RANGE WATERING PLACES IN THE SOUTHWEST 21 
WATERSHED AND DISTANCE FROM DIVIDE 
The watershed area must be sufficient to keep the water supply 
replenished so far as variable climatic conditions permit. No set 
rules can be laid down; limiting factors must be weighed in each case. 
Run-off will be retarded if the watershed is densely timbered or 
heavily grassed, if the slopes are gentle or almost level, and if the 
soil is loose and spongy. Conversely, a more rapid run-off may be 
expected if the area is untimbered or closely grazed and has steep 
slopes and compact soils or exposed bedrock. Sudden thaws, frozen 
soil, and heavy or long-continued rains increase the run-off. 
Other things being equal, the danger of floods tearing out dams 
built across drainage lines increases with the distance from the head- 
waters. In general, embankments must be more carefully con- 
structed below large as compared with smaller drainage basins to 
withstand the force of storm waters. For this reason earthen dams 
of any given strength built squarely across drainage lines should 
- be nearer the divide than is necessary if the reservoir can be placed 
_ to one side of the main channel. 
In a special study of 28 reservoirs, the distance from main divides 
varied from one-half mile to about 12 miles; and watershed areas 
not shared by other reservoirs, from 160 to over 7,000 acres. 
One small “tank” with a catchment area of only 190 acres has 
/ proved to be as permanent as others with watersheds 10 to 15 times 
larger, indicating that size of watershed is only one of the several 
influences which together determine the degree of permanence of 
water supply. The exceptional reliability of such reservoirs with 
very small watersheds is partly accounted for by the fact that rains 
seem to hit, year after year, certain areas more frequently than 
near-by localities. In locating reservoirs advantage should be taken 
of such storm areas wherever known. Reservoirs should not be 
placed so close together in any one drainage that the run-off reach- 
ing the lower ones is likely to be inadequate. 
CHOICE OF DRAINAGE LINE 
In choosing a site for a reservoir preference should be given to 
the valley showing the least erosion and the least tendency to erode, 
and draining a well-sodded watershed. Gullies washed down 
through the sod hasten the run-off. The erosion problem is be- 
coming so serious on many ranges, however, that any benefit from the 
increased run-off so obtained is more than offset by the cost of re- 
moving the greater amounts of mud carried into the storage basin. 
(Pl. X, fig. 1.) The channel slope should be flat above the dam; 
a small increase in steepness reduces materially the storage capacity 
of a dam of a given height. A long, deep basin should be sought. A 
level should be used to determine this, since the eye is an uncertain 
cuide. If suitable basins can be found just above narrow places 
in valleys where a relatively short dam will suffice, most water can 
be impounded at least expense. 
OTHER FEATURES OF SITE 
A watering place should be easy of access by livestock, with ap- 
proaches down gentle slopes and from as many directions as possible, 
