A 1984 strain net on the Snowdome of Blue Glacier showed that the surface slope is a good estimator of ice flow direction and divide location. Topographic maps from 1939, 1952, 1957, 1979, and 1984 show that the flow divide migrates within a zone up to 350 m wide, in response to changes in east-west gradient in snowfall. This zone encloses 6% of the Blue Glacier accumulation area and up to 10% of the year-end residual snow. An ongoing 28-year mass-balance study has used an extreme, westerly divide, giving systematically high net balance estimates. The correct catchment area, for a given balance year calculation, depends on the future migration sequence of the ice divide, with a time constant of about 30 years.