240 GUIDEBOOK OF THE WESTERN UNITED STATES 
Franciscan friar Francisco Garcés, traveling alone, reached the Colo- 
rado in 1771 near Yuma and crossed it on a raft. He crossed it 
again at that place in 1774 and 1775 with Anza’s expeditions. 
In the vicinity of Yuma, as elsewhere, the Colorado River meanders 
through a shallow channel in a wide trench excavated in the great 
desert plain that extends to the Gila Mountains on the east and con- 
stitutes the Colorado Desert and Imperial Valley to the west. The 
trench or alluvial flat is nearly 5 miles wide at Yuma, where it is 
bordered by long bluffs of sand and gravel 50 to 100 feet high. The 
upper part of Yuma is built on the bluff, which is here called “the 
Mesa.”” The trench also extends up the valley of the Gila River for 
many miles. The surface of the alluvial flat is nearly smooth, but in 
places it slopes slightly away from the river, owing to the low bank 
or levee built by the stream at times of freshet when there is con- 
siderable overflow in places not protected by artificial levees. South 
of Yuma there are extensive sloughs and oxbow ponds along the 
principal overflow channels. 
The Colorado River empties into the head of the Gulf of California 
in Mexico about 60 miles below Yuma (see pl. 35), and in fact this 
large water body is an extension of the Colorado Valley submerged by 
tidewater. The volume of the river varies considerably, and at times 
it is greatly swollen by freshets. The floods oceur mostly in early 
summer and are fed by winter rains and snows in the distant moun- 
tains. The highest summer floods have exceeded 200,000 second-feet 
(cubic feet per second). The ordinary maximum flow is 70,000 to 
100,000 second-feet, the minimum flow, 2,500 to 3,000 second-feet, 
and the average 10,700 second-feet. In August, 1931, the flow at 
Yuma decreased to 200 second-feet (U. S. Bureau of Reclamation). 
The total yearly flow at Yuma averages about 16,730,000 acre-feet 
(1902-1916) including 1,000,000 acre-feet or more from Gila River. 
The mineral content of the water ordinarily ranges from 1,000 
to 350 parts per million,” and it is estimated that the geBiirreeait in 
suspension is sufficient to cover about 100,000 acres 1 foot deep 
(100,000 acre-feet) annually, Considerable sediment is also moved 
along the bottom of the river. The material spread on the land by 
overflow has important fertilizing value. (U.S. Bur. of Reclamation).” 
% A very large amount of material is 
removed from the land and earried to 
the ocean by all rivers. Careful esti- 
mates based on analyses of river waters 
measurements of volume of flow 
have shown that every year the rivers of 
the United States carry to tidewater 
3 513,000,000 tons of sediment in suspen- 
sion and 270,000,000 tons of dissolved 
matter. - The total of 783,000,000 tons 
er 000 enbie 4 Ariz 
yards of rock, or a cube measuring 
about two-fifths of a mile on each side 
(Ms eubie mile). The total is equiva- 
lent to 610,000,000 cubie yards of sur- 
face soil. (See U Geol. Survey 
| Water-Supply Paper 234, p- 83, 1909.) 
also Bacon, J. L., Monthly 
Weather Review, vol. 59, p. 
297, 1931; Breazeale, J. F., Arizona 
Univ. Bull. 8, 1926; Raabe R. H., 
ona Univ. Bull. 44, 1 
