70 BULLETIN 1026, U. S. DEPARTMENT OF AGEICULTimE. 
pendent on the direct run-off from torrential rains, but most of this 
run-off is caught and stored. 
The majority of reservoir sites were natural depressions or basins 
on bench land which were developed by putting the outlet in a cut, 
throwing up an embankment along the lowest rim, and constructing 
short inlet and outlet canals, connecting with distributing canals. 
These sites were the most satisfactory to be found, and were de- 
veloped at a very low cost, running in one instance to $1.09 per acre- 
foot of capacity. Sites developed by the construction of a dam across 
a drainage channel which carries regularly little or no water are al- 
most as numerous. They differ from the ordinary stream-bed reser- 
voir in that they are filled from some nearby source through an inlet 
canal and their dams are rarely protected with spillways. Reservoirs 
in the channels of flowing streams are few in number, because more 
satisfactory and cheaper sites are usually available elsewhere. The 
majority of the sites developed were small, with rather steep slopes, 
the average capacity per foot of depth being close to 130 acre-feet. 
Bottoms vary from light soils through which there is considerable 
seepage to' a compact clay loam which is practically impervious. 
The dams are, almost without exception, earth fills, varying in 
height from 10 to 40 feet, set on earth foundations. In general the 
site of the dam was first cleared of all brush, roots, and stones, and 
then plowed, after which the material was put on in layers, levelled, 
sprinkled, packed, and then harrowed to form a bond with the layer 
above. Some of the first dams were carried up in layers as thick as 
5 feet, but as the practice improved the layers were reduced to a foot 
in thickness. The travel of the teams was depended upon, usually, to 
do the packing. 
An exception to the common type of dam in the valley is the 
Halligan Dam of the North Poudre Irrigation Co. shown in Plate 
XVI, figure 1. This is an arched concrete structure which im- 
pounds 6,428 acre- feet of water in the bed of the North Fork. Its 
length at the top is 350 feet and at the bottom 235 feet. The thick- 
ness ranges from 30 feet at the bottom to 3 feet at the top. The total 
height of the structure is 94 feet, and the depth of water stored is 
69.8 feet. A spillway is located in the middle of the dam. It is 
110 feet wide, 10 feet below the top, and has a curved lip designed to 
prevent the overflow from leaving the face of the dam under any- 
thing less than an 8-foot head. The lower 67 feet of the dam is 
cyclopean masonry, rock masses not exceeding 2 cubic yards in 
volume being imbedded in a 1:3:5 concrete, reinforced with steel 
bars. Because of the poor quality of the rock available the upper 
27 feet of the dam is of straight 1:3:6 concrete reinforced with bars. 
The total cost of the dam was $230,000, which is at the rate of ap- 
proximately $36 per acre-foot of capacity. 
