RANGE WATERING PLACES IN THE SOUTHWEST 35 
DIVERSION DITCHES AND HEADGATES 
Studies of properly built ditch-filled reservoirs show that they 
do not fill up with silt as rapidly as those in main canyons and 
are less liable to be washed out. For best results from a silt stand- 
point, ditches should be filled from one side of the current where 
the velocity of water is ordinarily less than near the center. Short 
diversion wings are therefore better at the intake than a dam which 
diverts the whole flow. 
The transporting power of running water decreases rapidly as the 
velocity is lessened. Hence, a small decrease in velocity means a 
greatly lessened quantity of material carried. This principle governs 
many of the suggestions already made relative to reservoir loca- 
tion or specifications. If the entrance to the diversion ditch is no 
lower than the main channel, and if the ditch is built on a very 
flat grade, the sluggish water will drop a considerable part of its 
load of heavier silt near the intake, whence it can be removed 
from time to time. Some of the fine silts do not settle for days or 
even weeks; these will, of course, be deposited in the storage basin. 
A flat gr ade of the ditch also reduces scour ing action on ditch banks. 
Where streams carry heavy loads of sediment and where perma- 
pent water is not essential small reservoirs may be supplied from 
ditches which partially encircle the bases of well-sodded hills but 
end before they tap the main drainage, thus collecting only the 
clearer run-off of the hills. 
Headgates of timber or of concrete or masonry in larger washes 
may be used in some places to regulate the amount and character 
of water entering reservoirs filled by diversion ditches and have 
some value for silt control. The first storm waters ordinarily carry 
a much higher proportion of silt and débris than do later waters. If 
the headgates are closed to exclude the first rush of the trash-laden 
run-off and then opened to admit the clearer water during the latter 
part of the flow, much silt will be kept out. Where an abundance 
of clear snow water is available in the spring the muddy flood 
waters from summer rains can be entirely cut off. It is seldom prac- 
ticable, however, so to operate headgates. 
CONSTRUCTION COSTS OF EARTHEN RESERVOIRS 
Many reservoirs are built under contract, usually on the basis of 
the yardage moved, but occasionally for a lump sum. In one locality 
in southeastern New Mexico 32 embankments were estimated to 
contain 130,000 cubic yards of dirt, or an average of about 4,000 
cubic yards ‘each. The total cost approximated Sod. 600, or an aver- 
age of about $770 each, slightly over 19 cents per cubic yard. Some 
of these were contract jobs, but most were built by the owners. All 
were built between 1903 and 1921. A few very small dams were 
included, and also two large ones of 13,000 and 15,000 cubic yards, 
respectively. A few were built 15 to 20 years ago at 10 cents a cubic 
yard of material moved, whereas from about 1908 to 1915 the aver- 
age contract price was ‘close to 15 cents per cubic yard. In 1919 
and 1920, when prices were highest, a few contracts were let at 35 
cents and many at 30 cents. During 1921 and 1922 prices showed a 
downward trend. Where rock work is required the unit cost is 
increased. 
