During this period the flow at Grand Coulee has varied from 17,000 second-feet to 492,000 second-feet, with an average of 109,000 second-feet, corresponding to an average annual run-off of 79,000,000 acre-feet.

There has been very little additional irrigation development on the upper tributaries of the Columbia River during the past 18 years, so that the flow as recorded in Table No. 1 represents the flow under present conditions. However, by the time the Columbia Basin project has been fully developed, additional irrigation development may deplete the present flow of the stream by about 1,000,000 acre-feet annually.

In the Gault report of 1924, the amount of such depletion due to future irrigation development above the dam site was estimated at about 900,000 acre-feet annually.

The irrigable areas and amount of depletion have been estimated by the Army engineers as follows (part 2, Appendix 2):

Above Flathead Lake..

From Priest Lake..

Location of irrigable lands

Between Flathead Lake and Lake Pend O'Reille.

Between Lake Pend O'Reille and Grand Coulee, exclusive of Spokane River.
From Spokane River.

Total..

In addition to the above lands there is an irrigable area of about 66,000 acres on the Rathdrum Prairie in Idaho which would receive a water supply from either Priest Lake or Lake Pend O'Reille. Making due allowances for this area and also for the areas now under irrigation in the Columbia River watershed, the estimated depletion for additional irrigation and incidental storage development have been taken as 1,000,000 acre-feet annually, as follows (units acre-feet):

Irrigation requirements.—On account of the variety of soil and climatic conditions on the Columbia Basin project, the crops grown and the water requirements will vary markedly on the different localities. On the whole, however, it is believed that crops and water requirements will be similar to those on the Sunnyside Division of the Yakima project.

The physical features of the two projects compare as follows:

1 Climatological record at Sunnyside.

Columbia Basin project

50.4°.2

62.2°.2

0.68 foot.2

0.30 foot.2

159 days.2

500 to 1,400 feet.

Largely deep soil, varying in texture from fine silty loam to sandy loam. Small areas of shallow sandy soil underlain by gravel.

Estimated 3.25 acre-feet per acre.

2 Mean of climatological records at Lind, Hatton, Wheeler, Ephrata, and Quincy. 2 Mean for period 1919 to 1930, inclusive.

From the above tabulation it is noted that while the Columbia Basin project area has slightly higher temperatures in the growing season and a little longer frost-free period than the Sunnyside division, it also receives more precipitation and the soils as a whole are more retentive of moisture than those on the Sunnyside division. In view of these facts the average irrigation requirements on the Columbia Basin project have been estimated at 3.25 acre-feet per acre annually.

Plans for the irrigation of the proposed project contemplate that all main canals and laterals having in excess of 100 second-feet capacity will be lined and also lining laterals below 100 second-feet capacity when indications point to excessive losses, thereby materially saving water that would otherwise be lost by seepage. Some water will still be lost, however, on account of regulatory waste, evaporation loss in the main canals, and laterals and seepage losses from the smaller laterals. The total amount of such losses in the distribution system has been estimated at 25 per cent of the water diverted into the main canals.

Opportunities for the reuse of return flow are not so good on this project as would be expected on an area of this size, due to the fact that the coulees which form the natural drainage channels of the project are deep and to recover such return flow would require additional pumping. The Gault report of 1924, estimates that a maximum of 512 second-feet could be reused out of Lind Coulee with the pumping plan fully developed with repumping lifts. For this report it is estimated that an average of 500 second-feet of return flow could be used with the fully developed project.

Based on the foregoing discussion the net diversion requirements from the Grand Coulee Reservoir, for irrigation water for the fully developed area, would be as follows:

TABLE NO. 2.-Net diversion requirements from Grand Coulee Reservoir for fully developed Columbia Basin project

(1) Total irrigable area, 1,199,430.

(2) Average of 500 second-feet, April to October, inclusive, distributed throughout year in same proportions as total diversions.

Grand Coulee Reservoir.-The irrigation plan provides for a reservoir to be created in Grand Coulee by the construction of two dams, which will serve as part of the main conduit and eliminate a very expensive portion of the main canal which would otherwise be necessary to convey the water past that vicinity, and will also provide regulatory storage as hereinafter explained.

At various times during the past 11 years geological examinations have been made to determine the suitability of the Grand Coulee Reservoir site and the probable extent of leakage therefrom. Unpublished geological reports of this site are available as follows:

July, 1921, by O. P. Jenkins and H. H. Cooper.

March, 1924, by Kirk Bryan.

October, 1930, by Henry Landes.
November, 1930, by Ira A. Williams.

December, 1930, by F. L. Ransome.

That portion of the Grand Coulee proposed to be used as a reservoir site has walls composed largely of basalt. Within the flow line of the proposed reservoir, except for the extreme ends of the coulee and at a few places along the side walls, the basalt is covered by unconsolidated talus slopes, sands, gravels, and silt terraces. The floor of the coulee, except for the southern end where basalt is exposed and near the northern end where some granite is exposed, is covered by silts which are underlain by sand and gravel.

