Spirit Lake was covered with logs as a result of the Mount St Helen’s eruption in 1980.
There is a process going on where the logs sink upright and petrify, so there’s a petrified forest building up on the lake bottom, but there are still, 30 years on, logs floating on the surface of the lake.
How long will it take until all the logs are gone?
While the water quality situation is quite different, I think The Old Man of the Lake in Crater Lake might provide a hint. Over 100 hundred years of floating around and still going strong.
Things that would affect the Spirit Lake logs differently: The water is more acid and turbid. The acidity might hinder growths that break down the logs, but also speed up direct break down. Also, Crater Lake is colder and has no significant input streams which would stir things up. The flow thru Spirit Lake probably brings in things which help decomposition.
When they sink, are the logs ending up in an upright position in the lake floor? (i.e. are we seeing polystrate tree fossils in the making?)
Yep. I just read the Geology article cited in the wikipedia page (which doesn’t mention how long the logs are going to stick around on the surface), and the gist of it is that before the eruption they didn’t really know how you ended up with things like the petrified forest in Yellowstone where all the trees were vertical. Now there’s just such a “forest” forming at the bottom of Spirit Lake.
Yes - the National Geographic article I was reading explained that since the root end of the tree is heavier, they tend to fall into an upright position, and then the petrification process starts.
The man-made lake I live on down here was created and flooded in the late 1930’s, early 1940’s. At the time, even though some 250,000,000 board feet of timber was harvested, probably twice that amount remained standing (actually, in the typical wisdom of the designers, they attached steel cables, and pulled many of the standing trees in an arc, and affixed them to the lake bed).
As of this date, some 70 years later, thousands of dead trees are back to standing, and the ones that have decayed enough to break off typically float nearly vertical, with the tops just barely submerged. Makes for interesting navigation to say the least. I think more people have lost props and engine feet on this lake then just about anywhere else.