30 BULLETIN 1358, U. 8. DEPARTMENT OF AGRICULTURE 
exclude stock is warranted. Fencing the water allows better control 
of stock and greater insurance against bog losses. Ability to close a 
permanent water for short periods is of great help when it is desired 
to lighten the grazing about it by ‘use of range adjacent to tem- 
porary waters. The entire closing, even for short periods, of a 
reservoir containing water necessitates careful riding to prevent 
hardship or death to those animals which drift back to their accus- 
tomed watering place. 
If the reservoir is fenced, troughs provide good drinking water, — 
eliminate the danger of animals bogging, allow the use of all the — 
water in emergencies, and obviate the necessity of building fences 
into the basin to water livestock from two or more pastures. Troughs 
should usually be placed from 25 to 100 yards downstream, above 
and to one side of the channel where they are out of flood danger 
and readily accessible to livestock. An arrangement of floats and 
valves to prevent wastage is necessary. Siphon systems have not 
been as satisfactory as direct-pressure pipe lines. Troughs have the 
disadvantages of increased cost and attention, danger of freezing in 
winter and of clogged pipes, and the fact that fewer animals can 
water in a given time. These disadvantages often outweigh the ad- 
vantages and will continue to limit the use of troughs. 
Burrowing animals sometimes cause severe injury to and even 
failure of dams. Gophers are frequently more numerous where dams 
are fenced. Compacted earth does not deter badgers and prairie- 
dogs. Rock squirrels inhabit rock or log facings. Where such pests 
occur aggressive trapping and poisoning are necessary to prevent 
damage and possible final loss of the dam. 
THE SILT AND MUD PROBLEM 
Whether a surface reservoir will prove economical depends in a 
large measure on its period of usefulness. One which quickly fills 
with mud is a poor investment. Furthermore, silt removal is often 
more costly than original construction. The rate of silting is in- 
creasing over a large portion of the Southwest. More careful con- 
sideration of the silt problem is, therefore, vital. 
The thickness of silt deposits in 30 representative southwestern 
reservoirs averaged approximately a foot a year, from the dates of 
first filling to 1922. 
TABLE 7.—Rapidity of silting of 30 reservoirs 
Original depth in 
font Age in years Depth of silt deposit in feet 
Total Annual 
Range | Average Range | Average a 
Range | Average | Range | Average 
2-20 ; 6.4 | 0. 14-6. 0 1 
4-30 15.3 1-34 gh 
ca | 
The silt load of run-off waters is increased by overgrazing. On 
one range where the total percentage of overgrazed areas was more 
than three and one-half times that for an adjoining similar range, 
