102 FOREST INFLUENCES. 



turbance. Here the comparative lengths of the affluents alone m ay- 

 become all important, since the simultaneous or uonsimultaneous 

 arrival of flood waters may determine the occurrence or nonoccurrence 

 of high floods. As far as forest cover is concerned in such cases, 

 deforestation in one of the side valleys and consequent rapid discharge 

 may become an advantage for the water flow in the main river, by allow- 

 ing its removal before the arrival of the flood waters of another affluent. 

 In view of these considerations it would, therefore, be folly to assign to 

 the condition of forest cover in the catchment basiws an all determinative 

 function. jSTevertheless, in general the influence of favoral)lc forest 

 conditions in the catchment basin upon river flow can not be doubted, 

 although it may become practically of no account in abnormal floods. 



The first cause of abnormal floods is the occurrence of abnormal rain- 

 I'alls or the sudden thawing of abnormal masses of snow. If the former 

 occur after the soil has been saturated, or the latter when the soil 

 remains frozen, the forest cover will be powerless to influence the run- 

 off and will shed the water as rapidly almost as the open ground, 

 although even the brief retardation of the confluence of water masses 

 which the obstacles of a forest growth cause may be of moment. 



But in its further course the drainage of this Avater, collected in the 

 rivers is favorably influenced by the presence of the forest, it having 

 prevented the formation and deposition of detritus in the river bed. 



The beneficial influence of the forest in case of abnormal floods can 

 then i^robably be claimed only in so far as it protects the slo])es 

 against abrasion and the formation of debris or detritus with which 

 the upper head waters are filled, and which carried down into the 

 rivers gives rise to sand banks and changes in the river bed which may 

 increase the dangers of the next flood. 



EXAMPLES OF THE IXFLCENCE OF FORESTS ON WATEKFLOW. 



Besides the detail experiments, which are to furnish explanation of 

 the physical laws upon which differences in water drainages occur, 

 there are constantly accumulated experiences. Especially in France this 

 question of forest influence on waterflow has been answered by practi- 

 cal demonstrations. To bring it, however, to a final solution, special 

 hydrographic surveys and statistics, such as are now contemplated in 

 Germany, will be necessary. A first attempt at such a work is the hydro- 

 graphic description of the lihine, a model work compiled by the agency 

 of the various governments whose lands border on that river, published 

 in 1889. This, however, was too comprehensive a field, and more de- 

 tail measurements and observations would promise more striking re- 

 sults. The conclusions from this survey, and a more detailed one of the 

 Hauensteiner Alp in the southern Black Forest mountains, are that the 

 comparative absence of damage from high floods in this Alp country, 

 when compared with neighboring valleys, may be ascribed to the forest 



