The Equalizing Influence of Forests on the Flow 

 of Streams and Their Value as a Means of 

 Improving Navigation 



By GEORGE F. SWAIN, LL.D., Late Professor of Civil Engineering in the Massachusetts 



Institute of Technology ; Now Professor of Civil Engineering in the Graduate 



School of Applied Science of Harvard University 



(Concluded} 



WHEN it comes to the question elusion that there would be no flow at 

 of extreme droughts, Colonel all in the stream. In other words, the 

 Chittenden takes a curiously author would have the mills at Law- 

 contradictory position to the one which rence and Lowell depend for their sum- 

 he takes in considering the matter mer flow, not upon keeping up the 

 of floods. Regarding the latter, it "springs and little streams" so far as 

 will be remembered, he considers that possible by increasing through the ef- 

 the forests may cause a combina- feet of forests the percolation into the 

 tion of the highest floods arising ground, but would have these mills 

 simultaneously from different tribu- trust to luck that the summer showers 

 taries ; with reference to droughts, would be so distributed over the differ- 

 however, he assumes just the reverse, ent tributary basins that when one was 

 namely, that the extreme low water on low others might be high, and he main- 

 different tributaries will not occur tains that in this way the low water 

 simultaneously. It seems clear that the would be greater than if all the little 

 extreme combination is as likely to oc- springs were kept up. This would, of 

 cur in one case as in the other. course, require most intelligent plan- 

 He admits "that, as a general rule, ning on the part of Jupiter Pluvius, for 

 springs and little streams dry up more it would not do to have these summer 

 completely than when forests covered showers, which are supposed to flow 

 the country," but he argues that, since rapidly from the surface, inaccurately 

 each spring is small, their drying up will timed or distributed over the basin. It 

 have little effect upon the main stream, does not seem necessary to pursue this 

 the flow of which will be kept up, if the suggestion further. 



region is deforested, by the rapid dis- Even a large drainage area, say 

 charge, over the surface, of the water 10,000 square miles, may well have its 

 from summer showers, which will oc- main stream possess a length from ex- 

 cur, first on one tributary and then on tremest source to mouth, measured on 

 another, in such a way as to furnish to the stream of considerably less than 

 the main stream always a low-water 300 miles. If the average velocity of 

 flow greater than if the springs could the stream is one mile per hour, which 

 all be kept up. If his argument be is low, it would take less than two weeks 

 carried to the very common case where for a drop of water to pass from the 

 no rain falls upon a given drainage extremest source to the mouth. Now, 

 basin for weeks, or for a much longer even in districts which have a summer 

 time than it takes for a drop of water rainfall, it frequently happens that even 

 to flow from the extreme source to the an area as large as that mentioned is 

 mouth, it would seem to lead to the con- without rain in part of it for months 



557 



