Mount St Helens Threw a Forest Into This Lake It's Still Floating

On 18 May 1980 Mount St. Helens took its north face off. The landslide that followed was the largest in recorded history, and part of it went straight into Spirit Lake. The water was driven out of the basin and up the far slopes, and when it came back down it carried the flattened forest with it. Forty-six years later that forest is still on the surface. When the wind packs it tightly it covers about a fifth of the lake — roughly 2.2 square kilometres of floating wood on a lake of about 11. It is not anchored to anything. It moves. This is what the raft is made of, why so much of it is still up there, what it has done to the lake underneath it, and why the US Forest Service is now proposing to bore a second tunnel through a ridge to keep the water where it is. No speculation, no dramatisation. Every figure in this film comes from a published source, and the ones people usually get wrong are flagged below. CHAPTERS 0:00 A forest floating on a lake 1:16 Spirit Lake before 1980 1:35 The north flank fails 2:16 How the trees got in 3:02 Watching the raft from orbit 3:23 Where the wind takes it 4:11 So why is it still floating? 5:39 This has happened before 5:57 The logs that did sink 6:44 Two cores of mud 8:05 The underside of the raft is alive 9:01 The raft finds the drain 9:33 The lake with no natural outlet 10:22 1985: a tunnel through Harry's Ridge 11:06 The tunnel is being squeezed 11:49 What the modelling does and does not say 13:02 July 2026: a plan for a second tunnel 13:41 Why access is restricted 14:24 Go back to the water THE NUMBERS, AND WHERE THEY COME FROM About a fifth of the surface when the wind packs the raft tight — measured log by log from 2017 satellite imagery and published by limnologists. Roughly 40 per cent in the first decade. The avalanche deposit raised the water surface by about 210 feet. Mean depth fell from 37 m to 23 m while surface area rose from about 5 km² to about 11 — a 108 per cent increase in area and a 30 per cent increase in volume. The old natural outlet at the north-west corner was buried under something like 276 feet of rock and shattered forest. By the summer of 1982 the lake had risen 54 feet above where the avalanche left it. The 1985 tunnel through Harry's Ridge holds the lake at about 3,440 feet of elevation. It was designed for 550 cubic feet per second and currently averages around 300. Two sediment cores taken in 2018 came up about 17 cm long. The lower layers accumulated rapidly in the months after the eruption; the upper sediments have accumulated much more slowly, preserving a record of the lake's recovery. Modelled failure of the debris blockage produces flow depths of about 60 feet at Castle Rock and 30 to 40 feet at towns farther downstream. These are modelled scenarios for a failure, not a forecast that one is coming. Two things this film deliberately does not say: that the logs are a mystery nobody can explain, and that the blockage is about to fail. Neither is supported by the literature. SOURCES Major, Grant, Sweeney & Mosbrucker (2020), "A Multidecade Analysis of Fluvial Geomorphic Evolution of the Spirit Lake Blockage, Mount St. Helens, Washington" — USGS Scientific Investigations Report 2020-5027 US Forest Service, Pacific Northwest Research Station — Spirit Lake outflow research and options assessment US Forest Service draft decision, 7 July 2026 — Spirit Lake long-term outflow National Academies of Sciences, Engineering, and Medicine (2018) — review of Spirit Lake outlet alternatives Peer-reviewed limnology of the log raft, the lake's recovery and the biofilm on the submerged wood USGS Cascades Volcano Observatory NASA Earth Observatory / USGS Landsat FOOTAGE AND IMAGES 1980 field footage: US Geological Survey, "Historical video footage of Mount St. Helens volcano, Washington, 1980-1982", DOI 10.5066/P94E623Y — U.S. Government work, no use constraints Archival film: US National Archives and Records Administration Photographs: US Geological Survey and USDA Forest Service, including Pacific Northwest Research Station and Gifford Pinchot National Forest Satellite imagery: NASA Earth Observatory / USGS Landsat Maps and figures: USGS SIR 2020-5027 All visual material used is Public Domain, CC0, or U.S. Government work unless otherwise credited. MUSIC YouTube Audio Library — Density & Time. If this is the kind of thing you want more of, subscribe. And if you know this lake, have visited Spirit Lake, or have worked around Mount St. Helens, tell me what I got wrong in the comments. Corrections get read, and they shape what gets made next.