when did mt st helens erupt before 1980
On April 17, 1857, the "Republican", reported that "Mount St. Helens, or some other mount to the southward, is seen to be in a state of eruption".
Mount St. Helens
On April 17, 1857, the "Republican", a Steilacoom, Washington, newspaper, reported that "Mount St. Helens, or some other mount to the southward, is seen ... to be in a state of eruption". 
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Mount St. Helens [SEP] European colonization and use of the area
The lack of a significant ash layer associated with this event indicates that it was a small eruption. This was the first reported volcanic activity since 1854. Before the 1980 eruption, Spirit Lake offered year-round recreational activities. In the summer there was boating, swimming, and camping, while in the winter there was skiing.