Changes in Ice-Flow Velocity and Surface Elevation from 1874 to 2006 in Rhonegletscher, Switzerland

We have studied changes in the ice-flow velocity and ice thickness in Rhonegletscher, Switzerland, over the period 1874-2006. The flow velocity field and surface elevation were analyzed in the lower half of the glacier using aerial photograph pairs taken in 1970/ 1971, 1981/1982, 1999/2000, and 2005/2006. We also digitized velocities measured by Mercanton (1916) in 1874-1910 by tracking stones distributed on the glacier. The results showed that the ice-flow velocity and ice straining conditions were strongly influenced by changes in the glacier geometry over the last 100 years. For example, the longitudinal strain rate near the current terminus has changed from tensile to compressive since the retreat of the glacier over a steep bedrock slope to a relatively flat region. The velocity decreased over the studied region from 1981 to 2006, which is in agreement with the ice thinning during the same period. However, the rate of the velocity change was smaller in the post-1990 period, because the effect of the thinning on ice flow speed was partly canceled out by the effect of steepening of the ice surface. The velocity change also implied that the magnitude of basal ice motion was influenced by changing subglacial drainage conditions and proglacial lake formation. Our unique data set contributes to a better understanding of ice dynamics under changing glacier geometry.