Sources of Meltwater                                                             meltwater title page

There are several ways to introduce meltwater into a glacial system. The sources are surface meltwater, basal, internal, and groundwater. Each source of meltwater plays a crucial role in the mechanics of the glacier, and how the glacier releases the meltwater as discharge.

Surface sources of meltwater accounts for the greatest amount of discharge from the glacier. There are several factors that influence surface meltwater; radiation, convection, and condensation. Radiation refers to solar radiation, which will change with the season, time of the day, latitude, altitude, cloud cover, exposure, humidity, and albedo of the glacier. The amount of incoming solar radiation is a major factor in controlling the surface melt of a glacier.
Convection refers to the movement of air across the surface of the glacier. Air that is next to the glacial ice will be quickly cooled to zero degrees Celsius. However, if convection is present, warm air will replace the cool air close to the surface of the glacier, and thus influence melt. Finally, condensation will influence the melting of glacial ice. If the relative humidity is such that the air is at dew point and is in contact with ice or snow, condensation will occur. This process of condensation is sufficient to melt a good amount of ice or snow, assuming the ice is close to melting point (Sharp,R.P.1960).

Basal and internal sources of meltwater also contribute to the amount of water within the system of the glacier. One source of meltwater is through the process of pressure melting. This process occurs when the glacier overlies areas where the ground has some heat within the bedrock. As a rule of thumb, warm glaciers are at the pressure melting point and cold glaciers are frozen to their bed. Therefore warm based glaciers will have plenty of meltwater within and under the ice itself. Most meltwater that exists within or under the ice exists at or near the base of the glacier. Finally, ground water can contribute to the overall system of water near the base of the glacier. Ground water sources to the glacier meltwater system are going to vary due to topographic or geologic constraints (Sugden,D.E. & John, B.S. 1976).

Now that meltwater is in the glacial system, the water must flow through the system and be released as discharge. The discharge of meltwater will vary from season to season, and from day to day. Glacial meltwater takes longer to flow through the glacial system, so that peak seasonal flow rates may last longer than peak season discharge rates for typical stream systems. Daily discharge rates vary because at night the meltwater flowing through the glacier freezes, thus lowering discharge. During the day this frozen water begins to thaw and reaches peak discharge in the afternoon. This is why in the morning near a glacier one may be able to cross a stream with no difficulty, while in the afternoon there is no way to cross the stream because the discharge is too high.

Drainage of Meltwater

Drainage of glacial meltwater can occur in three different places on or within the glacier. Supraglacial drainage occurs on top of the glacial ice. Englacial drainage occurs within the ice, and subglacial exists beneath warm based glaciers.

Supraglacial drainage is most common in the  ablation area of the glacier. For surface drainage to occur the surface must melt more water than can be absorbed by the ice. Stream channels usually form from slush swamps, where free meltwater ponds into a slushy swamp. Eventually this swamp grows and the water within the swamp begins to run off the glacier. The stream channels draining the glacier are usually of a dendritic pattern.  These streams exhibit a meandering course because of a lack of resistance to flow by the ice. However the surface meltwater streams will be influenced by overall relief of the glacial surface. Some streams may disappear into moulins and enter the englacial system. Some surface streams may occupy structural weaknesses in the ice such as cracks, and closed crevasses. A brief note about supraglacial drainage on cold based glaciers; cold based glaciers offer highly favorable conditions for surface flow, but meltwater tends to freeze if it descends into the englacial system (Benn,D.I. & Evans,D.A. 1998).

There are four basic characteristics of supraglacial streams. First, the drainage density will be high, because the drainage system lacks well-developed trunk streams. Second, drainage patterns will contain many sub-parallel streams that are controlled by the structural characteristics of the glacier and the consistent slope of the surface of the ice. Third, the density of streams decreases up glacier. Fourth, the channel patterns are highly changeable and unstable due to rates of ablation, and the opening and closing of crevasses.

Block diagram of the glacial drainage system.
Notice how the englacial system is a 3D network of channels.
Also notice how the supraglacial streams can be incorporated into the englacial system via moulins and eventually become part of the subglacial drainage system.

Within the englacial drainage system most of the water is from the surface. Cold-based ice does not allow an englacial drainage system to exist. This is because in cold based ice the englacial system is frozen shut. Warm based glacial ice is permeable to water in two ways. The first and basic way that warm ice is permeable to water is called primary permeability. This refers to the way that water is able to move through a 3-D network around the crystals of ice, also referred to as capillary action. The secondary permeability is a network of tunnels within the ice that exploit structural weaknesses within the glacier. Most of the water within the glacier moves about via secondary permeability. Water enters the englacial tunnels via moulins.
Within the secondary permeability of a glacier two zones of water movement exist. One is movement through an unsaturated zone of the glacier. In this unsaturated zone the water is allowed to move freely as controlled by gravity, and flow is usually downward towards the base of the glacier. The second zone is one where the ice is saturated with water. Basically one can think of a glacier as having a water table. Within this zone of saturation the flow of water is governed by pressure. Water will flow from zones of high pressure to zones of low pressure along the steepest gradient. An analogy to this high to low flow is to think of the wind patterns of the earth; wind flows form areas of high air pressure to areas of low air pressure, and water in a glacial system follows the same rules. Basically large passages or conduits have the lowest pressure. As a result many small passages will eventually lead into a large passage (Benn,D.I. & Evans,D.A. 1998).

