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Figure 2. --Grand Coulee Dam and surrounding area. The two powerhouses may be seen flanking the central spillway. Pumping plant is in the center of the view with the discharge lines leading to the Feeder canal which discharges into the Equalizing reservoir in the upper right corner of the photograph. CB 31527
canal of 9, 700 cubic feet per second capacity extends 6. 6 miles westward from the Long Lake reservoir to the bifurcation structure constituting the beginning of the East Low canal and the West canal.
It will be noted that a radical departure has been made from early plans for the canal system, which contemplated one high canal on the east side of the project and one canal covering the west side. A high east canal and a low east canal take the place of the former with only the low canal now under construction. The single high east canal, as originally planned, would have been long and on relatively rough ground. It would be expensive to build, requiring many siphons and tunnels of large capacity and would have passed through land not now demanding irrigation but used in periods of favorable rainfall and high grain prices for dry farming. The eventual saving to be made in construction costs by adopting the three-canal design will be measured in millions of dollars. Operating costs also will probably be lower. The change in design from a two-canal to a threecanal plan was officially approved in 1943.
The West canal has an initial capacity of 5, 100 cubic feet per second, an ultimate length of 80 miles, and will serve 281,000 acres. It extends westward, then around the north end of Soap Lake in a siphon 12, 912 feet long which has a maximum static head of 236 feet. The pipe is of reinforced concrete, 25 feet in diameter, at the two ends where the siphon is subjected to less than 100-foot static head; and of steel-lined concrete, 22 feet 4 inches in diameter, for the remainder of the length. The high-head section is 8, 264 feet in length. Beyond Soap Lake the canal skirts the hills north of Ephrata, and turns south near Quincy. It will continue south to a 9,150-foot tunnel through the Frenchman Hills, which will feed canals running east and west from the tunnel along the south slopes of the Frenchman Hills, to water the Royal Slope lying between the Frenchman Hills and the Saddle Mountains.
The East Low canal has an initial capacity of 4, 500 cubic feet per second, will ultimately be about 130 miles long, and will irrigate 252,000 acres. From its headworks, about 2-1/2 miles north of Adrian, the canal runs south along the east side of the Crab Creek valley. The location is along the upper margin of the flatter lands, across terrain offering no particular construction difficulties, although a number of siphons and one tunnel are necessary. This canal will extend in a southerly direction the length of the project, terminating in the Snake River valley about 8 miles northeast of Pasco. That part of the canal north of Lind Coulee will not be lined because any seepage loss will be recovered in the Potholes reservoir.
The Potholes reservoir formed by O'Sullivan Dam serves as a catchment basin for return flows of about one-third of the project lands; about 500,000 acre-feet of live storage will be provided, thus reducing the amount of water needed to be pumped.
Two canals will receive their water supplies from the Potholes reservoir. The Potholes West canal, of 235-cubic-feet-per-second capacity, to serve 13,600 acres of land in the lower Crab Creek area, will extend south and west along the base of the Royal Slope. The Potholes East canal, of 4, 600-cubic-feet-per-second initial capacity, will run generally south to the vicinity of Pasco, and will serve 254, 000 acres. A branch will extend west to water Wahluke Slope, along the south side of the Saddle Mountains. Construction conditions on these canals will not be difficult. Since seepage from these canals cannot be recovered, they will be lined with concrete throughout.
As a means of starting irrigation in the southern district before the main canal system could be extended that far, a temporary pumping plant has been built on the Columbia River, about 14 miles northwest of Pasco. With a lift of about 177 feet, it supplies water for about 6,000 acres of land. Later, this supply will come from the Potholes reservoir, by gravity.
The Burbank unit, on the south side of the Snake River, below Pasco, is included in the south district of the project. This unit, containing approximately 4, 500 acres, will be supplied by water from one or more pumping plants on the south side of the Snake River.
4. O'Sullivan Dam and Potholes Reservoir. - O'Sullivan Dam, called Potholes Dam until the name was changed officially on September 28, 1940, is the largest structure in the Irrigation division of the Columbia Basin project. It is an earth-fill structure located on Lower Crab Creek in sections 9, 10, 11, 12, and 16, T. 17 N., R. 20 E, Willamette meridian, about 22 miles south and 12 miles east of Ephrata, Wash. It is
about 5 miles south of the lower end of Moses Lake and approximately 12 miles west of the town of Warden. The dam forms the Potholes reservoir.
This structure involved the construction of about 19, 000 feet of continuous embankment, ranging in height up to approximately 160 feet above streambed; and outlet tunnel near the east end of the dam; a 500-foot-wide open-channel spillway between sections of the embankment; and a roadway extending along the full length of the crest of the dam. The dam consists of moistened and rolled embankments of clay, sand, silt, and gravel. The outer sections consist of sluiced gravel fill. The upstream and downstream slopes are covered with a layer of rock. A 30-foot gravel roadway was constructed along the crest of the dam embankment and adjoining rock cuts.
