Just off the shore near Lake Sammamish’s Greenwood Point, some 70 snags protrude from the water. It is not extraordinary to see snags in western Washington’s lakes, until you discover that these particular snags are over 1000 years old! This is the site of a large prehistoric landslide that sent an ancient forest into the waters of Lake Sammamish and drowned it.
Long before this landslide occurred, retreating glaciers deposited alluvium on Greenwood Point at a relatively rapid pace. This left a steep and unstable delta front, primed for failure during seismic activity. Evidence throughout the Puget Sound region points to a large earthquake along the Seattle Fault between 1000-1100 years ago, and radio carbon tests on the snags here date to the same time.
The Sunken Forest extends for about a half mile, divided into two clusters. The two clusters indicate that either there were two separate landslides, or that some of the snags have disappeared between the two clusters. The morphology of the adjacent shoreline consists of two small bays, which could be the headwall scarps left behind when the landslides carried these ancient trees into the lake from Greenwood Point.
What you see here at water level are the higher elevations of the drowned trees. Some of these trees are reported to be six feet in diameter where they sprout from the bottom of the lake. Imagine how large they must be! The portions exposed to the air have rotted, but the lake water has preserved the submerged portions. Many of the snags point shoreward, possibly thrown into that orientation as the landslide settled.
Lake Washington also has/had three sunken forests dating to the same time as the one in Lake Sammamish, and were likely created by the same earthquake. Two of them were off the shore of Mercer Island. These forests were first discovered in 1916 when Lake Washington was lowered during the construction of the ship canal. The trees presented a navigation hazard so in 1919 workers removed over 200 sunken trees and blew off the top 30 feet of the remaining trees with dynamite. Reportedly, the 1000 year-old trees had been so well preserved that they were used as timber. More recently, the Lake Washington trees were illegally harvested as timber by a log salvaging operation during 1991-1992. The owner of the salvaging company was found guilty of three counts of first degree theft, two counts of first-degree trafficking in stolen property and single counts of second degree trafficking in stolen property, filing a false document to obtain a log-patrol license and illegal use of proceeds from criminal profiteering.
To log this cache:
• Paddle or pilot your craft to these ancient trees. Now that you know how old and special they are, please show respect and do not molest them or moor your craft to them.
• Make an estimate of the exposed diameter of the biggest snag you can find and send that estimate to me. Do not post your estimate in your log.
• Take a photograph of yourself with one of the snags. If you are alone and this is not possible, take a photograph of your GPS unit or your watercraft with one of the snags. Post your photograph and tell about your experience in your log.
Read the logging requirements carefully! Logs that don't follow the instructions will be deleted.
Article about large Puget Sound earthquake
Article about Sunken Forest
July 11, 2012 article about a massive tsunami from the ancient quake