530 Bulletin 95 



Utah that stream unites with the Green River, the head waters of 

 which are in Wyoming. Below the junction of the Grand and the 

 Green the stream was called the Colorado, A year ago, by Congres- 

 sional action, the name of the Grand was changed to Colorado; pre- 

 sumably geography and, ultimately, public usage will adopt the new 

 name for the upper river. The principal tributaries below the junc- 

 tion of the Green and the Grand are the San Juan, flowing westerly 

 from the northwest corner of New Mexico ; the Little Colorado, which 

 drains the north side of the Mogollon Rim in Arizona; and the Gila, 

 which drains the central and southern parts of Arizona. 



In the upper basin, that is, the basin above the Grand Canyon, 

 there is a large area of land under cultivation, about 1,500,000 acres, 

 mostly on the headwaters and tributaries where diversions from the 

 streams are easily accomplished. The irrigation of the land, however, 

 requires comparatively little water, on account of the high altitude, 

 cold climate and short growing season, and part of the water applied 

 returns underground to the streams. An even greater area, now 

 idle, is susceptible of irrigation, part of it, however, at such high cost 

 as to make the projects of doubtful feasibility. Studies made by the 

 United States Reclamation Service indicate that the irrigated area in 

 the upper basin will be increased to 3,000,000 acres. 



In the lower basin, below the Grand Canyon, the areas irrigated 

 in 1920 included 39,000 acres between Needles and Yuma, mostly on 

 the California side; 54,000 acres in the Yuma project; 415,000 acres 

 in the Imperial Valley; and 190,000 acres south of the international 

 boundary line,— a total of 698,000 acres. This total is almost exactly 

 double the acreage irrigated in 1913, showing the rapid rate of in- 

 crease in the use of water in the lower basin. The possible extension 

 of irrigation in the lower basin has not been determined fully, but 

 conservative estimates indicate that the following additional areas 

 can be brought under irrigation : — 260,000 acres between Needles and 

 Yuma, 150,000 acres of which is on the Arizona side; 76,000 acres 

 in the Yuma project; 400,000 acres in the Imperial and Coachella 

 valleys ; and 630,000 acres in Mexico. 



WATER SUPPLY 



Engineers have methods, of comparative accuracy, for measuring 

 the quantity of water flowing in rivers. The record of the flow, day 



