Tweedsmuir Glacier surface elevations were measured on November 5, 2007, October 10, 2007, August 26, 2007, August 29, 2006 and June 6, 2001 with the UAF Geophysical Institute Laser Altimeter. The graphs below show surface elevation changes along the center of the glacier, comparing the various laser profiles and also comparing the Aug 2006 Laser data with the Feb 2000 Shuttle Radar Topography Mission DEM (SRTM). There have been no corrections for the seasonal differences caused by snow depth in these plots.

This first graph shows the elevation changes between the three laser altimeter profiles. The black data represents the pre-surge elevation changes from 2001 to 2006. Strong thinning is observed at the lower reaches, with some minor accumulation occurring at the higher elevations.

The blue data show the effect of the surge between 2006 and 2007. Ice from up high deflates while the lower glacier builds up. The "hinge line" is about 750 m elevation.

The red data show the continued evolution of the surge. The upper reaches continue to deflate while the lower glacier continues to build.

This graph shows the change in elevation between the two flights this fall, August 26 to October 10, 2007. Again, both profiles are very noisy due to the massive crevassing that is caused by the surge, and some of the elevation differences shown here are due to the presence of a crevasse in one profile but not the other.

The overall signal shows that the surge was perhaps more strongly active and ongoing than later in the fall (above).

This graph shows the rate of elevation change (vs. elevation) over the three most recent time periods between profiles.

When plotted this way, the most recent time period (blue) actually looks very active, just not so much in the lower reaches of the glacier.

This seems a little different from the idea that a surge propagates down glacier as a surge front. I was expecting to see progressively larger changes lower on the glacier as the surge developed, but instead we are seeing an increase in the rates of elevation changes from 500 m up all the way to near the head of the glacier. Snow accumulation accounts for some of this, but that effect would be greatest at the very top of the profiles where we see instead the lines converging again above ~1350 m.

This graph shows the rate of elevation change over some of the other time periods.

Note how slow the rate of change is during the non-surge period of 2001-2006 (blue).

This graphs shows the use of the SRTM as a baseline surface, and compares the laser to laser (Aug 2006-June 2001) elevation changes (red) with the laser to SRTM (Aug 2006-Feb 2001) elevation changes (blue) along the same centerline ground track

There is a cluster of positive elevation changes at low elevations: This is where a loop moraine moved down glacier and made an apparent elevation increase that doesn't represent an actual volume increase. The range between 200 and 400 meters elevation has a number of these loop moraines leading to the noisy response there.