Near Plymouth in Benton County, Washington — The American West (Northwest)

McNary Dam Tetrahedrons

Photographed by Clayton Pickett, May 14, 2026

1. McNary Dam Tetrahedrons Marker

Inscription. During the construction of McNary Dam a total of 3,271 of these 12-ton tetrahedrons were dropped into the Columbia River to complete the closure of the last cofferdam and divert the full flow of the river through the spillway. This method of closure was unique and its success made engineering history.
Closure operation started 10 October 1950
Last tetrahedron dropped 27 November 1950.

Topics. This historical marker is listed in this topic list: Man-Made Features. A significant historical date for this entry is November 27, 1950.

Location. 45° 56.561′ N, 119° 18.015′ W. Marker is near Plymouth, Washington, in Benton County. It is on McNary Road 1.8 miles east of Interstate I-82 when traveling east. Touch for map. Marker is in this post office area: Kennewick WA 99337, United States of America. Touch for directions.

Regionally, this marker is in Washington’s Columbia Basin and in Yakima Valley.

It is also in the American Mountain West

and in the Lewis & Clark Corridor.

Globally, it is in North America,

the Pacific Rim,

the Western Hemisphere,

the Western World,

and the Anglosphere.

Other nearby markers. At least 8 other markers are within 7 miles of this marker, measured as the crow flies: The Umatilla Rapids (approx. 1.6 miles away in Oregon); Cayuse, Umatilla and Walla Walla Homeland (approx. 1.6 miles away in Oregon); The Lewis and Clark Expedition (approx. 1.6 miles away in Oregon); Patímaša Tiičámna (approx. 1.6 miles away in Oregon); Strangers "from the clouds" (approx. 2.7 miles away); Umatilla Veterans Memorial (approx.

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3 miles away in Oregon); How Do You Make a Hat? (approx. 6½ miles away in Oregon); Naamí Tiičám Ku Waqíšwit (approx. 6½ miles away in Oregon).

Photographed by Clayton Pickett, May 14, 2026

2. McNary Dam Tetrahedrons Marker

Credits. This page was last revised on May 17, 2026. It was originally submitted on May 15, 2026, by Clayton Pickett of Richland, Washington. This page has been viewed 11 times since then. Photos: 1, 2. submitted on May 15, 2026, by Clayton Pickett of Richland, Washington. • Andrew Ruppenstein was the editor who published this page.