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 

 of extreme droughts, Colonel 

 Chittenden takes a curiously 

 contradictory position to the one which 

 he takes in considering the matter 

 of floods. Regarding the latter, it 

 will be remembered, he considers that 

 the forests may cause a combina- 

 tion of the highest floods arising 

 simultaneously from different tribu- 

 taries ; with reference to droughts, 

 however, he assumes just the reverse, 

 namely, that the extreme low water on 

 different tributaries will not occur 

 simultaneously. It seems clear that the 

 extreme combination is as likely to oc- 

 cur in one case as in the other. 



He admits "that, as a general rule, 

 springs and little streams dry up more 

 completely than when forests covered 

 the country," but he argues that, since 

 each spring is small, their drying up will 

 have little effect upon the main stream, 

 the flow of which will be kept up, if the 

 region is deforested, by the rapid dis- 

 charge, over the surface, of the water 

 from summer showers, which will oc- 

 cur, first on one tributary and then on 

 another, in such a way as to furnish to 

 the main stream always a low-water 

 flow greater than if the springs could 

 all be kept up. If his argument be 

 carried to the very common case where 

 no rain falls upon a given drainage 

 basin for weeks, or for a much longer 

 time than it takes for a drop of water 

 to flow from the extreme source to the 

 mouth, it would seem to lead to the con- 



clusion that there would be no flow at 

 all in the stream. In other words, the 

 author would have the mills at Law- 

 rence and Lowell depend for their sum- 

 mer flow, not upon keeping up the 

 "springs and little streams" so far as 

 possible by increasing through the ef- 

 fect of forests the percolation into the 

 ground, but would have these mills 

 trust to luck that the summer showers 

 would be so distributed over the differ- 

 ent tributary basins that when one was 

 low others might be high, and he main- 

 tains that in this way the low water 

 would be greater than if all the little 

 springs were kept up. This would, of 

 course, require most intelligent plan- 

 ning on the part of Jupiter Pluvius, for 

 it would not do to have these summer 

 showers, which are supposed to flow 

 rapidly from the surface, inaccurately 

 timed or distributed over the basin. It 

 does not seem necessary to pursue this 

 suggestion further. 



Even a large drainage area, say 

 10,000 square miles, may well have its 

 main stream possess a length from ex- 

 tremest source to mouth, measured on 

 the stream of considerably less than 

 300 miles. If the average velocity of 

 the stream is one mile per hour, which 

 is low, it would take less than two weeks 

 for a drop of water to pass from the 

 extremest source to the mouth. Now, 

 even in districts which have a summer 

 rainfall, it frequently happens that even 

 an area as large as that mentioned is 

 without rain in part of it for months 



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