Subsequent reworking of this sediment may lead to further channel aggradation, and aggravate overbank flooding during high river stages. Some smaller lahars may be triggered by the sudden release of water from cavities within or beneath the ice. Because outburst floods are unpredictable, you should be alert when visiting valleys with glacier-fed streams, particularly on unusually hot or rainy days. If you are near a stream and hear a roaring sound coming from upvalley, or note a rapid rise in water level, move quickly up the stream embankment, away from the stream channel and to higher ground. Outburst floods and the debris flows they often trigger pose a serious hazard in river valleys on the volcano.
Glacial outburst floods at Mount Rainier are unrelated to volcanic activity.
The best-studied outbursts those from South Tahoma Glacier are correlated with periods of unusually high temperatures or unusually heavy rain in summer or early autumn. The exact timing of outbursts is unpredictable, however. Commonly these sediment-water flows achieve discharges and velocities that produce catastrophic consequences more than 100 kilometers downstream from source areas. Given the current, relatively quiet, eruptive behavior of Mount St. Helens , debris flows and floods at present constitute the greatest hazards related to volcanic activity. The potential for mudflows and floods was increased by the existence of new ponds and lakes formed when the debris avalanche of May 1980 blocked parts of the preexisting drainage to serve as natural dams.