REPORT ON FORESTS. 143 



diverge widely from conditions which obtain in nature and are 

 never safe guides until abundantly verified. 



A word may be said as to another class of data which have 

 been much quoted as proving increased evaporation due to defor- 

 estation. I refer to records of river heights, such as those of Von 

 Wex on the Rhine or Berghaus on the Elbe. According to Von 

 Wex the average height of the Rhine at Emmerich from 1770 

 to 1780 was II Prussian feet 4.1 inches, while from 1825 to ^^35 

 it was only 6 feet 9.2 inches. It will be seen that this is a very 

 great change to take place in iifty years from deforestation 

 alone, so great as to put us on our guard against accepting it as 

 conclusive, for, if long continued, it must result in the entire 

 disappearance of the Rhine. These figures and Von Wex con- 

 clusions have been questioned by European authorities. We 

 wish to call attention to the fact that such a change in the 

 average height of rivers is very often the result of a change in 

 the conformation of the river-bed, which may take place at a 

 considerable distance below the point of observation. It may 

 also be due to erosion of the river-bed at the point of observation, 

 but the fact has not been pointed out, to our knowledge, that 

 precisely this effect would result in a change of the stream from 

 steady to flashy conditions, such as we have indicated to be the 

 real effect of deforestation. Thus in our own experience, in two 

 different months a certain stream discharged the same total 

 quantity of water per month, but during the first month it 

 reached a depth of 16 feet only one day, and stood at four feet 

 during the remaining thirty days, showing steady conditions; 

 while during the other month it reached 16 feet on three days 

 and stood at 4 feet on ten days, and about 18 inches on eighteen 

 days, showing a flashy condition. The average depth during 

 the period of steady flow was 4.4 feet, while during the flashy 

 month it was only 3.7 feet, although, as we have said, the total 

 discharge during each of the two periods was the same. This 

 illustrates the necessity of extreme caution in accepting meas- 

 urements of river height alone as proving a change in the 

 volume discharged by the rivers. Nothing can be accepted as 

 conclusive but actual measurements of the total volume of dis- 

 charge, which must be accompanied by measurements of the 

 rain-fall. 



