THE IRRIGATION SYSTEM
The Columbia Basin Project irrigation development embraces an area roughly twice the size of the State of Delaware. (See map in back.) Nearly half of this land is irrigable, with the remainder being rough and unsuitable or taken up by townsites, airports, reservoirs, and wasteways. Since the upper edge of the irrigated lands of the project is about 45 miles south and slightly west of Grand Coulee Dam, getting the water to the land, distributing it, and draining off the excess require an extensive system of waterways. Facilities have been constructed to provide irrigation water to farms comprising almost one-half of the ultimate 1-million-acre-plus irrigation system.
The supply and distribution system for the project's irrigation water starts at Franklin D. Roosevelt Lake, the reservoir behind Grand Coulee Dam, and stretches south and west to the city of Pasco at the southern tip of the project, 160 miles away. A network of thousands of miles of waterways traverse the project area, bringing water to the fields and removing the excess.
At present, there are more than 2,000 miles of canals on the project. Included in this 2,000 miles of waterways are 16 large canal siphons and 2 tunnels, which carry the water of the Columbia across coulees and through barriers of high ground with little loss of elevation. The largest of these irrigation structures is the Soap Lake Siphon, which carries the waters of the West Canal across the lower end of the Grand Coulee. It is actually an inverted siphon which dips from the high ground to the east of Soap Lake, crosses the coulee in a semicircle around the northern end of the lake, and comes out on high ground to the west. This concrete tube is 12,883 feet long and has an inside diameter of 22 feet 4 inches for the major portion of the distance, with the remainder 25 feet in diameter. Though it will hold 41 million gallons of water, engineers say the water in it will change every 16 minutes when the water in the canal is flowing at full capacity.
The project's irrigation water begins its long journey to the land when it is pumped up over the lip of the canyon wall at Grand Coulee Dam and flows into Banks Lake. The pumps lift the water at a rate of 4,300,000 gallons of water a minute. When the project is completed and all 12 pumps are working, this flow of water into the project's Feeder Canal will be almost doubled. Once in the Feeder Canal, the water flows along this 1.6-mile waterway to Banks Lake.
Banks Lake is an equalizing reservoir formed by sealing the north and south ends of an Ice Age channel in the Upper Grand Coulee. This 27-mile reservoir makes it unnecessary to regulate the pumping of water from Franklin D. Roosevelt Lake to meet daily irrigation requirements. Water can be pumped into this reservoir when both power and water are available and stored until needed for irrigation.
The dam at the north end of the reservoir is an earthfill dam, 1,450 feet long and 145 feet high. The south dam, known as Dry Falls Dam, is an earthfill dam also, 9,800 feet long, 123 feet high, and comfortably supports a two-lane transcontinental U.S. highway across its top.
The reservoir between these two dams is 27 miles long with a total storage capacity of 1,275,000 acre-feet of water. (An acre-foot is equal to 1 acre of water a foot deep, or 325,850 gallons.) Of the total volume of storage capacity of the reservoir, about 60 percent, or 761,800 acre-feet, is considered "active storage"water which is above the lowest outlet of the dam. This is roughly 245 billion gallons of water, enough to cover the entire State of Rhode Island almost a foot deep.
The "active storage" of Banks Lake will be replaced seven to eight times a year when the project is completed, depending on such factors as the amount of rainfall the land receives, evaporation, and other conditions which govern the demand for irrigation water in the basin.
Normally, pumping into the reservoir begins in May or June and continues intermittently through August or September. The final weeks of pumping build up a reserve of water which can be used during the early fall and the following spring before pumping is resumed.
From the equalizing reservoir the water moves south through outlet gates in Dry Falls Dam near Coulee City, flows through 2 miles of main canal, crosses Bacon Coulee in the 1,038-foot-long Bacon Siphon, and enters the 10,045-foot-long Bacon Tunnel bored through a plateau of hard basalt rock. Pouring out of the tunnel, the water next flows through two ancient lake-beds, a concrete-lined canal, and the bed of a prehistoric river.
Five miles below Bacon Tunnel, the water plunges 165 feet over a basalt cliff into the upper end of Long Lake Reservoir. Appropriately named "Summer Falls," the water pours over these cliffs only during the irrigation season. Normally, Long Lake Reservoir is used only as a waterway. However, if an emergency should call for a quick shutdown of the system, the reservoir can store water coming down the Main Canal until the system is in operation again or until the upstream headgates can be closed.
Continuing on its journey to the farms of the Columbia Basin, the water enters the Main Canal at the edge of the irrigable land. At this point it has traveled 15 miles from Banks Lake and is 44 miles south of the pumps at Grand Coulee Dam.
About 6-1/2 miles west and slightly south of Long Lake is the bifurcation works, where the Main Canal divides into the West Canal and the East Low Canal. The East Low Canal extends 87 miles from the bifurcation works down the eastern edge of the project and empties into the Scootenay Wasteway, which flows into Scootenay Reservoir. From the bifurcation works, the West Canal travels south and west to Soap Lake, where it jogs around the lake and crosses the Lower Grand Coulee via the Soap Lake Siphon, ending finally near Lower Goose Lake, 88 miles below the bifurcation works.
Much of the water in the Columbia Basin Project sees double duty before it returns to the Columbia River. It is used for irrigation, then drained off the land, recaptured by drains, wasteways, and natural channels, and then is used for irrigation a second time. Key structure in this economy operation is O'Sullivan Dam. This earthfill dam forms the Potholes Reservoir, directly south of Moses Lake in the center of the Columbia Basin. The dam is 3-1/2 miles long and rises 200 feet above bedrock. It has an "active storage" capacity of 379,500 acre-feet and covers about 45 square miles.
From Potholes Reservoir, the Potholes Canal flows southeast about 7 miles, then turns south to parallel the East Low Canal for about 15 miles. It passes west of Othello, then swings east to the Scootenay Reservoir, where it joins the waters of the East Low Canal. At the south end of the reservoir, the Potholes Canal flows south and then southwest to supply irrigation water to thirsty lands before ending 6 miles northwest of Pasco.
The Wahluke Branch Canal diverts from the Potholes Canal 6 miles south of Othello and passes through the Wahluke Siphon to serve the lands of the Wahluke Slope. This area slopes south from the Saddle Mountains to the Columbia River. It is about 40 miles long and varies from 5 to 10 miles wide. About 25 percent of this land, or approximately 42,000 irrigable acres, is in the safety control zone maintained by the Atomic Energy Commission. The Hanford Atomic Works is located across the river from the slope, and the control zone is maintained for safety in case of an accident that would cause radiation danger in the area.
These are the canals and reservoirs that form the main network of waterways that can now deliver water to almost half of the irrigable acres in the project area. Before the full project can be developed, there is still much work to be done. The East Low Canal, which will furnish water to the southeastern tip of the project, will have to be extended and the East High Canal, which will roughly parallel the East Low Canal and will serve water to the east of the area now irrigated by the East Low Canal, will have to be constructed.
Also, many miles of main and secondary waterways and related irrigation structures will have to be built throughout the project area. The pumping plant at Grand Coulee Dam will require 6 more pumps to lift the greater water volume into the irrigation system in order to serve the full project.
When completed, the total irrigated area of the Columbia Basin Project will be more than a million acres and will serve water to about 10,000 farms.
Last Updated: 01-Feb-2008