10/20/2026
"Too impressive to be told in everyday language," Lois P. Myers of the Portland News-Telegram wrote on November 21, 1935, of Bonneville Dam at night, on the Columbia River in Oregon.
The hard part was the river itself. At the main dam the channel was about 900 feet wide and 20 to 50 feet deep, and the water ran at 6 to 9 feet a second. The working season was only eight months, August to March, and every season had to end with the work in a state that could be left alone when the summer flood came.
So the Corps of Engineers chose to divert the river from the work site. Its plan, the Corps' own history says, "called for dividing the river in half and unwatering each half successively." First, a horseshoe of timber cribs went around the south half of the spillway site.
A crib was a huge timber box with open pockets. Built of 12 by 12 inch timbers bolted together in courses, the cribs generally measured 60 by 60 feet and stood up to 75 feet high, each roughly the size of a six-story apartment building. Crews built the lower part on shore, floated it out, finished it to full height in the river and sank it by dumping rock and watertight fill into the pockets.
But the bottom of the Columbia is not flat, and leveling it would have cost too much time and money. Therefore the engineers dredged off the thin cover of boulders and gravel, sounded the rock every two feet, plotted its contours and built each crib's base to match. In a current of 9 feet a second the strain on a crib's holding lines came to about 300,000 pounds, so the middle line was anchored straight onto bedrock near the center of the channel.
On August 27, 1935, the emptying of the south half began. Inside the horseshoe crews dug and poured the concrete piers of the spillway. After the 1935-36 low-water season it was time to swap. The upstream cribs of that first cofferdam were removed on March 16, 1936, and the Columbia ran between the piers, over the unfinished crest sections, while a second cofferdam went in for the north half between December 1935 and May 1936. The north half was finished in 1936-37.
This photograph, taken on February 16, 1937, caught the work still going. The spillway stands across the channel with the river running through its gates, and the powerhouse nearby is being fitted out: that February and March the first two turbines and generators began going in.
George Gerdes, chief engineer for the main dam, designed the cofferdams. They cost $2.5 million and took 8 million board feet of timber, and the Corps' history says that at the time it was "the largest cofferdam job attempted on a United States river."
About 3,000 people worked on the project at the peak, most of them from the relief rolls in Oregon and Washington. Before December 1935, over 300,000 visitors came to watch.
In September 1937 the Corps closed the spillway dam, the first of eight federal locks and dams on the Columbia and Snake Rivers. It still crosses the Columbia, a National Historic Landmark.
