LAND WATERS STREAMS 117 







depression thus started will be a cause of its own growth, and, as 

 before, the gully might develop into a valley. 



In the presence of sufficient rainfall, therefore, either hetero- 

 geneity of" slope or of material will occasion the development of 

 valleys. If lack of uniformity appears at but few points, there 

 will be but few valleys; if at many points, the number of valleys 

 will be large. Since it is unlikely that any great area of land ever 

 had perfectly homogeneous material and absolutely uniform slopes, 

 every considerable land area, affected for any considerable length 

 of time by abundant rain, has had valleys developed in it; and the 

 heterogeneity of material and slope is usually such that valleys 

 are developed at short intervals. 



The permanent stream. It appears from the foregoing dis- 

 cussion that a valley may be developed by the run-off of successive 

 showers. If supplied from this source only, surface streams would 

 cease to flow soon after the rain ceased to fall, and a valley might 

 attain considerable size without possessing a permanent stream. 

 How does the valley developed by the run-off of successive showers 

 come to have a permanent stream? The answer to this question 

 involves a brief consideration of that part of the rainfall which 

 sinks beneath the surface. 



If wells are sunk in a flat region of uniform structure and com- 

 position, the water in them is generally found to stand at a nearly 

 common level. If a hole 60 feet deep fills with water up to a point 

 20 feet below the surface, it is because the material in which the 

 well is sunk is full of water up to that level. When the well is dug, 

 the water leaks into it, filling the hole up to the level to which the 

 rock (or subsoil) is itself full. This level, below which the rock 

 and subsoil (down to unknown depths) are full of water, is known 

 as the ground-water level, or better, the ground-water surface, or 

 water-table. 



The ground-water surface fluctuates. It rises' during wet 

 weather, because more water sinks then; but several processes 

 conspire to bring it down again. (1) Where there is growing 

 vegetation, its roots draw up water from beneath ; (2) evaporation 

 goes on independently of vegetation; (3) the water is drawn out 

 through wells, mines, etc., and runs out as springs; and (4) it flows 



