1.0
Introduction
The objective of this paper is to encourage discussion regarding
the possible effects of Climate Change on northern aquatic habitats
in the Upper Yukon River Basin. The paper is speculative in nature:
it is to assist in the formation of, rather than in the testing
of, hypothesis'. The paper will deal with general effects only.
The nature of land and water is such that there will be numerous
exceptions.
A brief description will be given of the functional communities
to which fish species or stocks of the Upper Yukon River Basin may
belong. Fish habitat will be discussed and defined for the purpose
of the paper. Assumptions regarding climate change will be stated.
Potential effects of climate change on habitats will be described.
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2.0
Geographic Area
The Upper Yukon River basin in Canada is the geographic focus of
the paper. This does not include the Porcupine River Sub-basin due
to significant differences between the geomorphology and hydrology
of the sub-basin and that of the remainder of the Upper Yukon River
basin.
The Upper Yukon River basin is comprised of a number of principal
sub-basins (the Stewart, White, Pelly, Teslin, and Upper Lakes)
and a number of smaller tributaries. The more southerly rivers drain
terrain which was largely glaciated during the last period of continental
glaciation. The remaining sub-basins have headwaters that were glaciated,
but lower reaches which were not. All of the lakes of any significant
size in the Upper Yukon basin are related in some way to glacial
action.
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3.0
Fish of the Upper Yukon Basin
The Upper Yukon River Basin has a depauperate (limited in number)
fish species assemblage when compared to other drainage basins of
equivalent size. With the exception of three known species introduced
to the Basin by man, only those species which were able to survive
through the glacial period or to colonise the basin afterward are
present today. The colonisation is a dynamic process and is expected
to continue to be so.
The species may be grouped into the following functional communities:
Anadromous - this is generally considered to be limited to the
Pacific salmon. It may also include other species that have not
been documented, such as the Arctic lamprey.
Lake resident (spend all, or nearly all, of their lives in lakes,
with limited excursions into rivers or flowing waters) populations
- there is a general reduction in the number of species with reduced
lake and drainage basin size and with increased altitude. Large,
low elevation lakes will usually have larger species assemblages,
while high elevation lakes will have few species. In extreme situations,
small upland water bodies will have mono-cultures. A low elevation
lake may have resident populations of lake trout, lake (humpback)
whitefish, round whitefish, pygmy whitefish, broad whitefish, least
cisco, Arctic grayling, inconnu, northern pike, burbot, long nosed
sucker, slimy sculpin, lake chub and arctic lamprey. A moderate-sized
lake at a relatively high elevation may have only lake trout, lake
(humpback) whitefish, round whitefish, burbot, Arctic grayling and
slimy sculpin. A small high altitude lake may have a mono-culture
of Arctic grayling.
Lake/river populations - Many freshwater fish stocks are migratory
and either migrate between lakes using connecting rivers or migrate
into lakes from rivers or vice versa. The migrations (or movements)
tend to be during the open water period and to be associated with
such life history stages as spawning or summer feeding. The migrations
are generally poorly understood, with the exception of certain of
the species/stocks in the Southern Lakes.
Lake or river/stream populations - a small number of species leave
larger water courses to ascend significant distances up smaller
streams for summer feeding or spawning. Most return in the autumn.
These populations normally include Arctic grayling (which may also
spawn in the smaller streams), round whitefish, immature burbot,
and juvenile chinook salmon.
River or stream resident populations - this depends in part on
the size of any given stream during winter low flow conditions.
In very small streams, only slimy sculpin are expected to remain
resident in the winter (with the exception of juvenile chinook salmon
in non-natal streams). As streams increase in size, individuals
and perhaps stocks of other species are expected to stay in local
areas throughout the winter and perhaps for the majority of their
lives.
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4.0
Fish habitat
In Canada fish habitat is narrowly defined in law. It has also
been, in a general sense, narrowly defined in science. Resources
have been most available to study habitats which were disrupted
and stocks which had been depleted to such an extent that societal
pressure demanded restorative action. A result has been an applied
science which focussed on water bodies or courses and to the land
immediately around them.
This narrow focus has proven to be insufficient to maintain healthy
aquatic habitats. Significant investments have been made to protect
and, in some instances restore stream beds, banks, in-channel habitat
features and riparian plant communities. Too often the investment
was diminished or lost when out-of channel activities or occurrences
degraded or destroyed the habitats and/or the fish using them.
There are concerns with the transferability of the information
from the studies of disrupted habitats and depleted stocks. These
include the concern that use of disrupted habitats or the related
behaviour of fish (as individuals or as stocks) which are part of
depleted populations may differ from those which are utilising natural
habitats or are part of populations which are at natural levels
of abundance. The second major concern is that utilisation of habitats
varies geographically. This may be latitudinal, altitudinally, or
otherwise as a result of hydrological and geomorpological characteristics
of the habitats available.