Subglacial drainage is a complex issue and much of the discussion will be left for the next level. However it is important to realize that in a warm based glacier, water is flowing under the ice, whereas in a cold based glacier the glacier is frozen to the underlying bedrock, and therefore no meltwater exists. Subglacial drainage will have a profound influence on the velocity of the flow of the ice, the stability of the glacier, sediment transport, and erosion and deposition of sediment carried by the subglacial streams.
Two types of stream channels exist under the glacier; distributed and discrete channels. In distributed channels water is transported over the entire bed, or a large portion of the bed. Discrete channels have water confined to a few channels or conduits located under the ice (Benn,D.I. & Evans,D.A. 1998) .

It is important to realize that meltwater not only lubricates the underlying surface of the glacier, but if the subglacial water is under enough pressure it can offset the weight of the overlying ice and cause the glacier to slip, and maybe even cause the glacier to surge forward.

In many cases the drainage systems may not be able to completely drain the meltwater in the glacial system. In this case the meltwater must be stored within the glacial system. The glacier can store the meltwater on top of, underneath, within, or dammed by the ice.

Storage of Meltwater

Water accumulates beneath glaciers where regions of low pressure are surrounded by regions of high pressure. Water can also accumulate in a bedrock depression below the ice dome, such as a crater. A water cupola forms if the glacier bed is flat and the surface ice has a central depression, thus forming a relatively low-pressure area in the center of the ice. This low-pressure region under the ice then fills with water.Large reservoirs similar to those described above are known to exist under the Antarctic ice cap; one such reservoir is estimated to be 8000 square kilometers.

Englacial pockets of water form when a crevasse has been filled with water and the crevasse then closes, thus trapping pockets of water with in the ice. These pockets do not store large quantities of water. Supraglacial lakes form early in the melt season before the englacial system opens up. On cold based ice the lakes can persist during the ablation season without draining away. Debris laden glaciers, because of differences in albedo, lead to differential melting. This differential melting can lead to sinkholes, or water filled hollows. Finally ice cauldrons form from large amounts of geothermal heat rising through the ice and melting the surface of the glacier. Many times ice cauldrons will have water on top of the glacier as a supraglacial lake, and water will be present under the ice as a water cupola (Benn,D.I. & Evans,D.A. 1998) .
 
 

Supraglacial meltwater storage on a debris laden glacier
Mt.McKinely (B-1)Quadrangle, Alaska
1:63360

Ice dammed lakes form where the ice forms a barrier to regional drainage. This is common in many glacial systems. Ice dammed lakes are considered pro-glacial  (in front of the glacier), and will be discussed in another chapter.
 
 

Topographic map showing valley train (sandur)and an ice dammed lake. 
Kenai (D-8) Quadrangle, Alaska 
1:63360

Meltwater and Glacial Deposition

The realm of meltwater encompasses both fluvial and glacial processes. Thus, certain distinctive landforms are found in glacial ablation environments which can be formed either by the work of streams, ice, or both. Sediment which is deposited by glacial meltwater is termed stratified drift. This means that the sediment transported from the margins or on top of the glacier by water will be similar to that in
a fluvial system, and has a stratification not normally found in other types of till. The stratified drift is also sorted and rounded to a higher degree than most other types of glacial sediment (such as a moraine).

It should be noted that the sorting, rounding, and stratification of the sediment typically consists of sand and gravel-sized particles. The fines of the system (the silt and clay-sized particles) are commonly carried out of the system in suspension. This is why most outwash streams have a characteristically high mud content, resembling that of a river in spring flood. This is termed glacial milk . If the sediment is deposited by eolian (wind) processes, it is termed glacial flour, or loess.

Deposition by outwash streams is much more variable than that of normal fluvial systems. Most rivers are fed and sustained mostly by groundwater flowing into the system, whereas outwash streams consist almost entirely of meltwater, and therefore are tied much more closely to glacial ablation. For this reason, outwash streams are variable with local climate conditions, albedo of the ice and of the rock which surrounds the glacier, and even the time of day. For example, an outwash stream easily traversed in the morning may be a torrent of raging muddy water by late afternoon.

In order for meltwater to deposit, one of three things must happen. There must be either a decrease in water velocity, a decrease in the stream gradient, or an increase in the amount of sediment to the point that the stream can no longer carry the load. Probably the best way to visualize these concepts is to use a tool called Lane's Balance, presented below. The concept of Lane's Balance is that as sediment size or volume increases, or as slope decreases, aggradation (or deposition) will take place. If the opposite occurs (increasing slope or decreasing particle size or density), erosion would follow.
 
 
  Stream channels in a glacial meltwater system can take on many forms. Most common below the terminal moraine are braided stream channels, indicating that there is too much sediment being supplied for the amount of water flowing through the system to carry out. This will be discussed later (see sandur, shown below). However, some stream channels are dendritic (leaf-like), anastomozing, and a few even have entrenched meanders, especially on top of the ice itself.
 
  Listed below are some typical glacial meltwater landforms. The flowchart shows examples of several different types of glacial depositional landforms, divided into areas which were in direct contact with the ice (proximal), or areas beyond the margins of the ice itself (distal). As with any natural system, a flow chart is only designed to gain a perspective on the typical spatial distribution of a feature, and many variations from the norm are to be expected. However, the reader should gain a perspective as to how the following landforms were created, and where they are likely to be found. Each landform is hotlinked to a picture and description of the formation of the feature.

depositional landforms  top of page            proglacial & paraglacial 
meltwater title page