Methods

Fourteen plastic (PVC) stakes were drilled into the ice to measure surface displacement and ice ablation (Figure 1). Only one stake was drilled into clean ice and the remaining were covered with less than 90 cm of rock debris. Ablation was measured on a near-daily basis during the summer between August 13th and September 24th, 2004, and displacement was measured weekly. All stakes were measured again for ablation and displacement a year later on July 28th, 2005. Seven boulders were also surveyed for movement. The transverse elevation profiles (Dodge, 1964) were also remeasured. Although only one of the four end points was still marked, the missing endpoints were reestablished and, we believe, close to the original locations (Jackson, 2007). The profiles and stake locations were surveyed using a conventional total station. Additionally, we conducted a radar survey of glacier depth within the debris-covered area.

Area and length changes on seven of Mount Hood’s glaciers were compiled in a geographic information system (GIS) based on maps and historic terrestrial and aerial photographs. These sources date from 1901 to 2004. We calculated buffers around the glacier perimeters to define the uncertainty in area. Root mean square errors (RMSE) for georeferenced aerial photographs range from 3.6 to 9.6 m while buffers were defined at 20 m for ground-based photographs and 15 m for oblique aerial photographs. Details of the field and analytical methods can be found in Jackson (2007).