144 THE FLOOR OF THE OCEAN 



maximum of nearly 200 feet. Forel imagined two possibilities : 

 first, that the channel was dug by bottom currents; second, 

 that it is the result of preferential deposition of the silt where 

 the current loses velocity by friction against the quiet water on 

 each side of the current. The latter process would be analogous 

 to the levee-building along the lower Mississippi River. Forel 

 ultimately came to favor the second explanation of the trench. 

 On the other hand, a much later study led Dr. J. Romieux to 

 credit erosion of the channel by silty underflows and also up- 

 building of the channel's rim in the form of a sub-lacustrine 

 levee. 14 In any case the Swiss limnologists have agreed that 

 the bottom current persists to a distance of six miles from the 

 mouth of the Rhone. Their conclusion deserves strong empha- 

 sis in the discussion of submarine valleys. 



And Switzerland has a parallel case. The contoured map 

 of the Rhine delta under the surface of Lake Constance por- 

 trays a trench across the delta, with dimensions much like those 

 of the Lake Geneva channel. Be it noted, too, that each of the 

 Swiss channels has a slope which is less than one two-hundredth 

 of the average slope down the flank of a continent. 



For our problem, analogy becomes still more illuminating 

 when we watch the course of a muddy river that enters a long 

 reservoir, created by an artificial dam at the opposite end. 



An ideal case is represented where the Colorado River 

 penetrates Lake Mead, a reservoir with a length of about 120 

 miles, measured from the celebrated Boulder Dam that holds 

 up the water level. See Figure 74. Since progressive silting and 

 shallowing of the reservoir are inevitable, the Government 

 engineers are systematically measuring the proportion of solid 

 matter suspended in the water at the intake (point where the 

 river enters the reservoir), and as systematically tracing the 

 course of this weighted water after entry into the reservoir. 

 Cloudbursts upstream from the lake suddenly increase the 



