One major concern to people living downstream of Mount St. Helens is a breakout of any of these impounded lakes due to the instability of the debris dams blocking them. Flood waters from a breakout could be more catastrophic than the lahars of May 18, 1980. Gages, such as this Early Warning Gage on Coldwater Lake, have been installed at lakes and streams surrounding the volcano. These gages continuously monitor changes in water levels. Major increases or decreases in levels trigger warnings which are telemetered to the U. S. Geological Survey in Vancouver.
Spirit Lake is in middle right.
About 2,500 years ago during the Pine Creek eruptive period, a series of very large lahars traveled down the Toutle River from Mount St. Helens. The level of Silver Lake is now controlled by a dam. View is West-Southwest. Mount Shasta is on the skyline.