38 BULLETIN 1358, U. S. DEPARTMENT OF AGRICULTURE 
tains some fine silt. Water flowing down the channel or from weak 
springs collects in this pore space. <A pipe through the dam taps the 
impounded water and carries it to troughs below. 
For best results such dams should rest on a bedrock foundation 
and the lining of the storage basin above should be impervious. rock 
or clay. A watershed furnishing the proper filling material is vital, 
for if mud is deposited with the ‘sands or gravels the pore space will 
be reduced and percolation of the water hindered or practically | 
stopped. 
The pipe should be protected by a heavy metal screen at the in- 
take to prevent clogging. A sump or cavity should be built against 
the upper face of the dam with heavy durable timber or bowlders 
to protect the intake further and to provide a filtration chamber - 
surrounding it. 
A development of this kind has been in successful operation for a 
number of years on the Santa Rita Range Reserve near Tucson, 
Ariz. The pipe line extends about a mile “below the dam to a gal- | 
vanized water tank. In another development a gasoline pumping 
plant was used to raise the water from the sump above the dam, 
which was located in a rocky box canyon, to the mesa above. 
Since only a few dams of this type have been observed, very little 
information on cost is available. The cost of those studied ranged 
from 5200 to $15,000. Among the advantages of these débris- filled 
reservoirs or “ underground tanks ” may be mentioned: 
(1) Silt removal is unnecessary. They are therefore adapted to _ 
certain rocky washes in localities where vegetation is sparse, slopes 
steep, and surface wash of fairly clean rock particles difficult to con- 
trol. 
(2) Evaporation losses are reduced. 
(3) There is less danger of damage from high waters. 
Because of the high cost and the restricted number of favorable 
sites with impervious basins, narrow channels, and plenty of coarse 
filling material nearly free from fine silt, the use of this type of — 
water development is hmited. 
Small dams from 10 to 20 feet long and about 2 feet high de- 
veloped in the same manner as the larger ones, would be useful and 
practicable in many small arroyos and draws where a limited flow 
of fairly permanent water over impervious bedrock is covered by a 
layer of sand or gravel. A ditch extending up the channel from the 
dam and filled with coarse pebbles is effective in increasing the rate 
of percolation of the water to the intake of the pipe which leads to 
the trough below. In the absence of such dams, water in such places 
may sometimes be uncovered by a team and scraper and enough 
collected in natural depressions in the underlying rock or behind a 
temporary dam to supply stock during an emergency. 
Bryan (3) discusses these débris-filled reservoirs, and also the 
possibility of constructing artificial springs by blasting portions of 
adjoining cliffs into narrow gorges in order to form a porous dam 
behind which much wash material would collect. 
SUMMARY 
Livestock watering places have been extensively developed on 
southwestern ranges to supplement the scanty natural surface sup- 
SRP Se 2 eS SS we Se om a 
