January 16, 2013
Riggs Glacier for most of the 20th century was a tributary to the Muir Glacier in Glacier Bay Alaska. The two glaciers separated between 1960 and 1963 as noted by the USGS in their detailed research led by Bruce Molnia . The USGS chronicled the changes of this glacier from 1940 to 2004 with pictures from the same location by Bruce Molnia. The Muir, McBride and Riggs are still joined in 1950 and the Riggs Glacier is retreating out of sight into its own valley by 2004. McBride Glacier has had a faster retreat in the last decade.
Dan Lawson in a 2004 National Park Service brochure noted that Riggs Glacier was about 1.2 km wide at the terminus, had a 40 to 90 feet high ice face and was 15 miles long. The terminus that had been calving into Muir Inlet for the entire 20th century became mostly terrestrial during the mid-1980,s as an outwash delta built at the southern margin. In 2004 tidewater only reached the terminus during higher tides on either side of the bedrock knob that separates the glacier into two ice tongues. Riggs Glacier has been thinning progressively over the past two decades and is expected to continue thinning and slowly receding. Ice recession has been averaging about 20 to 30 feet per year for the last 5 years. This post examines changes as seen in Landsat images from 1990 to 2011.
In 1990 the two terminus tongues are still connected at the black arrow, and a prominent knob is just being exposed by retreat of the western terminus, red arrow. The terminus still reaches tidewater on the very northern margin. There is no vegetation evident on the knob separating the ice tongues. A tributary from the east at the orange arrow just reaches the Riggs Glacier. By 1999 the termini are separated the prominent knob is now fully exposed by retreat of the western terminus and the glacier is no longer tidewater except briefly at high tide. Vegetation has become to develop on the rock knob that separates the two ice tongues. By 2009 the glacier has retreated well back from tidewater. The delta building in front of the eastern terminus has expanded into the inlet some. Vegetation has spread over most of the rock knob that separates the two ice tongues. In 2010 in a late summer images the snowline of the glacier is at 975 m. The tributary from the east at the orange arrow has retreated 1 km from the Riggs Glacier. By 2011 the western terminus has retreated 600 meters since 1990 and the eastern terminus has retreated 800 meters from the black arrow where it used to connect to the main glacier. Examination of the snowline elevation on Riggs Glacier indicates that it now typically it is typically between 950 and 1050 m. This recent increase in the snowline is similar to the increase seen on Brady Glacier. This has led to the glacier retreat and it no longer reaching the sea.