Potholes reservoir covers an area of 32,000 acres at the normal water surface elevation of 1052.0. It has a total capacity of 675,000 acre-feet and a live storage capacity of 569,000 acre-feet. This is considered adequate for the ultimate development of about one million acres in the project area.
5. Purpose. - Potholes reservoir is used primarily for storage purposes. The reservoir benefits the irrigation in four respects: First, it will catch and impound for reuse the runoff from one-third of the project's irrigable area. The quantity of water thus recovered may amount to over 500, 000 acre-feet per year when the project is fully developed, which will materially reduce the amount of water pumped at Grand Coulee Dam. 12/ Second, it will make possible the irrigation of a large semiarid area in the southerly end of the project much sooner than if water for this development had to come from an extension of the East Low canal. Third, it will provide regulatory water storage in the central part of the project area. Fourth, it will provide early spring irrigation with relatively warm water for the lower, earlier lands in the southern part of the project, before water could be available from the upper main canals. The distribution of irrigation water to the 268, 000 acres of low land lying to the south will be through the following:
(1) Potholes East canal, 60 miles long, initial capacity 4, 600 cubic feet per second, serving 254,000 acres.
(2) Potholes West canal, 22 miles long, initial capacity 235 cubic feet per second, serving 13,600 acres.
12/ Superscripts refer to numbered items in appendix D, References.
6. General. - Extensive field surveys and investigations were made over a period of several years before a final irrigation plan for the Columbia Basin project was evolved. The problems for consideration on this project were unusually complex, owing chiefly to the characteristic Columbia River Basin topography and geology. Important factors considered in this study were the development of power and the necessity for pumping water to many of the higher project areas.
In selecting the irrigation plan, approximately 15 plans in addition to the original (Army) plan were considered. An estimate of the cost was prepared for each. O'Sullivan Dam is one of the several dams included in the recommended irrigation plan.
7. Regional Geology. - The O'Sullivan Dam and reservoir is situated ontheColumbia Lava Plateau in the southeastern portion of an area known as the Quincy Basin. The Quincy Basin is the largest of several structural basins on the project. It is bounded on the east by the gently rising surface of the plateau and on the south by a ridge known as the Frenchman Hills. The Frenchman Hills is an anticlinal arch with an east-west axis which, together with other features such as monoclines and faults, represent structural adjustments in the Columbia lava flows caused by post-basalt movements of the granitic floor upon which they rest .6/ ,7/ ,5/ ,9/
Subsequent to the structural adjustments fine-grained sediments known as the Ringold formation were deposited on the surface of the basalt flows in the Quincy Basin. These were later covered by fluvial and glacial outwash consisting of silt, sand, gravel, and boulders swept southward into the basin during the Pleistocene Period when the normal course of the Columbia River was blocked by glacial ice. The materials represent the debris derived from the erosion of the Grand Coulee and the other channels and coulees to the north.
The glacial and preglacial flows of water which reached the Quincy Basin escaped via several outlets. The largest and longest occupied of these was in the southeast corner of the basin at the east end of the Frenchman Hills anticline. Through this restricted area, a maze of channels, known as the Drumheller channels, were cut into the basalt strata; Crab Creek, the post-glacial relic of these older water movements, now drains the major part of Quincy Basin and flows through the deepest of the Drumheller channels. It is in this area that the O'Sullivan Dam has been constructed.
8. Dam Site Geology. - Bedrock in this area consists principally of basalt flows and interflow zones of the same general type as encountered elsewhere in the Columbia Basin project. Exposures of the Ringold formation sediments occur in Lind Coulee to the east of the dam site area, but not within the actual fpundation area of the dam. Overburden within the dam site area consists of a thin, discontinuous cover of windblown silt and sand which, together with sand and gravel, partially fills many of the potholes and channels cut into the basalt flows. Several of the channels in the southwesterly portion of the area contain deposits of sodium sulphate derived from the evaporation of the seasonal lakes. A prominent terrace of sand and gravel overlies the basalt and Ringold sediments to the north. Toward the northwest the surface of the reservoir area is covered by superficial deposits of sand and gravel and windblown sand. Beneath these overburden materials are the Ringold sediments and the basalt flows.
The basalt flows of the dam site area range from near-horizontal to gently warped except for a narrow zone of disturbance known as the Lind Coulee flexure. This zone, characterized by faulting and sharp folding, trends in a rather irregular fashion in a general east-west direction through the dam site area. Figure 3 illustrates the axis positions of three dam sites considered during the investigation and their relation to the Lind Coulee flexure. The geology of these individual sites is discussed in the following section in connection with site selection.
9. Dam Site Selection. - Early geologic and investigative work in the Quincy Basin for a possible dam site was done in 1918 by the United States Geological Survey. iiy. 2/ The results of these and later geologic investigations indicated that special foundation treatment would be necessary to insure against leakage, and that there was suitable embankment material in this area for the construction of the dam.