THE IRRIGATION AGE. 



41 



around the National Park. The rivers at this time, 

 when the lava was hot and sputtered and boomed, 

 fought their way under the lava and today they come 

 out along the 600 foot high walls of the Snake river 

 in the shape of huge springs as large as the largest in 

 existence. One collection of springs called the Thous- 

 and Springs, of 5,000 second feet flow, falls 200 feet 

 over the lava walls. The Snake river has also had its 

 troubles, and while it spread out and went over the 

 lava above American Falls and down as far as the 



north channel. There are therefore three dams, two 

 of which will have an extreme height of seventy-six 

 feet and the third sixty 'feet in height. These dams 

 will each have a width of twenty feet on the crest, at 

 a height of eight feet above high water mark in the 

 canals. They are being constructed of massive em- 

 bankments of rock, laid dry, having a minimum width 

 of 150 feet at base, reinforced at the upper side with 

 an embankment of earth having a base width of 300 

 feet, the total width of bottom being 460 feet, the 



GENERAL VIEW NEAR SITE OF TWIN FALLS DAM. 



head of the great canal, it finally got the best of it 

 here, and at the three islands where the dam, or rather 

 three dame are being built, it broke through the lava 

 and boiled and sputtered and fell into the large crack 

 of lava while cooling and now it falls in jumps of ten 

 to 200 feet at a jump, for sixty to eighty miles, leav- 

 ing a mesa from 100 to 800 feet above the bottom of 

 the Snake river canyon. This mesa or what is known 

 as the Snake river valley is a vast plain of sage brush, 

 without a break of any great depth, with lava ash soil 

 and sandy loam, making a most fertile foundation for 

 any kind of fruit or cereal which grows in great abund- 

 ance when the soil is softened by moisture. 



upper slope being four to one and paved with rock 

 rip rap. The aggregate length of the three dams is 

 about 1,100 feet, and the spill-ways will be over 900 

 feet long. The islands are to be used as spill-ways, 

 to take care of the excess of water beyond the capacity 

 of the canals. The top of the south island is to be 

 cut down to the bottom of the canals and provided 

 with a battery of ninety-nine gates, each 5x10 feet in 

 clear, extending across the island, a distance of 530 

 feet. It is safe to say no such assembly of gates 

 arrayed in one continuous line has ever been used upon 

 any irrigation works in America. These gates are 

 held in place by steel frames enclosed in concrete and 



TWIN FALLS DAM WHEN COMPLETED AND COMPANY'S HEADQUARTERS. 



THE DIVERTING DAMS. 



To get water up to the level desired to irrigate 

 the land, it is necessary even here to dam the river and 

 raise its normal level about forty-nine feet, or from 

 elevation 4048 to 4097, high water surface in canals. 

 At the point selected for damming the river there are 

 three channels with rocky islands between. Two of 

 these channels are only occupied at extreme high 

 water, the stream being ordinarily confined to the 



are expected to control the flow of the river at all 

 times, and have a capacity of 63,000 second feet, with- 

 out raising the water level above the high water mark 

 in the canals. The greatest flood discharge of the 

 river is 50,000 second feet and the maximum waste, 

 way provided in spill-ways, tunnels and canals is 

 166,000 second feet, 3 1-3 times the maximum. The 

 dams are provided with a central core of wood, 

 founded on bed rock and anchored thereto by concrete 



