RANGE WATERING PLACES IN THE SOUTHWEST 25 
less than 12 feet in depth 60 per cent had been dry a part of every 
year during the average of 9.5 years since construction, and only one 
had never gone dry in its six years of existence. Conditions for fill- 
ing that one were unusually favorable. Of the remaining 16 reser- 
voirs with depths of 12 feet or more none went dry every year, 
while 3, or 18 per cent, furnished water continuously and 5, or 31 
per cent, went dry a part of only one year in their average of 11.8 
years of existence. With all other factors favorable, reservoirs 
filled by storm waters and expected to furnish water permanently 
for comparatively large numbers of livestock should be as long and 
deep as possible and with a depth in the deepest part of at least 12 
feet. Shallower ones can seldom be depended upon for a continuous 
supply unless replenishment conditions are unusually favorable. 
THE RETAINING EMBANKMENT 
Faulty construction has been the cause of failure of many dams. 
Earth as a material varies greatly; consequently rules for its han- 
dling are not so rigid as in the case of wood or steel, though certain 
principles must be observed to get satisfactory results. The need 
for careful construction is greatest in the cases of dams thrown 
across drainage lines in the direct path of floods. Consequently the 
following suggestions refer primarily to such structures. Some of 
the essential features are shown in Figure 2. 
The plow-and-scraper method of building dams is the most 
feasible so far developed. Road plows of sturdy construction and 
both the fresno and the ordinary slip type of scrapers can be used 
to advantage. 
Material just damp enough to be plastic packs best. Over the 
Southwest as a whole most of the precipitation occurs during either 
the winter or the midsummer rain periods. Fall and winter months 
are best for earthwork construction where the climate is mild. 
During the summer rains, storm waters are more apt to tear out 
partly completed work. 
The first step in actual construction of earth dams should be to 
secure a good bond between the foundation and the new earth. 
Heavy vegetation should be removed, since its decay will leave 
crevices which encourage seepage. Several parallel furrows should 
be plowed lengthwise of the strip of ground upon which the em- 
bankment will rest, care being used not to injure the sod where it is 
planned to locate the spillway. For large structures engineers 
recommend a trench to be filled with clay, and this core built up a 
few feet into the mass. However, this expensive measure to pro- 
mote water-tight bonds between embankment and foundation has 
not been generally necessary for the success of moderate-sized dams. 
Earth for the dam should be obtained if possible from within 
the area to be flooded. The excavation should be 6 to 10 feet from 
the base of the dam to reduce slumping of material from the water 
face. A more compact and impervious dam results from spreading 
the material in layers roughly 6 to 12 inches thick. Repeated 
trampling by the scraper teams is very desirable and should be ex- 
tended to as near the edges as is feasible. 
The embankment should have a level top line. (PI. XI, fig. 1.) 
A slope-off toward the spillway is of course necessary unless that 
