RANGE WATERING PLACES IN THE SOUTHWEST 27 
its use on the water slope for protection against wave action 1s 
effective and may be economical. The use of small logs or slabs for 
facing both sides of a very narrow retaining wall of earth is not ad- 
visable. Such timber rots rapidly, and the expense is usually not 
justified. One disadvantage of all timber or rock retaining walls is 
that they attract burrowing rodents. 
A floating boom of logs fastened end to end has been used suc- 
cessfully to protect the dam against wave action. Fences of short 
boards driven into the dirt, wire fences backed by brush, brush held 
in place by stakes, wire, or rocks, and rock layers alone or to hold 
down brush mattresses, have all been used to retard wave cutting 
(14, pp. 30-32) (9, pp. 14-16) (3, pp. 145-149) (24). 
Many southwestern livestock watering reservoirs are shielded from 
the wind to a varying degree by timber or steep slopes; again many 
of them have so small a water area that large waves do not form. 
Wave cutting will be reduced if dams are so placed that the prevail- 
ing winds blow parallel to them or upstream. Certain large munici- 
pal or irrigation reservoirs have concrete or other expensive types 
of paving over the entire inner face, but such measures are generally 
too costly to be practicable for livestock water developments. In 
any case necessary repair work should be done before wave action 
or other soil-cutting agencies have endangered the dam. 
SPILLWAYS 
The success of a dam depends as much upon the provision made 
for carrying off overflow water as upon the embankment itself. A 
study of 69 reservoirs on the Sitgreaves National Forest. in Arizona 
(28) showed that practically all complete failures of dams could be 
traced to inadequate spillways. The force and volume of storm 
waters are commonly underestimated. The washes, arroyos, and 
small canyons that supply most of the livestock tanks of the South- 
west are dry during most of the year, but not uncommonly carry water 
for several hours or even days following rains. Empty reservoirs 
often fill in 80 minutes after a heavy storm, and some large ones have 
filled within 15 minutes. The spillway must, of course, be large 
enough to carry the full volume of water after the basin is full; 
otherwise a damaged or destroyed dam results. 
Figure 4 indicates the need for adequate spillway capacity. 
In this figur> is shown the necessity of knowing the peak flow. 
This may be approximated by a careful examination of high-water 
marks on trees and of little lines of drift débris along ledges or slopes 
for from one-fourth to one-half mile above the dam site, and by 
estimating at several points the cross section of the channel at high- 
water stage. Such cross sections should, of course, be taken below 
any side drainage of material size. As a margin of safety, a spill- 
way capacity of about double the average of these cross sections 
should be provided. The aim should be never to let the water rise 
higher than 2 feet from the top of the dam. A good rule to follow 
is to have the floor of the spillway at least 5 or 6 feet below the top 
of the dam in all but the smallest reservoirs, where 4 feet may be 
sufficient, 
