One major concern to people living downstream of Mount St. Helens was a breakout of any of the impounded lakes, such as Coldwater or Castle 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, were installed at lakes and streams surrounding the volcano. These gages continuously monitored changes in water levels. Major increases or decreases in levels trigger warnings which are telemetered to the Geological Survey's Cascades Volcano Observatory in Vancouver.
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