146 



THE FLOOR OF THE OCEAN 

 Table VII 



THICKNESS OF SILTY LAYER, MEASURED AT FOUR STATIONS ALONG THE 

 LENGTH OF LAKE MEAD (feet) 



of rock. Here, then, we have another clear proof that a silty 

 current does not lose its individuality by mixing with the over- 

 lying clear water or by the settling-out of the silt. In the case 

 of Lake Mead the bottom current persists for as much as 150 

 hours, one of the measured times for the traverse of 100 miles 

 in the lake. 



By identifying individual influxes of muddy water, the 

 engineers have been able to measure the average velocity of the 

 current between intake and dam. In one instance the rate was 

 found to be half a mile per hour — a notable speed for a current 

 following the average gradient of the floor of the reservoir, 

 namely, a gradient of only 7 in 10,000 or less than 1 in iooo. 16 



The Elephant Butte Reservoir, in the New Mexican part of 

 the Rio Grande Valley, receives the inflow of the Puerco River, 

 which in flood is charged with fine sand and mud to an extraor- 

 dinary degree. Here too the muddy water is seen to dive along 

 a sharp line of demarcation at the surface and then follow the 

 gently sloping bottom of the reservoir to the retaining dam, 

 the surface water remaining "perfectly clear." The sub-lacustrine 

 journey is about 35 miles in length. Although the measured 

 thickness of the silty current is comparatively small — only five 

 feet in one reported case, and, although the bottom gradient 



