A glacial outwash plain is a field of alluvial sediment deposited by the flow of glacial meltwater. Other types of outwash plains can form under different circumstances, but glacial outwash plains tend to be located downhill of glaciers that experience partial melting during warm seasons.
Glaciers form when snow deposited on the ground fails to melt completely and builds up to form a large mass of compacted ice. Because of the way they form, glaciers contain much more than pure ice. Dust, windblown debris and spillage from nearby landslides is incorporated into the glacier as fresh snow falls on it each winter. The glacier picks up still more debris as it slides downhill.
This acquired material tends to be broken down by the crushing force of the mass of ice until the glacier is rich with fine sediment mixed with ice. When the glacier melts, the water it releases carries much of this debris downhill with it. As the meltwater flows, it loses energy and drops the large stones it carries. Farther downstream, the water flows slowly enough to deposit fine sediment that can build over time to cover a large area. This field of sediment is the glacial outwash plain.