30 



STREAM- FLOW STUDIES 



Because of the increasing scarcity of small water supplies 

 in some parts of the West and the opportunity of obtaining them 

 by diverting canyon streams, there is need to know what happens 

 to the adjacent vegetation when the greater portion of its mois- 

 ture supply is taken away. For many water-loving trees and 

 shrubs, stream diversion often proves destructive. 



In addition to the usual factors of climate affecting the 

 water requirement of canyon-bottom growth there are the environ- 

 mental factors of type, density and distribution of adjacent vege- 

 tation, slope and depth of the soil mantle supporting the vegeta- 

 tive cover, and axial direction and general slope of the canyon 

 bottom as affecting its exposure to sunlight. The density of 

 growth affects the degree of shade and the amount of transpira- 

 tion, especially that of the under story cover. Slope and depth 

 of the soil mantle control, to a considerable extent, moisture 

 held in the side slopes of the canyon wall. 



The cardinal direction of the canyon axis is likewise 

 important, as is also the direction of slope of the mountain side 

 of which the canyon is a unit. In general a canyon stream extend- 

 ing in a northerly and southerly direction has greater exposure to 

 the sun, and its vegetation has greater transpiration opportunity, 

 than one running east and west. This is especially true of the 

 deeper canyons. 



Likewise the general direction of the mountain slope influ- 

 ences not only such climatic factors as humidity, rainfall, temper- 

 ature, wind movement, hours of sunlight, and melting or retarding 

 of snow cover, but also to some extent the variety and density of 

 the vegetation itself. Such differences, on opposite sides of 

 easterly and westerly mountain ranges, are commonly understood. 

 On the southerly side longer and more intense exposure to the sun 

 increases transpiration losses, snow melts more rapidly, and stream 

 flow decreases or dries up at an earlier date than on northerly 