Holistic visions of fish habitat reach into antiquity. In recent
times, increased understanding and awareness of the inter-relatedness
of the land and water has allowed a shift in emphasis in the definition
of fish habitat from the narrow to the broad. We increasingly look
to climate and landscapes to understand the functional attributes
of habitats. This is in contrast to the examination of the physical
attributes of habitats in isolation of the greater processes that
form and maintain them.
Fish habitat has generally been defined socio/culturally rather
than ecologically. In north-west North America, there has been a
general bias to salmonid species: the salmon, trout, charr, grayling,
and whitefishes. While this Family is associated with clear, cool
to cold water, there are significant differences in use of the various
habitats which comprise this larger habitat type. Other Families
such as the Catostomidae (suckers); the Cottidae (sculpin); the
Cyprinidae (minnows); the Esocidae (pike); or the Gadidae (cod)
are also present, and use different habitats. Climate change, in
changing the functional attributes of habitats, will shift advantage
from one species to another. The advantage - or disadvantage - may
be direct or indirect: as an example, in a given year chinook salmon
would not be able to migrate up a stream to spawn due to climate
change induced low water (direct disadvantage) and related beaver
activity (indirect disadvantage). The next summer, use of upstream
habitats by juvenile chinook salmon would be limited to such relatively
low numbers which have migrated upstream from the spawning areas
during high water periods when overland flow allowed upstream migration.
Resident fish in the stream such as juvenile or adult Arctic grayling
would not have to compete against dense populations of juvenile
chinook salmon (indirect advantage) and would have increased volumes
of habitat due to the beaver ponds (indirect advantage).
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5.0
Climate change
An assumption of this discussion paper will be that the amount,
type and seasonal distribution of precipitation will be unchanged.
Alternate scenarios - such as a change in either of the amount,
type or seasonal distribution of precipitation - would require an
expansion of the paper to address. The Upper Yukon River Basin is
large and complex. This paper will suggest general effects of climate
change on the various types of lands and waters. Please note that
the discussion deals only with generalities: it does not, and could
not, account for all effects.
5.1 Lake
systems with input from melting surface multi-year ice
5.1.1 Near
term
Estimates of net gain of the annual volume of water from melting
of multi-year ice from the Coastal Ranges ice caps/glaciers to the
upper yukon River basin are not readily available. Much or most
of the melt water flows to Atlin, Bennett and Tagish Lakes. The
water is then used to produce electricity at the Whitehorse Rapids
hydro-electical project. Kusawa Lake also receives glacial input.
In the near term, there will continue to be greater volumes of
water entering (and leaving) the Upper Lakes than can be accounted
for by annual precipitation. Elevated lake levels will extend beyond
the spring freshet. Possible effects include:
-
Altered seasonal volumes and areas of lakes.
The overall volumes or areas of most lakes will not be significantly
altered. The area of the biologically important littoral zone,
however, will be watered longer period every year.
-
Increased thermal complexity of lakes. During
warming in the spring and summer, the watered littoral zones
warm more rapidly than the water mass of the lake. Winds may
result in significant transfer of thermal energy into deeper
water environments. During cooling, the wetted littoral zones
lose heat more rapidly than the deep water areas. The cooled
nearshore water is unstable, resulting in intra-lake water movements.
These may take the form of overturn or of more subtle patterns
as the cool water moves downward.
-
Altered flow regimes in outlet rivers. Post-freshet
high flows are maintained into summer and autumn.
-
Altered wetted areas of watercourses draining
the glaciers and leaving the lakes during the post-freshet period.
Most channels in depositional areas are relatively wide and
shallow. Minor increases in flow may result in significant gains
in wetted channel area for a longer period.
-
Suspension of the establishment of riparian
vegetation on banks and bars of the outlet water courses due
to extended high water levels.
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5.1.2 Long(er)
term
In time the volume of
water from the melting ice cap and associated glaciers will decline
to or below those preceding the period of glacial retreat. Overflights
of glaciers indicate that many have already retreated to the extent
that they have reduced supplies (either in-stream or within the
glacier itself) of sediment to move and/or less energy to move the
sediments. Possible effects include:
-
Decreased flows and sediment levels and increased
lateral channel stability and temperature in streams draining
the glaciers.
-
Decreased sediment deposition in lakes from multi-year
ice melting upstream.
-
increased residence time (recognising that much
or most inflows flow through rather than into lakes) of water
in lakes,. This will be augmented by increased losses of water
to evaporation relative to outflow from the lakes. An increase
in the levels of total dissolved solids (TDS) in lake waters will
be a consequence.
-
Altered seasonality of elevated water levels in
lakes. Mid and late summer levels will be reduced in relation
to the present levels.
