THE INFLUENCE OF FORESTS ON CLIMATE AND FLOODS 239 



years except as caused by precipitation." 

 He draws this conclusion from the fol- 

 lowing facts : 



Average stage of the Ohio River at 

 Cincinnati, Ohio, 



Ft. 



1871-1889 17-3 



1890-1908 17-5 



Average precipitation in the Ohio 

 watershed : 



In. 



1871-1889 41-3 



1890-1908 41-^ 



Professor Moore is apparently will- 

 ing to draw definite conclusions from 

 insufficient data where this data indi- 

 cates to him that forests have no ef- 

 fect. He criticizes others for drawing 

 definite conclusions where the reverse is 

 indicated. Only on page 28 he has 

 made the remark, which is perfectly 

 correct, that: "Precisely the same 

 amount of rain falling in the two pe- 

 riods and no change whatever in forest 

 or cultivated area, might produce 

 largely differing results on floods ;" and 

 yet', because the average stage of the 

 Ohio River in the two periods corre- 

 sponds, in general, with the average 

 precipitation, he concludes that "the 

 average discharge has not changed ex- 

 cept as caused by precipitation." 



Now, the fact is, that Professor 

 Moore's figures give no indication what- 

 ever of the discharge of the river. He 

 is evidently ignorant of the fact that 

 a given river at a given place, when the 

 water stands at a certain height on the 

 gauge, may be discharging much more 

 or much less than it may be discharg- 

 ing at another time when the water 

 stands at precisely the same height. 

 Readings of gauge heights are very in- 

 accurate indications of discharge. The 

 discharge of a stream depends not upon 

 the gauge-height alone, but also upon 

 the slope of ike surface of the water, 

 not upon the slope of the bed of the 

 stream. A channel may have a level 

 bed for a mile, but if the depth of the 

 water at one end of the distance is 



greater than the depth at the other end, 

 there will be a rate of discharge, de- 

 pending upon the slope of the surface 

 as well as fhe depths. On the other 

 hand, if the bed of the stream is in- 

 clined, but the water surface level, so 

 that the depth is different at the two 

 ends, there will be no discharge. When 

 a flood wave comes down a river, the 

 front of the wave is steeper than the 

 back, and when the water reaches a 

 given height on a given gauge, and is 

 rising — that is, when the front of the 

 wave is passing, the slope of the surface 

 of the water will be greater and the 

 discharge greater than when the crest 

 of the wave has passed and the water 

 stands at the same height on the gauge 

 as before. It is, therefore, fundament- 

 ally wrong to draw any definite con- 

 clusion with reference to the fiozv of a 

 river merely from observations of the 

 gauge-heights. It is equally incorrect, 

 as Professor Moore does, to draw any 

 conclusions with reference to the in- 

 crease in number and violence of floods 

 from the number of days at which a 

 river stood above a certain stage. In 

 his testimony before the Agricultural 

 Committee. Professor Moore, when con- 

 fronted with these facts, said that it 

 made no difference whether the dis- 

 charge was measured by a gauge-height 

 or not, flood conditions were to be de- 

 termined by the number of days at 

 which the water stood above a certain 

 height. 



Now, let us suppose that from 1865 

 to 1885 on a given stream, there were 

 forty floods in which the water stood 

 above a given gauge-height at a given 

 place, an average of four days for each 

 flood, or 160 days in all; and that be- 

 tween 1885 and 1905 there were eighty 

 floods in which the water at the same 

 place stood above the same height an 

 average of two days each, or 160 days 

 in all, the same as before. According 

 to Professor Moore's discussion, the 

 conclusion would be that the floods 

 were not increasing because the total 

 number of days at which the water 

 stood above a given height had not 

 changed ! 



