299 



16 feet wide. The dam has a gravity section and is built on a 

 curve of 660-foot radius. The spillway consists of a run 400 feet 

 long" and a concrete-lined discharge trench approximately 900 

 feet in length with a capacity of 40,000 second feet. In the run 

 and trench lining are 25,400 cubic yards of concrete, all of which 

 is reinforced. 



This spillway run carries a movable crest of unique design 

 which permits the storage of water six feet higher than the fixed 

 crest, but drops automatically, very slowly, in case of flood, so 

 as to give the full capacity of the spillway when needed, and 

 rises automatically again when the flood has passed. 



A log conveyor with a capacity of 1,000,000 feet per day per- 

 mits the passage of logs to the mills below. These logs are 

 hoisted to the top of the dam by a cable lift and taken to the 

 river below the dam through a reinforced concrete chute 650 feet 

 long, the upper 400 feet of which carries a bull chain with spurs 

 or teeth set to hold against sliding. This carries the logs down 

 a 62^4% slope and delivers them to a gravity chute through 

 which they pass to the river. It is estimated that there is three 

 billion feet of timber in the Boise Basin above the dam that must 

 be handled in this way. 



Preliminary to the construction of the dam it was necessary to 

 construct a standard gauge railroad 17 miles long from Barber 

 Junction on the Oregon Spur Line to Arrowrock. This rail- 

 road has been in operation four years, and in that time it has 

 carried 80,000 passengers and about 14,000,000 ton miles of 

 freight. It is the only railroad in the country operated by the 

 federal government, and all tickets carry the signature of Presi- 

 dent Woodrow Wilson in facsimile. 



A 3000-horsepower hydro-electric power plant was built to 

 furnish power for the operation of the construction plant. This 

 has furnished all the electric power needed for construction pur- 

 poses, and in addition considerable of its surplus output has been 

 sold to local companies. Its total output since May, 1915, has 

 been almost 20,000,000 K. W. hours. 



A sawmill was operated for almost two years in the timber 

 about 17 miles above Arrowrock, and this furnished 6,750,000 

 feet, board measure, of lumber, all of which was used for the 

 building of the construction camp at Arrowrock, and to fill mis- 

 cellaneous requirements on the work. 



The excavation for the dam extended 90 feet below the river 

 bed to the granite foundation, and a diversion tunnel 500 feet 

 long with a cross-section 30x25 feet carried the river around the 

 work until the construction was far enough advanced to start 

 the storage of water. 



Regulating outlets in the dam are 20 in number, each being 

 four feet and four inches in diameter. They are controlled by a 

 58-inch balanced needle valve on the upstream face of the dam. 

 They are arranged in two sets of 10 each, the upper set being 



