104 



THE IRRIGATION AGE. 



DEPTH OF MEAN ANNUAL RUN-OFF IN INCHES. 



mean annual depth of rainfall. The two curved lines 

 show the conditions, the upper one for the high 

 mountain regions, the lower for the more gentle hill 

 slopes and rolling plains. Taking for example the 

 mean annual rainfall, of 40 inches, it has been found 

 that in high mountains the runoff may be expected to 

 be as much as 30 inches, while on undulating country 

 the runoff will be more nearly 15 inches. With a 

 rainfall of 25 inches the runoff from high mountains 

 may be about 12 inches, and from the lower plains 

 less than 5 inches; while with only 10 inches rainfall 

 the runoff from mountains is less than 2 inches and 

 from plains practically nothing. In other words, by 

 following along the upper curve the approximate run- 

 off corresponding to a given rainfall in mountains can 

 be obtained, while by taking the lower curve the sim- 

 ilar relation upon surfaces of less slope is shown. 

 These curves are somewhat arbitrary, and the values 

 actually obtained from observation might go beyond 

 them; but they express this broad fact, viz: That with 

 decrease of rainfall upon any given area there is a far 

 greater decrease of water available. 



FLUCTUATIONS OF STREAMS. 



It must not be supposed from studying these maps 

 and diagrams that the run-off is a certain invariable 

 quantity any more than is the rainfall. The rivers 

 fluctuate within wide limits, perhaps three or four 

 times as much water coming in one year as in the 

 preceding or succeeding year. The figures and dia- 

 grams deal solely with averages, and it must be borne 

 in mind at all times that while averages are useful for 

 obtaining broad views, there is never in nature any 

 such thing as an average year any more than there is 



an average man. To illustrate the wide variation 

 in quantities of water at different times of the year, as 

 well as the enormous differences in rivers themselves, 

 the following diagrams have been introduced, showing 

 a number of large rivers, both east and west. Dia- 

 gram No. 4 gives the maximum, minimum and 

 average quantities of water in the Connecticut, 

 Potomac and Savannah rivers of the east, in compar- 

 ison with tributaries of the Mississippi and a few 

 streams of the west. On this diagram the length of the 

 horizontal bar or block indicates the quantity of water 

 flowing in the river, whose name appears at the left. 

 The solid black, by its length, shows the least flow, 

 the black and shaded the average, and the open 

 portion, combined with the other two parts, the 

 greatest flood. 



FIG. 5. MAXIMUM, MINIMUM AND MEAN DIS- 

 CHARGE OF WESTERN RIVERS. 



In the case of the river at the top of the diagram 

 the upper Missouri in Montana, even in flood, it is so 

 small that it can scarcely be seen on this scale. The 

 Snake river, of Idaho, makes a better showing, but its 

 average discharge is scarcely perceptible. The great 

 flood of Salt river, Arizona, occurring in February, 

 1891, appears at about 300,000 second feet, but the 

 average discharge of this stream is too small to be 

 seen. The Sacramento stands comparison with the 