All of the geologists who reported on this reservoir agree that the most likely place for serious leakage to occur is at the southern end of the reservoir site, where a steep monoclinal fold occurs in the basalt. The inclined flows and the more permeable contacts between the successive flows along which water could percolate are exposed in the sides and bottom of the coulee. There is some disagreement as to the extent of such leakage; Cooper and Jenkins believe that the sharp folding of the basalt was accompanied by faulting along which water could readily escape; Williams believes that, while no general faulting occurred, the folding caused some fractures in the

adjacent basalt; Bryan, Landes, and Ransome recognize the possibility of leakage along the inclined contacts between the flows and apparently believe that such fractures as may be accompanied by the folding are superficial or will be sealed by the silt which covers the bed of the reservoir.

Ransome and Bryan believe that an underground hydraulic gradient exists from the plateaus toward the reservoir basin so that the pressure thus created would prevent the movement of water from the reservoir toward the sides. Mr. Williams, on the other hand, believes that a general hydraulic gradient exists from the east to west, so that while the raising of the water level would not be sufficient to reverse the gradient to the east, it would cause a steeper gradient to the west. He states that seepage along the west side of the reservoir would be limited by the permeability of the wall; in this connection he points out the possibility of fractures existing, especially at the southern end near the monoclinal fold.

All of the geologists contemplated a maximum flow line elevation of 1,552.5 feet in the reservoir, while present plans contemplate a maximum flow line elevation of 1,570 feet.

In view of the impossibility of determining in advance, the extent of the reservoir leakage, such leakage has been very conservatively estimated as 1,000 second-feet (corresponding to about 1 inch loss per day) for the purposes of this report.

With the project fully developed, the water surface elevation in the Grand Coulee Reservoir will fluctuate between 1,570 and 1,554.8 feet, thereby creating a storage capacity of 329,000 acre-feet which could be utilized to carry the irrigation requirements for short intervals in case it should become necessary to interrupt the pumps during the irrigaton season. This storage would be useful in carrying a large part of the irrigation demand during April of each year at a time when the proposed Columbia River Reservoir would be down to low levels, during periods of subnormal run-off. The pumping draft and power required for pumping would be reduced as a result of this useable storage in Grand Coulee Reservoir. A preliminary study of the joint operations of the Columbia River and Grand Coulee Reservoirs during critical periods of low run-off such as occurred during the winters of 1919-20 and 1929-30, shows that a minimum reduction in firm power output at the Columbia River Reservoir would occur if the Grand Coulee Reservoir were maintained at elevation 1,570 throughout the winter until the end of March and then allowed to drop to elevation 1,554.8 during April.

In May and the succeeding months there is always a surplus of water available at the Columbia River Reservoir, so that the pumping plants could be run continuously at full capacity and the storage in Grand Coulee Reservoir replenished as rapidly as possible.

While the operation of this reservoir could be varied from year to year to best fit in with the available power and water levels in the Columbia River Reservoir for that year, it has been assumed in these studies that the Grand Coulee Reservoir stages would vary as outlined above for each year. Based on the above discussion and the irrigation demands set forth in Table No. 2, the net amount of water to be pumped from the Columbia River Reservoir and

the elevations to which such water must be pumped are set out in Table No. 3.

TABLE NO. 3.—Net irrigation demands on Columbia River for fully developed Columbia Basin project

3 Corresponds to elevation of W. S. in Grand Coulee Reservoir except as noted. Minimum elevation to which water must be pumped fixed by conditions at outlet of pump discharge

Preliminary studies show that a dam in Grand Coulee, 10 feet higher than contemplated herein, would furnish additional storage which could be used to further reduce the pumping requirements from Columbia River during the winter months and thereby increase the firm power available at that site from 800,000 kilowatts to 840,000 kilowatts. However, in order to accomplish this, about 1,000 second-feet of additional pumping capacity and a corresponding increase in the power installation would be required at the Columbia River Dam to care for the additional pumping requirements to insure filling the reservoir during the period May to August, when a surplus of water is available in the Columbia River. In addition, raising of the water level in the Grand Coulee reservoir would tend to increase the possibilities for leakage of such reservoir. Before final plans are made for this reservoir more detailed studies should be made to determine the best capacity to which it can be developed safely and economically.

Columbia River reservoir.-The determination of the economic height for the Columbia River Dam requires the proper balancing of the costs for pumping into the Grand Coulee reservoir, the costs of power for pumping, the cost and value of power for commercial uses, the value of lands and power sites submerged by the reservoir and other important factors.

Based on preliminary studies of these factors, the "high dam" as proposed in the Army report was tentatively adopted and the studies reported herein are based on this dam, which will raise the

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