and 

 the results of its activity are seen on every hand. 



THE WORK OF RUNNING WATER 



Rivers are estimated to carry about 6,500 cubic miles of water 

 to the ocean annually. The average height of land is nearly half a 

 mile, and the waters which flow from the land to the sea therefore 



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