Small glacial outburst flood occurs on Mount Rainier - October 27, 2012
Contact: Scott Beason, Park Geologist, 360-569-6781
A small glacial outburst flood occurred at Mount Rainier National Park on Saturday, October 27, 2012 at approximately 9:00 pm. This event, while significant, was localized in scale, caused no damage to park facilities, and was not volcanic in origin. It likely originated from the Nisqually Glacier as result of moderate to intense rainfall. A stream gage located at Longmire registered a 2.8 foot (0.85 meter) rise in river water level (Figure 1) between 8:30 and 9:45 pm and the spike was seen on other gages downstream of the park. Field data (Figure 2) concurs with the stream gage and shows evidence of a several-foot surge of water in the Nisqually River.
While this event was small, it highlights one of the many hazards at Mount Rainier. It also shows that potentially destructive and hazardous events can occur even during "small" storms that are common in the fall and winter at Mount Rainier. Workers and visitors near rivers should always be aware of geologic hazards. Remember, if you feel the ground shaking or hear a freight train coming down the valley, get uphill as quickly as you can!
Where did this occur and when did it happen?
How did it form?
In the past, outburst floods have occurred during warm, dry periods in the summer (with usually high melt) and during fall storms. Rainfall intensity during the storm was likely the driver of this event. Precipitation intensity in the hours before the flood passed Longmire was in the 0.40 inch/hour range. This event occurred during a relatively small storm, compared to the size of storms the park has experienced in the recent past. Careful monitoring of rivers is crucial to detecting and understanding glacial outburst floods. This is the 1st step to prediction future events, a goal of the Geology Program.
What were the effects?
It is also possible that the lower Nisqually Glacier has changed. Field visits on Sunday to the Nisqually Vista Trail revealed a dark streak on the surface of the lower glacier that may be from a stream on top of glacial ice (a supraglacial stream). When the weather permits, we will investigate further.
How do we know it happened?
Several field visits occurred after the event and found "trim lines" in river sediment (Figure 2), evidence of the maximum height of water.
NPS, Laura Walkup
Do I need to be concerned?
It is important to note that we do not believe the glacial outburst flood was caused by a geothermal or volcanic event, however it underscores that caution is needed when near the river. This event could have been completely unnoticed if stream monitoring equipment had not been in place.
In the recent past, much larger glacial outburst floods have occurred in the park. Large glacial outburst floods occurred in the 1950s and caused flooding in Longmire on at least two occasions. Because of this, and the potential for future events in the park, we all need to be aware of volcanic and geologic hazards at Mount Rainier. If you are near a river and notice a rapid rise in water level, feel a prolonged shaking of the ground, and/or hear a roaring sound coming from up valley- often described as the sound made by a fast-moving freight train- move quickly to higher ground! A location 160 feet (50 meters) or more above river level should be safe.
Who do I contact for more information?