As snow accumulates and is buried, it is gradually transformed into glacial ice. This ice has little or no trapped air and spaces between ice crystals are reduced. Under tremendous pressure, it has the ability to flow becoming a glacier. The Mendenhall Glacier flows for 12 miles down the Mendenhall Valley to its terminus near the visitor center. The ice flows forward at an average rate of 2 feet per day, but at the very same time, it wastes away at a slightly faster rate.
Wastage occurs through melting or by large pieces of ice breaking off the face of the glacier.
When the rate of melting exceeds the rate of flow,, a glacier recedes. The Mendenhall Glacier has been receding since the late 1700's and currently retreats at a rate of 25-30 feet per year. When the rate of flow exceeds the rate of melting, a glacier advances. For the Mendenhall Glacier to begin advancing again, either more snow needs to accumulate in the icefield, or melting at the face of the glacier needs to decrease. A minor change in climate can bring about either change.