202 



Introduction to the Study of Science 



the masonry mass may have upon it even though it be ap- 

 parently a substantial rock basis. There are other conditions 

 which emphasize the experimental stage of dam building, and 

 indicate that present practices are no more than a suggestion 

 of the finally approved and permanent method. The general 

 form in use to-day is illustrated in the cross section of the 

 Arrowrock Dam, one of the latest achievements of the 



United States Reclamation Ser- 

 vice (Fig. 60). 



The Arrowrock Dam. The 

 difficulties met in the construc- 

 tion of this dam may be sug- 

 gested by the fact that in order 

 to find solid rock as a foundation 

 for the construction, excavation 

 had to be made through more 

 than ninety feet of rock of vol- 

 canic origin below the bed of 

 the river. The area at the base 

 of the structure is more than 

 200 feet in width and about 1100 

 feet in length ; from this one 

 may easily imagine the great 



mass of material that had to be removed for this part of the 

 work alone. The river at flood season has a flow of more than 

 18,000 cubic feet a second. The capacity of the dam is 

 244,300 acre feet, or about 79,800,000,000 gallons of water. 



The dam is to conserve some of the available water for the 

 generation of electrical energy which is used by the residents 

 of the section for light, heat, and power. Its cost is very small. 

 In some sections of reclaimed land in Idaho, electric energy 

 is utilized not only for lighting, but for heating, cooking, and 

 power purposes by about 50 per cent of the population. This 

 is an item of no small value in a region where transportation 

 facilities are limited and the cost of fuel is correspondingly high. 



FIG. 60. Cross section of the 

 Arrowrock Dam, showing excavation 

 for foundation, the general form and 

 dimensions of the dam. 



