SURFACE DRAINAGE. 157 



-INFLUENCE OF FORESTS UPON SURFACE DRAINAGE. 



Surface waters, like springs, may be considered from three i)oints of 

 view, namely their quantity, the course and manner of distribution, 

 acd finally their behavior when collected in rivers. All that has been 

 said regarding the conditions of nnderground drainage has, of course, 

 a bearing upon the quantity of surface drainage. The amount of sur- 

 face run-oft" is the complement of the amount drained oft' by springs, 

 audit follows that where surface drainage is the rule the supply to 

 springs is scanty, and vice versa. 



It is evident that the conditions for a superficial run off are to be 

 found, first, in the amount and nature of precipitation, and next in the 

 soil and surfiice conditions. A violent rainstorm will naturally furnish 

 more superficial run-oft' than when, the rain falling slowly, time is given 

 for the soil to abvsorb it; a snow cover, fallen on frozen ground, is apt 

 when melting to shed its water over the frozen surface without pene- 

 trating the soil. 



Nature of soil and soil cover and t opography determine, with equal 

 amounts of water to dispose of, what the nature of the run off will be. 

 An impermeable soil takes up sufficient water to make it plastic and 

 then sheds all additional water superficially; a permeable soil continues 

 to take up water and conducts it into the depth. This difference of 

 behavior must influence and determine largely the conditions of any 

 riverbed; for if it run for some distance through impermeable soil 

 even insignificant rainfalls will rapidly collect and swell the river, 

 while the jiermeable soil would have taken up and held all or parts of 

 the precipitation and would only gradually have given it up. 



The topography determines the rapidity of run-off and of collection. 

 The more diversified the country — cut into dells, coves, rills, and fur- 

 rows, steeper and less steep slopes — in the greater number of runs of 

 unequal length is the water collected, while the Ifess diversified the con- 

 tour the more water must be carried off in each run. Yet where the 

 diversity of configuration is accompanied by steep slopes the run oft' 

 may be so rapid that the valley river is filled more rapidly than the 

 river of the open pi ains country with even slopes of moderate inclina- 

 tion. 



Thus in some of the river valleys of West Virginia the watersheds 

 are scooped out into such an array of coves, gashes, and water courses 

 and minor watersheds, and so steep and rapid in descent that, in spite 

 of the forest cover, a raiutall of a few days will induce a rapid rise of 

 the rivers, while the same amount of rain will hardly wet the ground 

 in a prairie country like Iowa. 



As regards soil and surface conditions it is ()l)vious that the less 

 permeable the soil or soil cover the less the absorptive capacity of the 

 same, and the fewer mechanical obstructions are met the more water 

 runs oft' superficially. 



