288 N. S. SIIALER — BROAD VALLEYS OF THE CORDILLERAS 



soon as a river ceases to cut downward and begins to aggrade it passes 

 a critical point in its history and begins at once rapidl}^ to lose its 

 capacity to deal with the obstructive matter that comes to it. In pro- 

 portion as the bed rises, the rate of flow must diminish, with a conse- 

 quent and rapid loss of capacity to erode or carry debris. 



The degree of the obstruction of the channel of the main stream due 

 to the causes above mentioned depends on the distance of the point 

 where its tributaries emerge from the mountains into its valle3\ Where, 

 as in the case of Alder creek, Madison count}'^, Montana, the detrital fan 

 is about 4 miles in length, the greater part of the coarse debris is in the 

 existing regimen of the stream deposited on the slope, but little of it 

 entering Ruby river. In the lower part of its course the creek has a fall 

 of little less than 20 feet to the mile, and in its original condition it 

 found a devious and much obstructed way through a dense growth of 

 shrubby vegetation. It happens, however, that whenever a valley is 

 narrow, as it commonly is at the point where its stream cuts through a 

 range of mountains, tlie detrital fans are steep, and the discharge of 

 waste into the channel goes on rapidly. In this case the effect is to in- 

 crease the rate of shoaling of the river bed, so that by the consequent 

 diminution of the current a deposit of coarse detritus is laid down in the 

 stream bed. Occasional!}'' above these narrowed parts of the troughs 

 there occur considerable areas of swampy land. 



Tlie obstruction of the drainage of the broad valle3''S is in some measure 

 eftected by the action of the wind in l)earing dust from the more arid 

 parts of tlie area to the banks of the streams. It may be here briefly 

 noted that this dust is impounded in the rank vegetation which is com- 

 monly developed as a selvage along the banks of the water-courses, form- 

 ing a characteristic loose deposit. In the course of the frequent changes 

 in the position of the channel this wind-borne Avaste is apt to be washed 

 away, but it adds to the load of the overburdened stream and diminishes 

 the efficiency of its work. On the detrital fans this accumulation of dust, 

 partl}'^ by its direct obstructive effect and partly through its influence in 

 promoting a dense growth of vegetation, tends to force the torrents into 

 continued wanderings and thus to a wide distribution of the debris which 

 they lay down. 



On the basis above laid down we are led to construct a general hy- 

 pothesis as to the condition which brought about the present aspect of 

 the Cordilleran broad valleys. First we have to suppose- an ancient 

 long continued period of normal river action in which deep valleys of 

 the Alpine t3^pe were formed. Then a period, or rather a succession of 

 periods, of aridity in which the rivers were brought into the abnormal 

 state where they could not remove the debris which the torrents conveyed 



