Fluvioglacial landforms are landforms molded by glacial meltwater. This discharge of glacial streams, both over the surface (supraglacial) and beneath the ice sheet (subglacial), is higher in the warmer summer months. As subglacial water often flows under pressure, it has a high velocity and is very turbulent. This high velocity enables it to transport a large amount of material which would not normally be transported in a regular stream of similar size; boulders thus transported may remain as glacial erratics in the post-glacial terrain. As this material is transported it makes contact with the underside of a glacier and erodes vertically from below by abrasion. This erosion creates subglacial valleys. When there is a decrease in discharge of the glacial steams, deposition occurs, and is responsible for large groupings of landforms like eskers and drumlins in glaciated areas.
More active massive scouring occurs when ice dams of pro-glacial lakes fail catastrophically in a glacial lake outburst flood; the results can be seen, for example in the Channeled Scablands in the U.S. state of Washington, created by the cataclysmic Missoula Floods that swept periodically across eastern Washington and down the Columbia River Plateau during the Pleistocene epoch. The Shonkin Sag along the northern edge of the Highwood Mountains, Montana, is a channel formed by the Missouri River and glacial meltwater pouring from Glacial Lake Great Falls; it is one of the most famous prehistoric meltwater channels in the world.
- Axline, Jon and Bradshaw, Glenda Clay. Montana's Historical Highway Markers. (Helena, MT: Montana Historical Society), rev. ed. 2008:91.