Maximum speeds occur near the surface and along the centerline of the glacier. Flow rates are generally greater in summer than in winter, probably due to the presence of large quantities of meltwater at the glacier base. The position of the snout, or terminus, of a glacier may change as the relative quantities of snowfall and glacier melt change. If summer melt exceeds winter snowfall, the terminus retreats, whereas if snowfall exceeds summer melt, the terminus advances. These changes in terminus position do not occur instantaneously, but typically take several years or more to become apparent.
Glaciers are therefore sensitive indicators of climate changes.
Consequently, alternative data, which are obtained by mapping of terminus positions and surveying of glacier surface elevations, are commonly used. Changes in terminus position may actually be forecast by precise surveys of a glacier's surface elevation. For example, a rise in surface elevation, which reflects an increase in ice thickness, is typically followed within a few years or decades by terminus advance. The surface-elevation record at Nisqually Glacier is the lengthiest of any made in North America. The record, which was started in 1931, shows the glacier's dramatic responses to about half a century of small but significant climatic variations.