Overview

Palisades Dam is located on the Snake River about 55 miles southeast of Idaho Falls, Idaho. It is a large zoned earthfill structure 270 feet high, has a crest length of 2,100 feet, and contains 13,571,000 cubic yards of material. At the time of construction, this was the largest volume of material placed in a dam by the Bureau of Reclamation. The spillway is a 28-foot-diameter tunnel through the left abutment, with a capacity of 48,500 cubic feet per second. The outlet works and power inlet structures are controlled by a fixed-wheel gate at the entrances of the inclined shafts leading to 26-foot-diameter tunnels. The outlet tunnel conveys the water to the steel manifold transition section, where it is released to the stilling basin by regulating gates. At the lower end of the power tunnel, the water may be released to the stilling basin or to four penstocks and conveyed to the turbines for power generation. The capacity of the outlet works is 33,000 cubic feet per second. The dam creates a reservoir of 1,401,000 acre-feet capacity (active 1,200,000 acre-feet).

The powerplant is on the downstream toe of the dam on the west side of the river and initially had a total capacity of 118,750 kilowatts. The powerplant was uprated in 1994 and all four units were rewound increasing the nameplate capacity to a total of 176,600 kilowatts; 44,150 kilowatts each. As part of the mitigation for the powerplant uprate, a fish screen was constructed on Palisades Creek, a small tributary which joins the Snake River approximately 3 miles downstream from the dam.

Geology

Palisades Reservoirs covers about 16,000 acres on the floor of the Grand Valley, the northern portion of Lower Star Valley. The floor of the reservoir is a relatively flat plain underlain by a veneer of silt and fine sand and layers of sands and gravels. The total thickness of the overburden throughout the valley ranges from 5 to about 60 feet. The sand and gravel portion of the overburden is quite pervious. The overburden along the valley walls consists mostly of talus, which has accumulated along the base of the steeper slopes, and of outwash deposits from the side canyons. The talus deposits are accumulated rock debris and clayey silt soil, which appears to be semimpervious materials. The overburden on the main valley floor is underlain by a series of highly compact, uncemented, clays, silts sands and gravels. Surface exposures are found only at a few cuts along the riverbanks. In general the beds are rather lenticular, their strike is parallel with the north-southwest trend of the valley and are rather tight. The left wall is underlain by steeply inclined, consolidated sediments which are much older than the clay-silt beds. The right wall of the valley rises as a very steep slope from the dam upstream for about 500 feet, then as a cliff for several hundred feet to the rim of the valley.

General

Dimensions

Hydraulics & Hydrology