BUILDING THE WOELD'S HIGHEST DAMS 705 



higher than the Shoshone dam in Wyoming, C7 feet higher than Roosevelt 

 dam in Arizona, and 54 feet higher than Croton dam in New Yorli. Its curved 

 crest will have a length of 1,000 feet. Its foundation will cover an acre of 

 the bed rock. The diamond drills went down 90 feet below the river bed before 

 finding the solid granite bed rock, and numerous test pits, shafts and tunnels 

 were driven in the canyon walls above the water line. The dam will be con- 

 structed of rubble concrete, with as many boulders incorporated as can be done 

 economically, probably 20 per cent of the whole mass. About 500,000 cubic 

 yards of masonry will be required. Outlets will be provided at several eleva- 

 tions and starting with a full reservoir the upper openings will be put into 

 commission first. Those lower down being used as the water level drops, so 

 that it will not be necessary to operate any of the gates under very great 

 heads. Provisions will also be made for penstock openings for a power house, 

 in case it may be advisable later to develop power at this site. 



A spillway with a normal capacity of 40,000 second feet will be built at 

 the north end of the dam, with a concrete lip 700 feet long following the 

 contour of the ground in a direction generally at right angles to the line of 

 the dam. The water will fall over this spillway into a concrete lined trench 

 parallel to it, by which means it will be carried well beyond the end of the 

 dam and allowed to cascade over the granite cliffs into the canyon of Deer 

 Creek, and thence back into the river. From the records of floods in Boise 

 River it is believed that 40,000 second feet will more than cover any flood that 

 will occur above this point, but the capacity of the spillway may be crowded to 

 40,000 second feet, and the dam will be so built that water may flow over it to a 

 depth of two or three feet without endangering the structure. 



One of the most important problems confronting the engineers was taking 

 care of the river during construction. Two hundred and twenty-five thousand 

 cubic yards of material must be excavated below the river bed. It has been 

 decided to divert the water through a tunnel under the lava bench at the 

 south end of the dam. This tunnel will be 500 feet long, 35 feet wide, and 

 25 feet high, the arched top having a ten-foot rise. The bottom and sides will 

 be lined with concrete and the top with timber. This waterway will cai'ry 

 the flood waters of ordinary years. Five years will be required to complete 

 the masonry work on this dam, and in the meantime the canal system will 

 be extended to cover 243,000 acres of exceedingly fertile land in the vicinity 

 of Boise. 



THE ENGLB DAM. 



While the Engle dam will be only four-fifths as large as the Arrowvock 

 in cubical contents, its storage capacity is more than twelve times as great. 

 It will rise 205 feet from foundation to top of parapet wall, will be 1,400 feet 

 long on top, and will contain 410,000 cubic yards of concrete rabble masonry. 

 At the bottom the dam will be 180 feet in thickness, tapering up to 20 feet at 

 the top. It will create the largest artificial lake in the world, a lake 40 miles 

 long, from one to five miles wide, and containing 2,538,000 acre-feet of water. 

 At a gaj) on the west side of the valley several miles above the dam a spillway 

 800 feet long will be built at a height of 100 feet above the river bed. 



