Quake may have sped up rotting

Official says shaking likely freed lilies on Offut Lake

JOHN DODGE, THE OLYMPIAN

THURSTON COUNTY -- The mysterious floating mats that cropped up at Offut Lake after the Nisqually Earthquake might not be so mysterious after all.

The gaseous masses at the lake's east end are in the same area that a herbicide was sprayed last year to kill water lilies, said Jim Bachmeier, Thurston County manager of utility development and special services.

Bachmeier said it's common for the vegetation, once it starts to decompose, to resurface as floating mats during the next spring or summer.

"It's possible the earthquake accelerated what would have occurred naturally," Bachmeier said.

Other South Sound lake residents, including Eugene W. Smith of Munn Lake, said he's seen similar floating mats near his residence during past years.

State Department of Natural Resources geologist Pat Pringle said Bachmeier's explanation makes sense.

However, Pringle said he also witnessed smaller, muddy mounds that rose up a foot or so above the lake surface after the earthquake.

The mounds emitted streams of gas composed of methane and carbon dioxide when they were prodded.

Pringle said the floating mats and volcano-like mounds were the result of bottom sediments settling and liquefying during the earthquake.

Bachmeier notified the Thurston County Commission earlier this week that his office received calls after the earthquake of new "islands" appearing on Offut Lake.

A site visit by county officials, coupled with a review of herbicide applications from the state Department of Ecology, suggested the islands were sediment-covered root masses of previously treated water lilies, Bachmeier said.

Typically, the mats settle back to the lake bottom, or to the shoreline where they decompose, Bachmeier said.

They pose a minimal public safety problem, but could be a temporary nuisance to boaters or lakefront property owners, Bachmeier said.

On the web:

Washingtonlakes.com.

Earthquake links.

Earthquake stories archive.

The Olympian Copyright 2001

back to main Earthquake Stories index