RANGE WATERING PLACES IN THE SOUTHWEST 29 
than where soil has been disturbed. Salt is occasionally put in 
unsodded earth spillways to encourage grazing animals to pack the 
bottom by trampling, though this is seldom advisable because of 
the additional concentration of stock around the water. In a few 
cases earthen spillways have been planked to retard erosion, but 
this is rarely practicable. Linings of heavy rock or bowlders, or 
brush weighted with rock, have also been used. These are of value 
to prevent floor cutting only so long as the waterfall at the lower 
end of the spill does not cut back under them. To stay this cutting, 
brush and wire structures, held in place by rocks and by deep-driven 
iron rods or durable wooden posts, are of value (Pl. XIV, fig. 1) 
(14, p. 33). The use, at the foot of each fall, of little check dams 
of 18 or 20 inch woven wire on deeply set posts and, if needed, 
mattresses, formed by layers of anchored bundles of brush or of brush 
held flat by stretched woven wire, would not add greatly to the cost. 
Where eroding spillways are endangering big expensive. reservoirs 
(Pl. XIV, fig. 2) they merit more consideration than has generally 
been given them. 
If possible, the spillway should be located so that the overflow 
will not wear against and undermine the end of the dam. Where 
this can not be prevented, special protection such as well-embedded 
rock (Pl. XIII, fig. 1) or timber cribbing is usually necessary. 
Occasionally a large tree or bowlder at the end of the dam will serve 
as a valuable buffer. A galvanized wire-mesh cylinder filled with 
rock has: afforded fair protection, although a low concrete wall 
joined to underlying bedrock may be necessary to halt undercutting. 
In emergencies rapid cutting may be checked by sacks of earth or 
sand piled together at the proper point. 
NUMBER AND LOCATION OF SPILLWAYS 
Where a dam directly across the drainage is flanked by slopes of 
about equal steepness, one spillway around each end of the dam may 
be advisable, and if sufficiently large will add to the safety of the 
structure. A steep slope on one side of the valley often makes it 
impracticable to have more than one spillway, however. Where a 
diversion ditch is used to fill a natural depression or an old lake bed 
no spillway may be needed. A ditch which fills a reservoir on a flat 
usually is broken between the intake and the storage basin by heavy 
flows. Even under these conditions a spillway should be provided at 
the reservoir itself. 
From the standpoints of lessened silting and greater economy and 
safety it is an advantage to locate the spillway near the upper end of 
the body of water, either by using the natural lay of the land or by 
extending the dam upstream. (PI. I, fig. 2.) Natural spillways in 
well-sodded or rocky saddles in one of the ridges that form the rim 
of the water basin, entirely apart from the dam, should be utilized if 
possible (fig. 2 (c)). 
PROTECTION OF RESERVOIRS FROM LIVESTOCK AND BURROWING ANIMALS 
Tramphng of new earthen dams is to be encouraged, since it settles 
and packs the freshly worked dirt. Trampling after the dam is well 
settled results in gradual reduction in the width at the top, so much 
so that in some cases the expense of fencing the embankment. to 
