SURFICIAL ICE VELOCITIES OF THE LOWER NISQUALLY GLACIER AND THEIR RELATIONSHIP TO OUTBURST FLOOD HAZARDS AT MOUNT RAINIER NATIONAL PARK, WASHINGTON, UNITED STATES

Mount Rainier, Washington is the tallest of a number of glacier-clad volcanic peaks within the Cascade Range. Standing at 4,392 m (14,411 ft) and having 25 named glaciers, it contains more glacial ice than any other single peak in the continental United States. Glaciers are the headwaters for a number of rivers emanating from Mount Rainier, many of which flow to the Puget Sound, approximately 60 km (37 mi) away. Glaciers on Mount Rainier have unleashed damaging outburst floods (jökulhlaups) in the past, often without warning. Previous observations have indicated an empiric link between changing glacial velocities and glacial outburst flood hazards. Specifically, the lower portions of the glaciers were often stagnant when outburst floods occurred. Additionally, the current Nisqually glacier extent is similar to glacial extent in the 1950s when an outburst flood impacted Longmire, one of the visitor destinations and work areas in the park.

This study explores whether the surficial velocity field of the Nisqually glacier is changing and if ice velocity changes can be used to indicate glacial outburst flood potential. Multiple sites on the lower Nisqually glacier were surveyed weekly during the summer/fall of 2011 and 2012. The surficial velocity field of the lower 1 km² (0.4 mi²) of the glacier was calculated from repeated measurements. Between 2011 and 2012, observed velocities in the upper portions of the study area accelerated while the lower portions of the glacier slowed or remained the same. Shortly after the on-glacier study concluded in October 2012, we observed a short-lived anomalous 1 m (3 ft) rise in river stage at Longmire during a rainstorm on the upper mountain. Field evidence corroborated the stage record and indicates a small outburst flood (approximately 15 m³/s (525 ft³/s)) was released from the glacier during this rain event.

Our work contributes to the understanding of glacial outburst floods and provides an additional line of evidence that changing ice velocities precede outburst floods. Further study of glacial velocity fields could provide a predictive methodology for glacial outburst floods in similar terrain. These results are critical for employee, visitor, and infrastructure protection in dynamic environments such as that at Mount Rainier.