Fluvioglacial processes and landscape development

Cold environments are subject to temperature fluctuations that can convert water to ice, and back again. When (melt)water is in its liquid state it can transform glacial environments through both erosional and depositional processes. Collectively these are known as 'fluvioglacial processes'.

Fluvioglacial landscapes are areas that are the result of the actions of glacial meltwater. The processes of fluvial erosion create a distinctive landscape with features unique to this environment. The landscapes can be categorised into contemporary and relic landscapes.

Contemporary fluvioglacial landscapes are found in areas where glaciers currently exist: on the fringes of polar areas and in the many alpine environments, such as the Himalayas, the Rockies, South Island New Zealand, Iceland and the Andes.

Erosional processes

There are four main types of erosion:

• Hydraulic Action - the sheer force of the water erodes the bed and banks of the meltwater channel. The meltwater will force air into fissures in the bed and banks of the channel resulting in pressure cavitation, causing material to be loosened. This material will then be carried away by the stream in times of high discharge to be deposited in an outwash plain.
• Abrasion - stones in transport within the water are thrown at the bed and the banks of the meltwater channel, eroding them. Where the bed and banks of channels are formed from soft rock or sediment, this process will occur rapidly. This process can over-deepen and widen the meltwater channel.
• Corrosion - where weak acids within the water react with the rocks, bed and banks of the channel. This erosional process will only take place when the rock type is affected by acids, such as limestone.
• Attrition - rock fragments in transport are thrown into one another during channel turbulence. This will reduce the size and smooth the shape of material. Material will change from angular glacial material to rounder, smoother material; evidence of fluvial erosion and subsequent deposition.

Depositional processes

Deposition occurs when glacial ablation periodically reduces so meltwater streams have a smaller discharge. It also takes place with increasing distance from the ice front as meltwater load increases while energy input is dispersed. When discharge decreases, so does velocity and the stream’s energy will be reduced. Lower energy results in the meltwater stream depositing its load. Fluvial deposition is sorted, with the larger, heavier materials being deposited first and smaller materials further away, and fine materials at the furthest extent.

Landforms

Erosional landforms:

Meltwater channels:These form where the meltwater from a glacier follows a pre-existing river channel. The large volume of water released from the glacier has high levels of energy together with considerable load in the form of rock fragments released from the ice mass which results in rapid erosion taking place. The channels are over deepened, particularly by abrasion, to form meltwater channels. These may also take the form of glacial overflow channels.

Moulin:A moulin is an erosional feature (rather than a landform) which occurs on the surface of a glacier. Meltwater erodes by abrasion through the ice creating this feature, which is a circular inlet down which meltwater enters the body of the glacier via deeply eroded vertical shafts. They are responsible for considerable quantities of meltwater flowing within the body of the glacier and contribute to many depositional fluvioglacial landforms.

Depositional landforms:

Outwash plains:These are formed in front of a glacier and are where material is deposited over a wide area, carried out from the glacier by meltwater. Discharge occurs from both the melting snout of the glacier and the emergence of meltwater streams from within the body of the glacier. The finest sediments are carried further away from the glacier. Coarser materials are deposited nearer to the snout of the glacier as the meltwater drops these first as its energy declines.

Eskers:an esker is formed when there is a sub-glacial meltwater channel flowing within the body of the glacier and sediment is deposited within this channel. After the ice age when the glacier melts, a ridge of sediment is left behind representing where the previous meltwater tunnel deposits descended to the valley floor as the surrounding ice melted. It looks like a winding ridge that follows the general route of the glacier and consists of coarse sand and gravel and visually, may be likened to a medial moraine.

Kames:These are accumulations of partially-sorted material found at the front of a melting or stationary glacier. These mounds build up in height as a glacier melts and meltwater streams carry material from within and under the glacier to be deposited immediately in front of the glacier into meltwater lakes. The lakes are often formed from dammed meltwater ponding up between the retreating glacier snout and terminal or recessional moraines. The deposition occurs as the meltwater flow loses energy upon emergence from the ice mass. The process continues and material is repeatedly deposited on top of the growing kame and may form a kame delta.

Kame terraces are formed in a similar way but rather than in front of the glacier they are generated along the sides of the glacier. Meltwater streams flow along the convergence of the glacier’s lateral edge and the valley side. They deposit material on the bed of their temporary channels which, when the glacier retreats fully, collapses to leave a ridge of partially sorted and rounded material to slump along the valley side.

Kettle holes:Kettle holes are formed when large blocks of ice calve from the main glacier onto an outwash plain. As the glacier retreats the block of ice is left stranded. The ice then gets surrounded and possibly buried by subsequent meltwater deposits and outwash. Eventually, when the temperature increases and the ice block melts it leaves a large depression in the ground that the ice occupied. These are known as kettle holes. Where the depressions subsequently fill with rainwater, they are known as kettle lakes.

Varves: Varves are successive layers of fine sediments deposited by meltwater streams into glacial lakes. During the summer months when discharge is higher, more sediments flow into the lake and deposits accumulate more rapidly. Coarse material in particular, such as sand and silt, flows into the lake during the summer melt and is deposited on the lake bed. During the winter where there is little or no discharge in meltwater streams, finer material and organic matter within the lake will sink to the bottom. This gives a distinct series of layers to the sediment.