156 FOREST INFLUENCES. 



While we have here cousideied condition;; under which spring's are 

 formed, there are also conditions under which their formation is ex- 

 cluded ; such might be found in extended plains or low hill lands, with 

 a compact, impermealde soil, which may give rise to pools and morasses, 

 but not to springs. Plateaus of fissured limestone dolomites or of com- 

 Ijact gneisses or granites may also be poor in springs, their waters sink- 

 ing at once to such depths that no discharge is met in the immediate 

 neighborhood of the catch-basin, or else shedding the water at once 

 superficially. 



The most direct influence of a forest cover ui^on the discharge would 

 be noticeable on the surface springs, since in these the catchment area 

 and the place of discharge lie close together, while the underground 

 run is not only short, but lies near the surface, and hence experiences 

 most sensibly the effect of the j^rotection against evaporation which 

 the forest cover offers. Deforestation here would no doubt i*educe or 

 cut off discharge eutirel5^ 



In cavern springs an influence could be exercised only in the indi- 

 rect manner, by the increase of filtration over the catchment-basin. The 

 same pertains to fissure springs, whose sources of supply are usually 

 quite removed from evaporative influences, and only where these come 

 nearer the surface or when the spring is only small, may the removal 

 of the shade of forest cover reduce the outflow. 



With reference to ground-water springs, which come to light at a 

 considerable distance from the catchment-basin, the conditions of the 

 latter as far as the influence, increase, and preservation of water suji- 

 plies, and of the area over and under which the waters run collect, is 

 of considerable importance, while the surface condition of the area 

 within which the spring lies {a e of Fig. 61), if of impermeable strata, 

 is of less consequence, except that a forest growth may lower the ground- 

 water level by transpiration, should the water quantities furnished 

 from the catchment-basin not be continuous and sufficient. If these 

 strata consist of permeable soil, they would act as a second catchment- 

 basin, and the effect of the soil cover upon the quantity of drain waters 

 (precipitation, evaporation, and transpiration) would be directly no- 

 ticeable. W"e have seen that the tendency of the forest cover — trees, 

 foliage, litter, moss — is to change a certain amount of surface draiimge 

 into subterranean drainage, or, in other words, to reduce the surface 

 waters where they have fallen. Eventually, however, the subterra- 

 nean waters come to the surface again, and add their stores to the 

 surface waters that are carried away in open runs, brooks, and rivers. 

 Finally, then, all the water that falls on the catchment basin, except that 

 which is returned to the atmosphere by transpiration or evaporation, 

 becomes surface water; but the manner in which it runs off is the im- 

 portant pointo 



