SOUTHERN PACIFIC LINES 241 
An important engineering project in connection with the Colorado 
River is the Boulder Dam, in Black Canyon, 200 miles above Yuma 
(about 15 miles below Boulder Canyon), which was started in 1931. 
It will completely control the waters of the river and not only maintain 
the supply as needed, but prevent floods and greatly diminish the 
amount of silt. According to printed statements of the United 
States Bureau of Reclamation the dam will be a curved gravity 
structure about 1,180 feet long and will contain approximately 
3,500,000 cubic yards of concrete requiring about 5,500,000 barrels 
of cement. Its height will be 707 feet above bedrock; this will raise 
the water surface about 582 feet, or to 1,229 feet above sea level. 
The reservoir, 115 miles long and with an area of about 145,000 acres 
(227 square miles), will hold 30,500,000 acre-feet of water. It will 
take a year and a half for the river to fill the reservoir under ordinary 
conditions of flow. The cost of the dam will be about $70,600,000, 
not including a 1,200,000-horsepower electric generating plant 
($38,000,000), the revenue from which, together with the charge to 
irrigators for the water, is expected to cover the interest and finally 
repay the cost. An all-American canal 75 miles long, provided for 
by an allotment of $38,500,000, will be built through the sand hills 
that begin 10 miles west of Yuma, to replace the present canal, which 
for 35 miles is in Mexico. Its cost also must be repaid by the i irriga- 
tion under it. This canal, with a bottom width of 134 feet and a 
depth of 22 feet, will supply a much larger volume of water than is 
now flowing in the old canal, which is the largest one in operation in 
this country, and will marae for greatly increasing the irrigated area 
in Imperial Valley. The water will be taken from the river at a point 
5 miles above the present Laguna Dam, a few miles above Yuma. 
It is estimated that a branch 130 miles long to provide for irrigation in 
the Coachella Valley and increasing the area irrigable under this 
project to 900,000 acres, will cost about $11,000,000. Los Angeles 
i also receive some of the water (1,500 cecadifekt. which will be 
taken out at Parker and carried through long aqueducts and tunnels 
by way of San Gorgonio Pass. 
From a point 6 miles west by south from Yuma the middle of the 
Colorado River is the boundary between the United States and 
Mexico, Arizona extending about 16 miles farther south than Califor- 
nia. By the treaty of Guadalupe Hidalgo, in 1848, the original 
international boundary followed the Gila River to its junction with 
the Colorado and thence was a straight line west to a point on the 
Pacific Ocean 1 marine league south of the southernmost point of the 
port of San Diego. By the Gadsden Purchase the southern boundary 
east of the Colorado River was shifted to its present location, which 
touches the Colorado at a point 20 English miles below the junction _ 
of the Gila. North of Yuma the Colorado is for many miles the : 
: ee line between Arizona and California. ce 
