92 LANDS OF THE ARID REGION OF THE UNITED STATES. 
wrought by man on the surface of the earth appear to be adequate to the 
production of the observed effects. The destruction of forests, which has 
been immense in this country for the past fifteen years; the cropping of the 
grasses, and the treading of the soil by cattle; the destruction of the beaver 
dams, causing a drainage of the ponds; the clearing of drift wood from 
stream channels; the draining of upland meadows, and many other slight 
modifications, all conspire to increase the accumulation of water in the 
streams, and all this is added to the supply of water to be used in irrigation. 
Students of geology and physical geography have long been aware 
of these facts. It is well known that, under the modifying influences of 
man, the streams of any region redeemed from the wilderness are changed 
in many important characteristics. In flood times their volumes are excess- 
ively increased and their powers of destruction multiplied. In seasons of 
drought, some streams that were perennial before man modified the surface 
of the country become entirely dry; the smaller navigable streams have 
their periods of navigation shortened, and the great rivers run so low at 
times that navigation becomes more and more difficult during dry seasons; 
in multiplied ways these effects are demonstrated. While in the main the 
artificial changes wrought by man on the surface are productive of bad 
results in humid regions, the changes are chiefly advantageous to man in 
arid regions where agriculture is dependent upon irrigation, for here the 
result is to increase the supply of water. Mr. Gilbert, while engaged dur- 
ing the past season in studying the lands of Utah, paid especial attention 
to this subject, and in his chapter has more thoroughly discussed the diverse 
special methods by which inerease in the flow of the streams is caused by 
the changes wrought by man upon the surface of the earth. His state- 
ment of facts is clear, and his conclusions are deemed valid. 
