One of the greatest concerns during a future eruption from Mount Jefferson is the possibility of large lahars entering reservoirs on either side of the volcano, namely Detroit Lake to the west and Lake Billy Chinook to the east. Such lahars entering these lakes could possibly cause large waves or rising water to overtop the dams causing them to fail. Such a scenario would have catastrophic effects downstream. Smaller lahars (volumes much less than reservoir volume) would likely result in a slow rise in lake level, similar to what happened when a lahar entered the Swift Reservoir near Mount St. Helens in 1980.
Another way to generate lahars, both during eruptions and during periods of quiescence is by failure of glacial moraine dams that impound high-altitude lakes. Such failures have occurred recently in the past at Mount Jefferson, which could happen again in the future because there are many such lakes on the volcano. The sudden release of floodwaters from one of these lakes could move quickly down valley and mix with loose volcanic rock to form a lahar.