|
Sources
|
|
| Surface
Sources |
| The majority of the water added
to the meltwater system comes from the surface. Any precipitation
that falls on or near the glacier, including the snowfall that may accumulate
or ablate will eventually contribute to the meltwater. At the surface,
the water is melted by incoming solar radiation. Although snow has
a high albedo and therefore relfects some of the radiation that can melt
the snow, it is the main reason the is will melt. Debris laden glaciers
will absorb the radiation and melt the snow. |
|
| Subsurface
Sources |
| Meltwater that comes from beneath
the surface of the glacier is not as voluminous, but still an important
part of the system. The melting of water below the surface is due
to heat generated from ice deformation, geothermal heat, and any increase
in pressure. Dissolved gas and solids in the water may also encourage
the melting of ice. In the case of subglacial and englacial channels
and tunnels, friction of the water can cause the ice to melt depending
of the morphology of the tunnel, and the amount of energy the water within
it has. |
|
|
Storage,
Flow, and Drainage
|
|
| Supraglacial |
| Supraglacial streams flow in a similar
manner to streams in fluvial systems in that their major controls are gravity
and ice topography. They are located above the ice, and are fed by
ice and snow melting at the surface of the glacier. Water at the
surface is either stored there, enters the englacial system through moulins,
or runs off and contributes to the proglacial system. Only when the the
temperare at the surface of the glacier is warm enough for an extended
period of time will there be water at the surface of the glacier. |
|
| Englacial |
| Englacial water flows through crevasses
or other fractures in the ice. It's morphology is similar to caves
and passages in limestone. Water can flow directly from the surface,
or be diverted into intricate systems of anabranching channels. A
study of subglacial and englacial channels was conducted by Seaburg et
al. (1988) using dye tracing at Storglaciaren, Sweden. Dye was poured
into a moulin in the lower part of the ablation area, and the time for
the dye leave the system by way of a meltwater stream was recorded.
They found that there was a significant amount of time between when the
main cloud of dye came out to the time no dye was seen in the water.
The hypothesized reason for this lag time was due to dispersion by way
of a system of englacial passages (Seaburg et al., 1988). When the
englacial channels are cut off from atmospheric pressure, cryostatic pressure
becomes the driving mechanism of the water in the englacial system.
Water is no longer constricted to flow down hill, but rather down gradient.
The weight of the overlying ice creates differing pressures in the ice.
The water will find the path of least resistance. |
|
| Subglacial |
Water in subglacial channels flows
in response to atmospheric pressure where it is open to the atmosphere,
as well as to cryostatic pressure where it is closed off from the atmosphere.
There are a variety of different ways water can exit the subglacial system.
| Rothlisberger channels (R-channels)* |
 |
| Nye Channels (N-channels) |
 |
| Water Film |
 |
| Linked cavities |
 |
| braided canal network |
 |
*Eskers are a special type of R-channel.
They are subglacial channels that are seen as ridges on the landscape after
a glacier has retreated. Eskers flow from areas of high potential
to low potential. They are an indication of the location of the equipotential
lines in a glacier.
When eskers are closed off from the
atmosphere and respond to changes in cryostatic pressure, they will flow
over ridges, and not follow topography. The shape of the esker in
the ice is a function of the pressure gradient.
| Sharp crested eskers |
 |
Debris is deposited along the sides
of the channel where the flow velocity is the lowest. Preferential
melting of the crest of the channel carves out the top. |
| Multiple-crested eskers |
 |
Where the topography of the bedrock
influences the location of the esker, the channel may migrate downslope.
Debris is deposited as the esker moves, and two or more crests are formed
on the esker. |
| Broad-crested eskers |
 |
Water will refreeze in the channel
if the energy of the water cannot keep the temperature above freezing.
The channel will broaden because only the parts of the channel that are
in contact with the flowing water will not freeze. |
** adapted
from Shreve, 1985 text
|
|
| Ice
Dammed Lakes |
Ice dammed lakes occur where a surface
topography feature causes ice water to build up behind ice. The ice
keeps the water from draining. The different stages of ice dammed
lakes were observed in Alaska by K. H. Stone (1963). First is little
of no water, second is a system of discontinuous ponds. When the
ice begins to encroach into the lake, stage three is reached. Stage
four is a smaller ice dammed lake with floating ice masses. When
the water reaches a maximum height for the dam, or the hydrostatic pressure,
the water will break through the dam. Depending on the stability
of the damming ice, the water will exit by way of a system of englacial
passages, or through crevasses. Water builds up by melting either
by frictional heating of the ice, or yearly melting and runoff. The
lack of passages in the ice means the water find different means of emptying
the lake. This process involves a build up of hydrostatic pressure.
A small void between the ice and the bed is filled with water. Because
water is virtually incompressible, the proximal portion of the glacier
is lifted off its bed, and eventually, the water is drained from the lake
(Desloges, et al., 1989). If this process occurs rapidly, a glacial
outburst flood is the result.
The presence
of ice dammed lakes can be an indication of glacial advance and represent
recent retreat. |
|
|
Erosion
and Deposition
|
|
| Abrasion
and Crushing |
| Although the ice at the base of
a glacier does a large amount of erosion of at the bed of a glacier, some
of the erosion that can take place is due to meltwater, and the sediment
it carries in suspension. The abrasion at the bed of a meltwater
channel are the same as in a non-glacial stream, but at higher rates because
of the amount of sediment in a glacial system. The velocity of the
water in a channel dictated the size of particles in the channel, as well
as the force that those in suspension have when they strike the bed of
the channel. |
|
| Cavitation |
| When the Cavitation is the process
by which gas bubbles created by turbulence of flow collapse, and damage
the surrounding bedrock. As water flows over obstructions, turbulence
is created, and gas bubbles form. Theses gas bubbles increase in
size until the pressure inside them becomes too great, and they collapse.
The amount of damage these bubbles do is dictated by the volume and content
of the gas, the compressibility of the fluid (Drewry, 1986). The
amount of cavitation depends on a number called the Euler
Number. It is the ratio of inertia to pressure in the water (Drewry,
1986). Each time a bubble collapses, more turbulence is created as
more rock fails. This inturn increases the amount of cavitaiton that
occurs in a meltwater channel. Cavitation may become more common
with variations in stream discharge due to changes in seasons. |
|
| Meltwater
stream depostion |
A. Suspended sediment
B. Bedload
C. Bedforms
D. Terraces (proglacial) |
|
| Dissolution,
Precipitation, and Mixing |
A. Dissolution and precipitation
(Benn and Evans)
B. Chemically based
mixing models and rated of erosion (Brown et. al. 1998; p 160)
1. Surface area
2. Hydrogen Ion concentration |
|
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