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77. Outlet pipe (56-inch inside diameter)
78. High-pressure gate
79. Butterfly valves (48-inch)
80. Other metalwork

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Title Frontispiece 1. Platoro Dam location map 2. Geologic map 3. General plan and sections for Platoro Dam (2 sheets)

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4. Material distribution chart

5. Hydrograph of Conejos River below Platoro Dam site

6. Area-capacity-discharge curves for Platoro reservoir,

spillway, and outlets

7. Plan, profile, and sections for outlet works 8. Outlet works structures

9. 56-inch-diameter outlet pipe

10. 4- by 5-foot high-pressure gate

11. 48-inch butterfly control valves as initially installed

12. Dumping material during initial construction of temporary cofferdam

13. Two large bulldozers teamed together to construct the cofferdam

14. The entire flow of Conejos River diverted through the outlet pipe

15. Looking upstream at trashrack structure and conduit

16. Diamond drilling of grout holes to form the grout curtain

at stations 10+50 and 10+90

17. Excavation of grout cap trench in left abutment of dike

18. Stripping activities on the foundation of the river section of

the main dam

19. Vicinity map showing borrow areas with test holes and profile lines

20. A 2-1/2-cubic-yard shovel used to work a 30-foot face in

borrow area 6 21. Loading operations in gravel area 2 22. Embankment placing operations during early stages of construction 23. Completed embankment viewed from same point as preceding



Placing zone 2 and zone 4 material in the upstream part of the
river section

25. Loading zone 1 material in a pothole downstream from station 14+00

26. Large pneumatic tamping machine being used to compact zone 1

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27. Equipment used at outlet portal of tunnel for concreting


28. Pumping concrete into grout cap trench of the right

abutment by means of a pumpcrete machine

29. Structural steel used to support sections of the horseshoe

portion of the outlet tunnel

30. Placement of reinforcement steel in the high-pressure

section of the outlet tunnel

31. Workmen placing reinforcement steel in spillway bridge 32. Final excavation of spillway channel and completed spillway bridge 33. 48-inch butterfly control valve with improved operating mechanism,

scheduled for installation in the fall of 1954

· 34. Completed outlet valve house, with both butterfly valves (initial

installation) discharging at full opening and 57-foot head



1. General Description. - Platoro Dam (fig. 1) is located on the Conejos River 1 mile west of the town of Platoro, Colo., at an elevation of 10,000 feet above sea level. It is the principal feature of the Conejos division of the San Luis Valley project; it will provide supplemental water to 85, 150 irrigable acres in the San Luis valley and will alleviate flood damage in the flood plain of the Conejos River.

The reservoir formed by the dam has an area of about 950 acres and contains about 60,000 acre-feet of water at full capacity (not including superstorage). The stored water is released back into the river through outlet works as needed for diversion to district lands about 40 miles downstream.

The'dam (fig. 3) is an earth-fill structure 1,540 feet long, with a maximum height of 165 feet. The spillway section consists of an open rock cut between the dam and the dike; the section is 50 feet long, with a crest elevation of 10,034.0 feet, and has a capacity of 3,000 cubic feet per second with the reservoir surface at elevation 10, 041.0.

The outlet works is on the right abutment. Progressing downstream, it consists of a trashrack structure, a concrete-lined high-pressure tunnel, a gate chamber, a concretelined horseshoe tunnel housing a 56-inch-diameter steel pipe, and a valve house structure. Flow is controlled by a 4- by 5-foot high-pressure emergency gate in the gate chamber and two 48-inch butterfly valves in the valve house. The invert elevation of the tunnel inlet is 9911.5; this is sufficiently low to allow the reservoir to be completely emptied when required, and there is therefore no dead storage.

Platoro reservoir is the highest of any formed by a Bureau Dam. In fact it is believed to be the highest reservoir of its size in the world.

2. Site Investigations. - After the adoption of the Rio Grande Compact on March 18, 1938, which paved the way for development of the waters of the Rio Grande River above Fort Quitman, Texas, the United States Corps of Engineers and later the Bureau of Reclamation investigated sites for irrigation and flood control dams on the Conejos River in the San Luis Valley project. During these and earlier investigations a total of 12 sites were investigated.

Early planning compared the advantages of a single main-stream (Counsellor) reservoir against those of a two-reservoir (Platoro-Mogote) plan of development. By July 10, 1946, exhaustive studies had eliminated the main-stream reservoir scheme from consideration 15), and the object of later planning was to determine the most economical arrangement of Platoro and Mogote reservoirs. A summary of the status of investigations at these sites was made on July 29, 1946 16/. Up to this time, the Platoro Dam site is frequently referred to as "Upper Conejos Dam Site No. 6.".

By April 30, 1947, cost studies had determined that, in view of current construction cost, Platoro Dam, with a capacity of 60,000 acre-feet, was the only feature of the project that could be economically justified 5/. Accordingly, designs and estimates were prepared for its construction.

3. Regional Geology. - Platoro Dam is located in the upper reaches of the Conejos River in the San Juan Mountains. The regional geology of the area is best described by listing the major stages in its history. The basic pre-Cambrian complex of igneous and metamorphic rocks was covered with great thicknesses of Paleozoic and Mesozoic sediments consisting of limestone, sandstones, and shales. At the end of the Cretaceous Era a broad dome was elevated in the present general position of the San Juan Mountains of today. Several cycles of erosion and elevation took place culminating in a widespread outwash deposit locally known as the Blanco Basin formation of Oligocene Age. During the Miocene Age, great thicknesses of volcanic rock were extruded over the area. These

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