70 BtTLLETlis" 1026, IJ. S. DEPAETMEl^T OF AGEICtJLTTJEE. 



pendent on the direct run-off from torrential rains, but most of this 

 run-off is caught and stored. 



The majoritj' of reservoir sites "vrere 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 j)lowed, 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 laj^ers 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 Xorth Poudre Irrigation Co. shown in Plate 

 XW, figure 1. This is an arched concrete structure which im- 

 pounds 6,428 acre-feet of water in the bed of the Xorth 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, aiid 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. 



