Flood of 1948

McNary Dam: The Flood of 1948

One of the major challenges of dam building is to hold a river back, using cofferdams to control its flow during construction. But the massive Columbia challenged the Army Corps of Engineers almost immediately. The 1947 winter left a large snow pack in the mountains north of the Columbia, and a quick thaw in May of 1948 flooded areas from Grand Coulee Dam to Astoria, Oregon. The river topped the first cofferdam at McNary almost as soon as it was built.

Above. The Columbia River Tug, Winquatt, stuck in ice during the winter of 1947. Courtesy of Keith Rodenbough

The tank farm at the port of Umatilla is inundated by the Flood of 1948. courtesy of Keith Rodenbough

Facing a dilemma, the Corps initiated studies on methods of river closure. The agency decided that "tetrahedrons" could control the river. The following excerpt from a Corps report describes the problem and the solution:

"The cofferdam was constructed of steel cells 60 feet in diameter, 40 to 60 feet in height with a gap in the upstream leg across a natural deep channel about 240 feet wide, through which most of the low flow of the river (60,000 to 70,000 cfs) passed. To plug this hole was the challenge. The possibility of constructing steel cells was investigated and abandoned first. Large timber cribs were very seriously considered, to be floated into place and sunk. In fact, this scheme, as against others, caused considerable alteration in the 'rear echelon' design studies carried on in Portland during 1950. A 'brainstorm' of carefaully placing an old Liberty Ship across the opening and sinking it was another idea. Otto Lunn, directing the work of the rear echelon, always alert to better ideas, and a student of technology, after studying experience at Passamaquoddy, opted for utilizing stone carefully dropped. Model studies were made, shaped analyzed, and the decision was made to use 3,000 pre-cast concrete tetrahedrons weighing 12 tons each."

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