20 BULLETIN 1858, U. S. DEPARTMENT OF AGRICULTURE 
attention, and greater certainty of filing during dry years, since 
the storage basin catches the entire flow of the stream. Ditch-filled 
reservoirs have the advantage of lessened risk of damage from floods, | 
reduced costs of and difficulties with wasteways, and a modified silt | 
problem. The most successful watering places ‘of this t type are lo- | 
cated on flats or mesas some distance from the stream that supplies | 
the water. Although the extra work on ditches adds an item of — 
cost, this may be offset wholly or in part by less expensive flood 
protection works necessary at the reservoir itself. | 
RESERVOIRS FORMED BY EARTHEN DAMS 
Earthen embankments are generally the most feasible and eco-— 
nomical type of construction on southwestern ranges and are there-_ 
fore emphasized in the following discussion. Concrete and masonry | 
dams have a limited place on the range and are discussed briefly. 
CAREFUL LOCATION IMPORTANT 
Where conditions permit, a watering place near the center of the — 
area to be grazed is to be preferred. The source of water for filling » 
reservoirs, however, limits the choice of location. All factors should 
be carefully considered in order te insure permanency of both the | 
water supply and the dam, with a minimum expense. An ideal site | 
is seldom found, but among those on any particular range where | 
this type of development is suitable there is usually one that is | 
more favorable than the rest. . 
SOIL 
Soil is an important item in obtaining both stability of the dam _ 
and water-tightness of the reservoir. Successful storage basins are — 
found in eranitic, sandstone, or volcanic regions, and also in soils” 
derived mostly from limestone, though it is “often difficult to build 
successful ones where openings in limestone are near the surface. 
Loose, open soils of coarse texture, such as fairly pure sands, gravels, 
or cinders and those with a large content of soluble mineral salts, or | 
subsoil layers of such materials, should be avoided because of ex- 
cessive losses from seepage. Test holes should be made with a sub- 
soil auger or otherwise. Clay has been successfully hauled in to 
improve the soil mixture, but ordinarily this will not pay. Fortu- 
nately these extreme types are usually localized, and over great ex- 
panses of the Southwest soils commonly prevail which are satis- 
factory, especially after trampling by animals. 
In general, the most suitable soils for dams are clays with a 
woodly proportion of sand or gravel. Kngineering tests and experi- 
ence have shown clearly the advantages of mixed mater ials, varying 
from fine to coarse. It appears that the best soil is a cohesive mix- 
ture of 1 part of clayey material to 2 or 3 parts of gritty material, 
(14, p. 92), combining weight, stability, and water- tiohtness. Such 
a mixture reduces pore space ‘and @ives a more compact and stable 
mass with greater unit weight and usually with sufficient water- 
tightness Go. pp. 14-17). Clays crack badly upon drying and are 
apt to slump when wet. They are accordingly not advised for large 
dams. 
