St Helens Survey 2025

Mt St Helens Survey 2025

Eric Gilbertson

Results: St Helens has shrunk by 0.4ft since Sept 2024. Current elevation is 8,320.7ft +/-0.1ft NGVD29 (datum summit elevations have historically been reported in).

Results recognized by the peer-reviewed scientific journal Progress in Physical Geography (accepted for publication and will be online soon)

St Helens has dropped from number 101 to number 102 now on the WA highest peaks list.

I’ve been resurveying Mt St Helens each fall for the past few years to track its elevation change over time. In 2023 I discovered that it had lost so much elevation that it was no longer one of the highest 100 peaks in Washington. It had been losing elevation at a rate of 0.3ft per year very linearly (R squared value 0.98) since 1989, and left the top 100 list around 2021. In 2024 it was tied for number 101 with Switchback, the same elevation to the nearest inch based on dGNSS surveys I conducted.

I survey in the fall in September or October because I suspect the elevation change is due to erosion, and the erosion likely stops once the summit gets frozen in snow and ice. This generally happens by mid October. I am setting a precedent of surveying this time of year so I get consistent measurement dates each year going forwards, at the lowest-elevation time of year. Past measurements in 1989 were with a trigonometric survey, and several Lidar measurements were done in 2002, 2009, 2017, and 2018. These are generally accurate to about +/-1ft, while dGNSS measurements are accurate to +/-0.1ft. As far as I know my dGNSS measurements in 2023, 2024, and 2025 are the only ones that have been taken of the summit.

This year it looked like snow starting early October, so I got a last minute permit for the weekend, perhaps the last snow-free weekend of the year. I decided to go to the summit for sunset when the views would be good and nobody would bother the equipment set up. I planned to do a two-hour survey with a standard 2.0m antenna rod and tripod using a Trimbel DA2 dGNSS device.

I got to the trailhead by 4:30pm Saturday and was soon hiking up. After a few miles I passed the last person descending and soon had the mountain to myself. I reached the summit by 7pm, just when lighting was getting good for sunset. The highest location looked different than it had in 2023 and 2024, as I verified with pictures of my surveys from then. This indicated the summit might have eroded down.

I used my Abney level to identify the highest rock, and had the receiver mounted by 7:15pm. I then hung out taking pictures and enjoying the view. I planned to take a 2-hour measurement because this pretty much guarantees 0.1ft vertical accuracy, and is the standard time for blue-booking static survey measurements.

By 9:30pm I started packing up, and reached the trailhead a bit after midnight.

Results: St Helens has shrunk by 0.4ft compared to Sept 23, 2024. It is now 8,320.7ft +/-0.1ft NGVD29. I’m reporting the elevation in the NGVD29 vertical datum since that is how historical elevations of St Helens have been reported, so this allows for fair comparison over time. The elevation loss is likely due to erosion on the steep north face, and may be exacerbated by cornice collapse in the spring.

The summit location moved horizontally by 1.5ft to the SE.

Note: USGS sources still use outdated elevations for St Helens,  and mix up or don’t mention datums the numbers are reported in. Datums differ by 4.5ft vertical on St Helens, so this is significant. For instance, https://volcanoes.usgs.gov/observatories/cvo/Historical/LewisClark/Info/summary_mount_st_helens.shtml  still uses the height 8,364ft, which was from 1982 in NGVD29 datum and is a very outdated elevation. (St Helens has eroded down 43.3ft since then).

The source https://www.usgs.gov/volcanoes/mount-st.-helens quotes the 1982 post-eruption height as 8,365ft , then cites the 2009 Lidar survey height of 8330ft as the most recent elevation. The source does not mention, though, that these are in different datums, so cannot be fairly compared (8365ft is in NGVD29, and 8330ft is in NAVD88). Also, the 2009 measurement is now very outdated.

In summary, St Helens has eroded down 43.3ft since the 1982 8364ft post-eruption measurement. It’s current height is 8320.7ft NGVD29, and it has shifted from 101 to 102 on the list of highest peaks in WA.

(For reference, this height converted to the current standard datum, NAVD88, is 8,325.2ft)

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