Mazama eruption is linked to valley's soggy soil

EUGENE, Ore. — New research by a pair of Eugene scientists shows the long-ago eruption of Mount Mazama, whose remains form the rim of Crater Lake, can be blamed for the wet, soggy earth of the Willamette Valley.

The ash from the mountain's massive eruption apparently formed the layer of clay that underlies much of the valley, keeping water from soaking deeper into the earth.

Karin Baitis, a soils scientist with the Bureau of Land Management in Eugene, tied the clay layer to the Mazama eruption in a paper she delivered Thursday at the Northwest Anthropological Conference.

She and geochemist Michael James of James Geoenvironmental Services stumbled on the origin of the clay as they were doing separate surveys of soil characteristics in areas around Eugene.

Until now, no one thought ash from the massive eruption of Mount Mazama made it into Western Oregon. Previous studies suggested it all went north and east of the mountain, where layers hundreds of feet thick have been found.

"We were really surprised with our findings," Baitis said after her presentation. "No one ever thought we had anything like this here."

Before Baitis and James made their discovery, some scientists thought the layer of clay in the valley was alluvial material swept in by the great Missoula Floods about 13,000 to 15,000 years ago.

Those massive events occurred when an ice dam that held back a colossal ancient lake in what is now Idaho collapsed, releasing floods that surged across Eastern Washington, down the Columbia River and into the Willamette Valley.

But when Baitis and James began analyzing samples they collected, that idea just didn't pan out. The material was just too fine, not the mixture of larger and smaller bits characteristic of a flood and not something that came from weathering of existing materials over time.

So she and James worked with researchers at the University of Oregon and the U.S. Geological Survey to analyze their samples, which didn't support any of the existing theories and only added to the clay mystery. But it wasn't until they later saw the same clay turn up in an archaeological dig near Woodburn that they made the connection to Mount Mazama.

The two researchers said they haven't nailed down the entire story of how the Mazama eruption affected the Willamette Valley and realize there's likely to be some controversy over some of their theories.

But both are sure that they've found a connection between one of the largest eruptions in the Northwest's long volcanic past and present-day life in the Willamette Valley, something that substantially changes the region's history books.

"We've got the evidence," Baitis said. "It's just so remarkable. I think we have a different history than what we've thought."