-
Reduced period during which the upper littoral
zones will be dewatered. The degree of effect will be heavily
dependent on the structure (i.e. size, shape, depth, etc.) of
the lake. In fjord-type lakes, with steep sides, wave cut littoral
zones and rapid drop offs at the outer lip of deltas, this reduction
may be significant. In shallow lakes, where the littoral zone
is extensive the effect may be much less marked.
-
Altered thermal regimes of lakes. There will be
variations in temperature within lakes due to the changes in the
seasonal water levels. There will also be changes between lakes
due to differences in the structure of each lake. The expected
trend to overall warming may not occur in all lakes or in any
given lake at all times. The energy flux from a water body is
greatest when the water surface is not frozen. If a lake remains
unfrozen longer is then subjected to winds, colder temperatures
at depth could occur than would be the case if the lake surface
were frozen. The simple curve of the thermocline could become
complex, and the intersection with the point of maximum density
at a greater depth.
-
Altered flow regimes in rivers flowing between-
or from headwater lakes. Post-freshet flows will be reduced as
the net gain from the multi-year ice melt component of the annual
hydrograph declines. The spring freshet will become dominant or
more dominant. Summer and autumn flows will be much lower. and
significant areas of aquatic habitat will be dewatered.
-
Reduced erosive capability, and ability to transport
sediment, of rivers draining glaciers. There will be less sediment
deposited in lakes. There will be a trend to decreased turbidity
of rivers and lakes, particularly during the period following
spring freshet.
-
Increased stability of river channels both up-
and downstream of the lakes in response to the reduction of flow
and energy. There will be a decrease in lateral channel migration.
Emergent vegetation will be able to establish itself on the upper
parts of bars. Increased in-channel complexity both up- and downstream
of the lakes will also result.
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5.2
Lakes without input from melting glacial ice - all sub-basins
Most lakes in the Yukon are associated with past glacial events.
Many are located in basins which were eroded below the surrounding
land surface. Some are also impounded by moraines at one or more
points or are associated with flooded drum and kettle topography.
There is a wide range of lake surface area relative to the size
of the drainage basin upstream. Effects of climate change will reflect
all of these variables and others (i.e. altitude, aspect, local
precipitation regimes, etc) and will vary accordingly. Possible
effects of climate change include:
-
Reduced total inflow of surface waters. This may
be due to increased evapotranspiration from the land surface resulting
from higher temperatures during the growing season, to increased
sublimation due to higher air temperatures and wind velocities
during the winter and spring or to more subtle effects. The relative
input from surface and sub-surface flows will also vary from lake
to lake and from year to year.
-
Increased evaporation from the surface of the
lake.
-
Reduced outflows from lakes. This will increase
the potential for the construction of dams by beaver at the outlet
of smaller lakes, or on larger lakes with small drainage basins
relative to their surface areas.
-
Increased potential for freezing of the outlets
of some lakes. A result could be the dewatering of stream channels
downstream of the outlets. Significant mortalities of aquatic
organisms (including the incubating eggs of autumn spawning fish)
due to asphyxiation and freezing may occur. Winter inflows to
the lakes will continue irrespective of the blocking of the outflows.
The lake may become charged (lake level above the level of the
outlet). When the outlet ice obstruction is overtopped or otherwise
removed, flows downstream may be significant.
-
Reduced recharge of aquifers located upslope
of lakes, where the ground water discharge areas are directly
or indirectly tributary to the lakes. In time, this will result
in decreased inputs to the lake.
-
Increased ground water residence time. There will
be an accordant trend toward increased total dissolved solids
(TDS) and decreased dissolved oxygen levels in the discharged
ground water.
-
Altered density of ground water discharges. During
periods of high vertical stability in the lake (i.e. under ice)
the density may be such that the discharged ground water forms
a discreet water mass. This water mass may form layers or pools
that are anoxic and/or otherwise toxic. If the discharge waters
are more dense than the lake water (through increased salinity
or other dissolved substances), they will flow downslope and pool
in deep areas of the lake.
-
Increased residence time of water in lakes. As
lake volumes remain relatively constant but the inflow to- and
outflow from the lakes decline, a trend to increased age of water
in the lakes will occur.
-
Reduced lake levels, or decreased length of time
that the littoral zone is watered. This will allow encroachment
of vegetation onto foreshore/ seasonally flooded areas. More subtle
physical effects will vary between lakes depending on their size,
shape and function.
-
Reduced flows in outlet streams and rivers. Many
of the non-glacial fed lakes are relatively small or have small
drainage basins relative to their surface areas. Surface flows
may be significantly reduced or may cease in some situations.
-
Increased numbers of outlet streams dammed by
beavers. The numbers and the longevity of the dams will increase.
