On March 30, 1806, the Lewis and Clark spotted and named Mount Jefferson, after the then current President of the United States who had funded their expedition. " ... we made 22 Miles only to day the wind and a Strong current being against us all day, with rain. discovered a high mountain S E. Covered with Snow which we call Mt. Jefferson...."
Mt. Jefferson (10,495 feet) last erupted 15,000 years ago and has an eruption history dating back hundreds of thousands of years.
It's still considered active and capable of large explosive eruptions.