CONTINENTAL TERRACES AND SUBMARINE VALLEYS I45 



FIGURE 74. MAP OF LAKE MEAD (SUPPLIED BY H. T. KELCH OF THE SOIL 

 CONSERVATION SERVICE, WASHINGTON). 



volume, velocity, and silt-content of the torrential river. At the 

 intake the time of arrival of water with maximum muddiness 

 is noted. Some days thereafter the same water, still charged 

 with a load of suspended silt, appears at the dam, 120 miles 

 downstream from the intake. That the muddy current hugs 

 the gently sloping bottom of the lake has been proved by 

 sampling the water from surface to bottom and at various 

 vertical sections between intake and dam. The thickness of the 

 silty current running along the bottom is variable, as shown 

 by Table VII, taken from a publication of Mr. H. N. Eaton. 13 



We note that a silty current along the bottom, measured on 

 July 20 and 21 of the year 1937, was 20 feet at Virgin Canyon 

 near the upper end of Lake Mead and 37 feet at Boulder Can- 

 yon, farther down the Colorado River valley. The greatest 

 thickness found was 117 feet, this time at Boulder Dam, the 

 outlet of the lake. 



Above each muddy sheet the reservoir water, hundreds of 

 feet thick, was almost entirely free from suspended particles 



