RANGE WATERING PLACES IN THE SOUTHWEST Al 
care in regard to favorable factors and be as long and as deep as 
| possible, with a depth of at least 12 feet. To reduce seepage the 
_ bottoms should be well trampled by animals. 
In building the embankment desirable methods and specifications 
include (a) thorough packing of earth, which should be damp; 
(D) water-tight bond between old and new dirt; (¢) “borrow pit” 
above the dam; (d) smooth and level top line, at least 5 or 6 feet 
above the spillway floor in large reservoirs; (¢) outer downstream 
) slopes in the ratio of 1144 (horizontal) to 1 (vertical) or 2 to 1, and 
- water slopes of 214 to 1 or 3 to 1, depending on materials and size 
of embankment; (7) tops at least 10 feet wide; (g) facings or riprap 
if practicable; and () location so that prevailing winds blow par- 
allel to dam or upstream. 
Inadequate spillways have been responsible for most failures in 
_ dams. The spillway should have a capacity double that required 
to handle the largest known volume of storm water, and be designed 
to prevent the water level from ever rising higher than 2 feet from 
the top of the dam. A wide, flat-bottomed spillway should be pro- 
vided, and unless in solid rock it should not slope off abruptly im- 
» mediately below the dam. If not well sodded or in hard soil or rock, 
/ it should be protected against erosion by paving or by firmly an- 
‘ chored mattresses constructed of rock, brush, and wire. 
More efficient handling of the mud problem is necessary if surface 
reservoirs are to continue to be profitable watering places. It is 
) usually advisable to clean out basins which contain much silt, except 
where other favorable sites are handy and where the expense of 
cleaning out accumulated silt will be larger than building a new 
dam. 
Measures to slow down silting include (@) prevention of over- 
erazing and maintenance of unbroken vegetative cover over the 
watershed; (6) especial protection of the channel immediately 
above the reservoir by fencing a long pasture up the channel where 
practicable and locating corrals and other facilities below the dam; 
(c) location of spillways near upper end of reservoir; (d) settling 
basins and check dams; and (e) ditch-filled reservoirs where feasible. 
The average cost of 592 southwestern reservoirs, formed by 
earthen dams, was $695, with extremes of $20 and $15,000. Mainte- 
nance may amount to from 10 to 15 per cent of the original cost 
yearly, and is decidedly higher where the immediate watershed has 
been overgrazed. 
Masonry or concrete dams may prove economical where adequate 
storage basins are found above narrow places in rock-bottomed 
canyons. ‘Though original cost is high, such structures if properly 
built are not often damaged by floods and repair costs are low. 
Silting, however, must be a minor factor. 
Gravel and sand-filled reservoirs are formed when coarse sands, 
gravels, or cinders wash into the storage basin behind a concrete 
or masonry dam. The stored water is drawn off by pipe lines. Such 
reservoirs have proved successful but are limited to localities in 
which the material washed along by storm water is coarse, clean 
sand or gravel with very little mud. Where small flows of water 
trickle along over bed rock beneath gravels or sands small concrete 
dams a foot or so high may be used to raise the water enough to 
permit piping it out to troughs. ; 
