Exploration activities at Palisades Dam site were conducted during 1934 and 1935. These activities were resumed during the years 1941 and 1942 and then were temporarily suspended during the war. Field work was resumed in March 1945 and detailed surveys and explorations were made. In 1947 preliminary designs and estimates were made showing the costs for a dam with two different outlet capacities. One design had a 20,000-second-foot outlet works and the other had a 30,000-second-foot outlet works.

The project was reauthorized by Congress in 1950 by Public Law 864, 81st Congress, substantially in accordance with the "Supplemental Report, Palisades Dam and Reservoir Project, Idaho" (Bureau of Reclamation, Boise, Idaho, June 1949). This reauthorization was sought because of:

(1) Proposed changes in the operating plan in the interest of flood control and in consequence a substantial increase in the allocation to flood control.

(2) Increase in size of power installation.

(3) Desirability of reporting to the Congress on the negotiations with the water users concerning winter water-saving agreements.

In order to facilitate early generation of power it was decided to issue a separate contract for the excavation of the tunnels. This contract was awarded and notice to proceed was issued on December 17, 1951. Specifications for the construction of the dam, spillway, outlet works and powerplant were prepared, and a contract was awarded on May 8, 1952 (chap. IX).

Figure 2 shows the Palisades Dam reservoir area.

4. Physiography.

The upper Snake River watershed is about 22,000 square miles in size and occupies the eastern part of Idaho and adjacent lands in western Wyoming. The eastern half of the Snake River plain--a broad desert plateau--and its flanking mountains are the major land features of the project area. The Snake River plain slopes from east to west and has a general elevation of 5, 000 to 3, 000 feet above sea level. Figure 3 shows hydrographs (years 1911 through 1920) of the south fork of the Snake River.

The main stem of the Snake River heads in the southwestern part of Yellowstone Park at an elevation of nearly 10,000 feet. It flows south for 130 miles in Wyoming and then turns sharply to the northwest in passing through a deep canyon to Idaho and the Snake River plain. The drainage area above Palisades Dam is 5, 110 square miles.

The dam is located at a narrow constriction of the river valley where the course of the river has been abruptly changed by a large mass of igneous rock. The floor of the valley at the dam is approximately 1, 200 feet in width and is contained by steep walls. The left canyon wall rises on a 60 percent slope to a point 1, 130 feet above the valley floor. The right canyon wall rises on a 50 percent slope nearly to the crest of the dam (elevation 5630), where a remnant of a terrace occurs. From this terracelike slope the hillside rises on a slope averaging about 40 percent for a vertical distance of nearly 1,000 feet.

5. General Description of Features. (a) Reservoir.-- The reservoir will have a total capacity of approximately 1,417,600 acre-feet allocated as follows:

2

PARTIAL RUNOFF 5,401,000 AF

JAN FEB

MAR APR MAY

JUNE JULY AUG SEPT OCT. 1920

NOV DEC.

NOTES

APPROVED

BRAWN.

Joint use storage includes storage for irrigation, flood control, and power. The normal water surface of the reservoir is at elevation 5620. Surcharge to maximum water surface, elevation 5621, in combination with a spillway capacity of 48, 400 second-feet and an outlet works capacity of 47,000 second-feet (at elevation 5620) is provided to protect the dam against the inflow design flood having a peak of 105,000 second-feet and a 30-day volume of 3, 320,000 acre-feet.

The normal water-surface elevation of 5620 is considered the elevation at which the area to be flooded in Wyoming is contained within the channels of the tributaries entering into the Snake River or within the Snake River itself. This is of importance in that it avoids the complications that would arise from interstate negotiations. Another important factor limiting the normal water-surface elevation to 5620 is that any appreciable increase of water surface would inundate the REA Powerplant at Freedom, Wyo. This generating plant is the sole source of power for the Lower Star valley area in Wyoming, a prosperous farming community which lies to the east of the Palisades Reservoir. These two reasons were considered adequate for adopting and maintaining a normal water-surface elevation of 5620, even though subsequent studies indicated that additional allocation of reservoir storage could be made.

an

(b) Dam.-- Palisades Dam contains over 13,000,000 cubic yards of embankment. The crest of the dam at elevation 5630 is 40 feet wide, approximately 260 feet above the bed of the river, and 2, 100 feet long. The structural height (crest of dam to lowest point of foundation) is 270 feet. The embankment is constructed of four zones: interior zone of impervious fine-grained material, intermediate zones of relatively impervious transition material, outer zones of pervious material, and a rockfill zone at the downstream toe. The upstream slope is protected by 3 feet of riprap, and the downstream slope is protected by a cobble and boulder blanket of variable thickness.

The upstream face of the dam has a 3 to 1 slope from the crest to elevation 5520. Below this elevation, it slopes 4 to 1 to elevation 5450, and then 6 to 1 to the berm at elevation 5400 which forms the upstream toe of the dam. The downstream face of the dam has a 2-1/2 to 1 slope from the crest to elevation 5550. Below that elevation it slopes 3-1/2 to 1 to elevation 5450 and then 6 to 1 to the original ground surface. A 30foot-wide access road is provided at about elevation 5396. This road provides access to the powerplant and switchyard from the highway. The left portion of the downstream face immediately upstream from the powerplant is warped from the 6 to 1 slope on the maximum section to a 3-1/2 to 1 slope below elevation 5450. The steeper slope was allowed because drill logs indicated that the further upstream the powerplant was located, the better the foundation would be. This change in section was satisfactory from the standpoint of the earth embankment because the foundation in this area is more competent than that in the river channel.

A 300-foot-wide cutoff trench is provided under the central impervious zone, and a 100-foot-wide cutoff trench is provided at the upstream toe of the embankment. The impervious zone extends, as a blanket, from the interior of the dam to the upstream cutoff trench.

On the right abutment, the upstream slope of the dam is curved and extended upstream to form a blanket. The downstream slope of the dam is curved and extended downstream on the right abutment as a supporting blanket.

abutment.

The spillway, outlet, and power tunnels are all located in andesite at the left

(c) Powerplant.-- The powerplant is located at the west end of the downstream toe of the dam and east of the outlet works (fig. 6). The powerplant is of the indoor type constructed of reinforced concrete except for the superstructure which is of structural steel framing with reinforced grouted brick walls and precast concrete roof slabs. The powerplant building is divided functionally into three main areas: the main generating unit bays, the service bay, and the machine shop.

Power is generated from the release of water stored in Palisades Reservoir where it is impounded by an earth dam. The water enters the power inlet structure at

the upstream left abutment of the dam, flows through a 26-foot-diameter concrete underground power tunnel, through a steel penstock manifold in a concrete anchor block, and through four 12-foot 6-inch diameter penstocks to the 39, 500-horsepower Francis turbines which drive the generators rated at 30,000 kilovolt-amperes each.

The fenced switchyard is located about 500 feet downstream from the powerplant

on the right bank of the tailrace at elevation 5397.50.

6. Cost Summary. The following tabulation summarizes the total estimated cost of construction of Palisades Dam and appurtenant structures based on information submitted by the regional office on December 4, 1959. A listing of contracts and purchase orders showing the total project cost is given in appendix A.

1/Includes actual costs to October 31, 1959, plus estimated costs through June 30, 1960, based on form PF-2B, date 8-14-59.

2/In appendix A, Government camp is included in construction contracts but is not

separately identifiable.