INFLUENCE ON WATER FLOW. 163 



cover, occupying 51 i:>er cent of its area, and the general observation is 

 confirmed that the presence of well kept large forest areas at the head 

 waters in the mountains has a favorable influence upon the water 

 stages in the water channels and per cent of erosion and formation of 

 detritus to an appreciable extent. 



One interesting detail hydrographic survey, which has in view to 

 establish relation of forest cover to river, is published by Danckelman. 

 It refers to the river Wupper, an affluent of the lower Rhine from the 

 Khenish state mountains. It was made in connection with the con- 

 struction of a dam to regulate water supplies. Three catchment basins 

 were under consideration, two of these, although containing 32 and 

 39 per cent woodland, are reported as devastated and especially robbed 

 of their soil cover of litter and humus, the third containing nearoU per 

 cent of well-kept dense forest. 



From the detailed observations and measurements there was not ob- 

 servable any influence of forest conditions on the average daily and min- 

 imum flow, but in case of high water the daily flow was most decidedly 

 influenced, namely, a diminution or retardation occurred in the well- 

 wooded basin: in July, by 55 per cent; in August, by 34 per cent; in 

 November, by 28 per cent; in March, by 21 per cent. It is stated in 

 addition that the well-forested basin had a larger amount of rainfall 

 and steeper slopes, a narrower valley, and was, therefore, comparatively 

 more unfavorably situated. 



An interesting note as to the amount of retardation which may be 

 produced by the artificial means employed in the French Alps for reg- 

 ulation of waterflow, namely, forest planting in connection with overflow 

 dams, is given in M. Mathieu's work Reboisement in France. 



The two basins of Faucon and Bourget were visited by a terrible 

 downpour of rain of twenty-five minutes' duration. In the upper moun- 

 tains there fell 42 millimeters, in the lower regions 12.3. The torrent 

 of Faucon (which was in a devastated, deforested condition, but other- 

 wise topographically similar to that of Bourget) was at once filled with 

 flood waters which were estimated to consist of 60,000 cubic meters of 

 water and 180,000 cubic meters of rock material or detritus, the flood 

 subsiding in two hours. 



In the torrent of Bourget, which had been reforested and corrected 

 in its bed, a simple, somewhat turbulent run of water was observed, 

 which at the overflow reached the licight of 45 centimeters (18 inches) 

 and lasted about three hours. The report continues: 



These facts show the iiuportauce of the forest cover. Thanks to the dense forest 

 growth planted, the flood waters, divided in nnraberless runs and retarded constantly 

 in their movement over the declivities in the upper basin, arrive only successively 

 and little by little in the main bed, instc^ad of those formidable masses of water and 

 debris which, rapidly agglomerated, rush into the channel ; the brooks called to re- 

 place the torrents receive only pure water; flood waters flowing off gradually and 

 made harniless by the regulation of the torrent bed and of the slopes. 



