438 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 5 



feet, and an " outer diversion tunnel " that has been driven through 

 the cliffs past the dam. The largest battleship could be floated in 

 the channel if the inclined shaft were damned at the portal. It is 

 also of interest to note that if the spillways were operating at their 

 capacity of twice the highest river discharge ever recorded at Black 

 Canyon, the water would pass into the outer diversion tunnel at a 

 velocity of 2 miles per minute while the energy expended would 

 exceed 11,000,000 horsepower. However, a discharge of this volume 

 will never occur, except in the most extreme emergency, for floods 

 equal to the severest ever experienced on the river can be restrained 

 by the outlet works and spillway gates to a flow past the dam. of 

 not more than 75,000 cubic feet per second, or approximately one- 

 fifth of the spillway capacity. 



Four structural steel floating gates, a hundred feet in length and 

 of circular segment section, are installed on each weir. Ordinarily 

 the gates lie in recesses in the weirs, but, as the reservoir rises, they 

 may be lifted as much as 16 feet to regulate the flow into the spill- 

 way channel. Each gate is hinged on the reservoir side and, although 

 weighing 500,000 pounds, it is a buoyant vessel and can be raised by 

 allowing water to enter the weir recess and float the gate to the 

 desired height. It may be lowered, conversely, by allowing the 

 water to flow from the recess into the spillway inclined shaft. With 

 gates in lowered position, the weir crest is 27 feet below the dam 

 crest. 



Water will be taken from the reservoir for downstream require- 

 ments, production of electricity, and regulation of the reservoir sur- 

 face below the weir crests of the spillways, through the gates located 

 in the four intake towers, the steel pipes in the penstock and outlet 

 systems, and the needle valves of outlet works, or the turbines of 

 the power plant. 



The four intake towers that are located just upstream from the 

 dam are notable examples of the possibilities presented for combin- 

 ing artistry and usefulness, of building for beauty as well as for 

 strength and utility. These graceful concrete spires resemble huge 

 fluted columns and have all the appearance of memorial monuments 

 rather than serviceable structures designed for the particular pur- 

 pose of regulating the flow of water from the Boulder Canyon 

 Reservoir. 



Actually each tower is a hollow concrete cylinder of 29 feet 8 

 inches internal diameter and 75 feet average outside diameter from 

 which 12 fins project radially, the openings between the fins being 

 spanned by steel trash racks. A hoist house of more than four 

 stories high sits atop each tower and contains electrically operated 

 hoists for raising and lowering the cylindrical gates that are installed, 



