478 ANNUAL EEPORT SMITHSONIAN INSTITUTION^ 1915. 



that has been constructed in this country (pi. 5, fig. 1). The per- 

 manent crest of concrete is erected at a relatively low level and above 

 this at 60 and 70 feet intervals rise concrete piers containing the 

 operating mechanism for the structural steel rollers. The latter 

 extend between the piers and may be lowered to form a water-tight 

 junction with the concrete crest, diverting water into the project 

 canal, or may be quickly raised to pass flood water over the weir. 



From the diversion dam the main canal follows what is locally 

 known as the " high line," and piercing several hills by tunnels, 

 proceeds in a general westerly direction, passing north of Grand 

 Junction, Fruita, and Mack, and supplying about 43,000 acres of 

 land above the older private canals of the valley. An additional 

 10,000 acres may be watered by pumping with power developed at 

 drops in the canal. The land is particularly suitable for fruit grow- 

 ing and capable of producing crops of high value. 



There remain for construction part of the main canal and dis- 

 tribution system, the power and pumping works, and possibly some 

 drainage works. Irrigation will begin on this project in 1916. 



UNCOMPAHGRE VALLEY PROJECT, COLORADO. 



Here the Reclamation Service has built the Gunnison Tunnel to 

 bring water from the Gunnison River to the valley of the TJn- 

 compahgre to supplement the meager flow of the latter stream. A 

 immber of canal systems heading in the Uncompahgre will distribute 

 the water to about 140,000 acres. Irrigation has been practiced here 

 for many years and the principal private canals have been purchased 

 or absorbed in the Government system to permit change or enlarge- 

 ment in a comprehensive development of the possibilities for irriga- 

 tion in the valley. Work is in progress on the canal system and this 

 now reaches 65,000 acres, of which 40,000 are being irrigated. The 

 crop production has steadily grown, approximating a million dollars 

 in value in 1915. Deciduous fruits are successful on the irrigated 

 lands and good yields are obtained from alfalfa, potatoes, wheat, 



and oats (pi. 13). 



BOISE PROJECT, IDAHO. 



One of the largest projects nearly completed is the Boise in Idaho. 

 This is about equal in area to the Salt River and involves the storage 

 and diversion of the waters of Boise River. The reservoir is formed 

 by the Arrowrock Dam (pi. 5, fig. 2; pi. 6, figs. 1 and 2), the highest 

 in the world, a rubble concrete arch rising 350 feet above the lowest 

 point of the base and measuring 1,075 feet along the crest. The inac- 

 cessibility of this site and the large amount of material to be hauled 

 in for the construction made it economical to build a 17-mile railroad 



