Geologic Secrets of America's Tallest Single Dune: Bruneau Sand Dunes, Idaho

Summary

Geology professor Shawn Willsey visits Bruneau Sand Dunes State Park in south-central Idaho to explain the dune’s unusual height, the movement and makeup of its sand, and evidence of the ancient Bonneville Flood.

Why the dunes are in Eagle Cove

• The dune field lies in a low, basin-like area called Eagle Cove, bordered by higher ground and located near the Snake River.
• The Snake River once followed a large meander. Erosion along the river’s outer bends gradually narrowed the strip of land between two parts of the channel.
• After the narrow neck was breached, the river took a more direct route and abandoned the meander. The former channel left a low depression where sand could accumulate.

How the dune grows—and why it is so tall

• Wind pushes sand grains up the windward slope by rolling, bouncing, and skipping them.
• Near the crest, grains pass onto the more sheltered leeward slope, where they collect. When that slope becomes too steep, sand avalanches downward.
• Bruneau is a reversing dune: winds arrive from different directions, so the windward and leeward sides can switch. Sand can be pushed back across the dune and deposited on top of earlier deposits, helping the dune grow taller.
• Regional storm systems can bring winds from the east ahead of a front and from the west after it passes. Even moderate winds can move sand near the crest.
• The dune is described as North America’s tallest single-structure sand dune. Other dunes may be taller overall, but are supported by features such as mountains or other dunes; Bruneau’s large sand mass is not braced against such a structure.
• Sand avalanches form visible lobes on the steep leeward slope, while ripples and moving grains show that wind continues to reshape the dune.

What the sand is made of

• Bruneau’s sand is darker than many familiar beach or dune sands and has a “salt-and-pepper” appearance.
• The dark grains are weathered fragments of basalt, a volcanic rock common in southern Idaho.
• The lighter grains are quartz. Shawn points to quartz-rich granite and rhyolite in the Owyhee Mountains as a local source.
• River systems, including the Snake River, and the Bonneville Flood could break up and transport rock material. Wind then helped carry and deposit sand in the dune field.

Evidence of the Bonneville Flood

• On the east side of Eagle Cove, Shawn examines a beige, finely layered deposit that is finer than the dune sand and mostly silt-sized.
• Its thin, closely spaced layers are called laminations. The deposit also contains ripples.
• The ripples appear fairly symmetrical, which Shawn says is consistent with sediment being moved back and forth rather than transported predominantly in one direction.
• During the Bonneville Flood, water inundated Eagle Cove. As floodwater slowed in the broad depression, larger material would have been deposited first, while finer particles suspended in the water settled out to form the laminated silt.
• The deposit preserves evidence of the flood in a landscape now dominated by wind-shaped dunes.

Speakers or sources featured

• Shawn Willsey — geology professor and on-camera narrator. No other speakers are featured in the subtitles.