para&proglacial

Proglacial environments and paraglacial activities are found at or just beyond glacial ice margins. Landforms are considered proglacial if they are proximal to ice margins. Paraglacial activities occur as the result of glacial retreat but they are usually independent of ice. Pluvial lakes are ancient lakes of great size that formed in closed basins (like the bolsons of the American Southwest) as a consequence of glacial climates and the surplus of available moisture over evaporation and transpiration.

fig 1 Map showing proglacial Lake Missoula (black), the Cordilleran Ice Sheet (blue), and the Spokane Floods (red)

The proglacial environment encompasses all features next to ice margins. A common proglacial feature at both ice sheet
and alpine glacier margins is a lake. Proglacial lakes are created when glacial meltwater is dammed by some sort of barrier, commonly terminal or lateral moraines. Another type of proglacial lake, an ice-dammed lake, forms if an ice mass blocks a river or stream valley. Ice-dammed lakes (like Lake Missoula) are notoriously unstable because the ice can float if the trapped water gets deep enough, thus can generate huge floods (above). Outwash channels, covered in depth in the Meltwater section, are also a major feature of proglacial environments.

fig 2 Glacial Lake Missoula shorelines above Missoula, Montana (photo courtesy of William Locke)

Some of the largest lakes in the earth's history - for example, Glacial Lake Agassiz - have been ice-dammed lakes. Glacial Lake Missoula (fig. 1) in western Montana is one of the most famous examples. It formed when the Cordilleran Ice Sheet dammed the Clark Fork River downstream of the present day city of Missoula. Glaciologists have speculated that Glacial Lake Missoula filled up and catastrophically drained as many as 40 times. At the lake's maximum size, it would have been roughly the size of Lake Ontario, containing 2500 cubic kilometers of water. Though the ice-dam would fail before the water was ever deep enough to flow over the top, the lake would have reached a maximum depth of around 600 meters. These high magnitude floods caused by glaciers have been given the Icelandic name jokulhlaup. Jokulhlaups are not always caused by ice-dam failure, but in general they are unpredictable and can cause much destruction. Evidence of the fluctuating lake level still exists on valley walls above and around Missoula, MT (fig 2.) Shorelines from the ancient lake mark the hillsides in a series of parallel, horizontal bands.

fig 3 The Columbia River Gorge, Washington. The entire gorge would have been filled with water during the floods (photo courtesy of William Locke)

The water draining out of Lake Missoula, commonly known as the Spokane Floods, rushed down the Clark Fork River, flowed into the Columbia River in central Washington, and eventually emptied into the Pacific Ocean (fig 1). In eastern and central Washington, where the Spokane Floods crossed the Columbia Plateau, the water spread out and formed a large, hummocky surficial region known as the Channeled Scablands. Viewed from above , the scablands clearly show the marks of enormous river channels. Waterfalls larger than Niagara Falls formed where the floods plunged over basaltic cliffs. A satellite image of Washington State shows the scale of the floods' destruction across all of eastern Washington. The yellow area on the SE corner of the image is the loess-capped Palouse region - the gray areas are the Channeled Scablands. Take a look at the USGS page on Glacial Lake Missoula.
Many other proglacial lakes existed across North America at the margins of the Cordilleran and Laurentide Ice Sheets. During the most recent ice age, the Great Lakes (satellite image) were, for a time, proglacial lakes. Another enormous lake that even made the Great Lakes look small, called Glacial Lake Agassiz, spread over 2 million square kilometers of North America. It was formed by the Laurentide Ice Sheet damming of rivers that flowed northward into Hudson Bay.

Glaciers have the ability to severely alter the earth's surface. The U-shaped valleys that are left by glaciers (fig 4) are much different in size and shape than preglacial V-shaped valleys. Once deglaciation begins, the valley environments begin a slow return to their preglacial states. This is activity is called paraglacial because it is independent of, but related to glaciers. The paraglacial environment consists of changes in three major systems: gravitational, fluvial and aeolian.

fig 4 The Lauterbrunnen Valley, Switzerland returning to its preglacial state (Photo courtesy of Michael Gartland)

Mass Wasting. Glacially carved valleys have steep, high walls and broad, flat bottoms (fig 4). Unstable by nature's standards, these walls are influenced by gravitational forces, or mass wasting. The slope of the wall depends on the material through which the glacier cut; in bedrock it is not uncommon to have slope angles of 90 degrees. No matter the angle of the slope, it is usually far beyond the angle of repose. In order to return to preglacial conditions, at or below the angle of repose, the valley walls often produce rockfalls or landslides, and become mantled with talus - debris at the angle of repose.

Fluvial. The fluvial aspects of the paraglacial environment are so pervasive that they are covered in detail in the Meltwater and Depositional Landscapes pages.

After material has fallen to the valley bottom through gravitational forces and then transported fluvially, it comes under the influence of aeolian forces. Katabatic (mountain) winds pick up silt from glacial outwash plains and deposit it thousands of kilometers away as loess. Loess covers roughly 10% of the world and 30% of the USA (predominantly along the Mississippi River.) The deposits can be as thick as 100 meters. Loess is unique because it has the capability to form vertical, and often unstable, cliffs. The Loess Hills page has some good images of loess terrain in Iowa.

fig 5 The Wei River in the Loess Plateau, China (photo courtesy of I-Ming Chou, USGS)

fig 6 Image of The Great Salt Lake, Utah. The Salt Lake is the shrunken remnant of Pluvial Lake Bonneville (image courtesy of The United Sates Geological Survey) - click on image for bigger version

Pluvial lakes may also be considered paraglacial. Glaciers form because of colder (and locally, wetter) climates. These same conditions decrease evaporation and may restrict plant growth, thus decrease transpiration. As a result, stream runoff may fill natural depressions or basins and form a pluvial lake. Numerous pluvial lakes have existed on the earth from time to time. Today either smaller remnants of these lakes remain or, in some cases, all of the water has been entirely evaporated.

One of the largest pluvial lakes to have existed on North America was Pluvial Lake Bonneville. Filled by meltwater from the northern and central Rocky Mountain glaciers, it at one time covered much of the state of Utah. In the 10,000 years since the last glacial maximum, most of Lake Bonneville has dried up and become the much smaller Great Salt Lake. A satellite image of Utah shows the present location of the Great Salt Lake and the flat, arid areas to the south is where Lake Bonneville once existed. In addition to Lake Bonneville, numerous other pluvial lakes were located throughout the Basin and Range region and in Africa and Australia.

PROglacial and PARAglacial