532 Bulletin 95 



by day, month by month, shows the extent of the water supply and 

 its fluctuations, and furnishes a basis for the design of engineering 

 works. On the Colorado River and its tributaries, many gaging 

 stations, at carefully chosen locations, have been kept for varying 

 periods of time, some of the records extending over twenty-five years. 



The records of stream flow at Yuma have been kept since January, 

 1902. The gaging station is below the mouth of the Gila River and 

 below the Yuma diversion dam, but above the head gates of the Im- 

 perial Canal. The average annual flow at the gaging station for the 

 period 1902-1920 was 17,300,000 acre-feet. Had the present irrigated 

 area been under irrigation throughout the period of the record, the 

 average annual flow would have been about 16,000,000 acre- feet. 

 The average flow at Boulder Canyon is practically the same amount, 

 since diversions and losses between Boulder Canyon and Yuma are 

 balanced by the inflow of tributaries. 



Most of the water comes from the upper basin. At the junction 

 of the Grand and the Green, the average annual discharge of the 

 Grand is 6,900,000 acre-feet, and of the Green 5,500,000 acre-feet. 

 The Green and Grand and San Juan rivers together, though draining 

 less than two-fifths of the area of the Colorado basin, furnish 86 

 percent of the total water supply. 



By far the greater part of the precipitation in Colorado and 

 Wyoming is in the form of snow. During the winter the snow 

 accumulates to great depths. The melting of the snow during the 

 spring months produces a long period of high water, the annual 

 flood, which lasts from two to three months and reaches its highest 

 point at Yuma usually in June. During the June flood of 1909, the 

 flow at Yuma reached 150,000 cubic feet per second. On June 27, 

 1921, all previous June records were broken by a flow of 186,000 

 cubic feet per second. The low water season begins in August and 

 lasts from three to seven months. The minimum flow at Yuma has 

 been below 4000 cubic feet per second during several low-water 

 seasons. 



The Gila River drains an area of 57,000 square miles. While 

 the average annual discharge of the river is not great, it is very 

 variable. In 1916 the discharge of the river at its mouth was 4,500,- 

 000 acre- feet; in some other years the total has been less than 100,- 



