182 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



miles in length, or even more. Each must be designed, built, watched, 

 and maintained individually. 



In the description that follows only a few of the larger feature^ 

 are mentioned, and it must be borne in mind that on most of the 

 projects there are hundreds of miles of small distributing ditches and 

 thousands of minor works such as farmers' headgates and flumes, 

 each of which is important to some one man or group of men. but 



Fig. 1. — Principal irrigation projects in the western United States. 



which forms merely a part of the highly elaborate system of control- 

 ling, diverting, and distributing the water. 



Arizona, Salt River Project. — The principal feature of this project 

 is a storage dam at Roosevelt, Ariz. (pi. 3), creating a reservoir with 

 an area of 25.5 square miles, and a capacity of 1,284,000 acres of 1 foot 

 in depth. The Roosevelt Dam is of rubble masonry 280 feet high, 

 235 feet long on the bottom, and 1,080 feet long on top. Its purpose 

 is to regulate the flow of Salt River. When needed for irrigation 

 the water is allowed to flow down the river from the dam for* 40 miles, 



