440 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1935 



It then flows downstream to the power-plant turbines or continues 

 farther downstream past the turbines to outlet works, which open 

 into the river channel. Each of the two downstream towers is con- 

 nected with a battery of six 84-inch needle valves in the canyon wall 

 outlet works, and each of the two upstream structures with the six 

 72-inch needle valves contained in the downstream plug of an inner 

 diversion tunnel. Connections between the 30-foot pipe line and 

 the power-plant turbines are through 13-foot diameter steel pipes. 



Foundations for the towers are on rock shelves cut in the canyon 

 walls 250 feet above the old river surface, and the high points of the 

 structures rise 56 feet above the dam crest. Two towers are on each 

 side of the canyon, the center line of the downstream ones being 

 approximately 135 feet from the dam face and the other two 185 feet 

 farther upstream. Bridges join the upstream and downstream towers 

 with each other and with the dam. 



Placing the tower foundations so far above the old river bed 

 produces a silt pocket, and thus clear water can be supplied at all 

 times to the powerhouse turbines or outlet works. Silt deposition 

 in the reservoir will be heavy, but will be relieved by upstream de- 

 velopment, and present indications are that not more than one- 

 tenth of the reservoir volume will be filled with silt at the end of 

 50 years. 



The flow of water from intake towers to powerhouse or outlet 

 works is carried in plate steel pipes installed on concrete piers in 

 the lined tunnels. Anchors and thrust blocks are provided at sev- 

 eral locations where the entire space between the pipe and tunnel 

 walls is filled with concrete. 



The outlet works containing the 72-inch and 84-inch needle valves 

 for by-passing water around powerhouse turbines are located in 

 plugs in the inner diversion tumiels and in valve houses placed on 

 benches in the cliffs 160 feet above the old river channel and down- 

 stream from the powerhouse. A means of passage to the plugs is 

 provided by a concrete lined adit whose canyon wall portal is near 

 the downstream end of the powerhouse, and access is gained to 

 the canyon w^all valve houses through a shaft and elevator from the 

 plug adit. 



A steel Stoney gate, whose leaf is 35 feet high by 52 feet wide and 

 6 feet maximum thickness, is located at the downstream portal of the 

 inner diversion tunnel to cut off the inflow from the river, whenever 

 the requirement for maintenance work make such procedure desirable. 

 The gate, weighing 260,000 pounds, is counter- weighted and is raised 

 and lowered by two electrically operated hoists. 



Immediately downstream from the dam lies the huge concrete and 

 steel structure of the powerhouse. This is a U-shaped building 