There will be intermittent periods when virtually all flow is
cut off in the outlet streams below beaver dams. This may result
in significant mortalities of aquatic organisms from asphyxiation,
predation, temperature stress, or freezing.
-
Altered characteristics of spring outflows from
the lake. If the channel downstream of the lake is ice filled,
the flows may be diverted overland. This may result in new stream
channels being eroded into the flood-plain downstream and in erosion
and transport of significant levels of sediment fo aquatic and
flood-plain habitats.
-
Altered water quality in the outlet stream. If
the surface flows from the lake cease, the watercourse downstream
will at some point be watered as a result of overland flows (summer);
confluential flows (all seasons); and ground water discharges
(all seasons). Ground water may be low in oxygen or totally anoxic.
In under-ice environments, this may result in the destruction
of all or some life history stages of many or most aquatic organisms.
Ground water may also be acutely or chronically toxic.
-
Altered quantity and quality of water in the hyporheic
zone. This is the saturated zone below and around the surface
of the stream bottom. It provides habitat for invertebrates and,
to a much lesser extent, the incubating eggs or early life stages
of some fish.
-
Increased stability of stream channels downstream
of lakes. There will be a trend to lower and less frequent channel-forming
flows, and increased lateral channel stability.
-
Reduced sediment erosion and transport, particularly
sands and silts. There will be a consequent trend to lower levels
of turbidity.
-
Increased colonisation of margins of streams and
rivers by emergent vegetation. There will be a general downslope
encroachment of vegetative zones, and a greater degree of shading
of the water surface.
-
Decreased insolation of the water surface. The
decreased surface area of the outlet stream and increased shading
will result in a net decrease of solar energy to the water surface
in these locations.
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5.3
Water courses independent of upstream lake influence
An effect of climate change common to most watercourses is the
net gain of water from melting multi-year ground ice (associated
with permafrost). This will probably be most significant in smaller
streams without lakes in their upper reaches. Melting ground ice
is most obvious in areas with low surface relief, fine grained soils
and no surface drainage. The parklands between Whitehorse and Haines
Junction, on the Aishihik Road or those upstream of the Rawlinson
Creek fan on the Nordenskiold River are examples. As a general sequence,
the surface of dry meadows subsides and a pool appears in the centre.
Subsidence accelerates, and the land surface collapses radially,
forming vertical walls. Trees collapse inward; finally, the pond
may dry as subsurface aquicludes are breached and the waters drain
into regional ground water pathways. The Siberian term for these
features is "alas".
A feature that may be associated with alas' is the melting of
near-stream ice. Some streams appear to have ice rich, fine grained
soils paralleling or surrounding portions of the channel. As the
ice in these soils melts, the land and vegetation along the banks
collapses toward the stream, and the channel may become incised.
If the alas', either alone or in association with near surface
ice, are common and near small streams, the quantity and quality
of the water in the streams may reflect the melt water. Water from
melting permafrost tends to be high in organic material resulting
in a high biological oxygen demand.
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5.3.1 Watercourses
with glacial input.
Glacial input is defined, for the purposes of this discussion,
as being the dominant channel- and flood-plain forming mechanism
for the subject watercourse. Typically, a glacial stream has least
one and perhaps many melting glaciers in the upper drainage basin.
Highest sustained flows usually occur in the mid- to late summer.
Inorganic materials are carried down the stream as suspended sediment
or as bedload. Levels of suspended sediment are variable and depend
on the nature of the material entrained in or located on the glacier
and the volume of flow at any given time. The stream bed and valley
bottom are aggrading (being built up), sometimes aggressively. The
rate of aggradation determines the stream channel form. Unstable,
braided channels occur where rates are high and single channels
where rates are low.
The present increase in glacial melt waters carried by streams
entering lakes has been noted and addressed above. Possible longer
term effects to glacial streams may include:
-
Reduced supply of sediment from the actual melting/recession
of the glacier. Heaviest sediment loads result from glaciers which
are overlain by sediments (i.e. volcanic ash) or have recently
surged. Older or inactive glaciers may have relatively low sediment
yields.
-
Reduced volumes of flow, resulting in lower available
stream energy.
-
Increased
lateral channel stability. This will be most marked in areas with
braided channels.
-
Reduced erosion of bed and banks, and less transport
of sediment (both in regard to total volume/weight moved and to
the particle size(s) which can be moved). This will result in
less deposition of sediment into downstream water bodies and courses.
-
Altered location and structure of riparian and
floodplain vegetative communities. Emergent vegetation will colonise
much or most of the previous braided flood- plain. A modified
successional phase will probably follow, leading to a general
spruce climax forest interspersed with more hydrophilic species
in or near areas of ground water discharge.
-
Reduced volume of ground water flow due to decreased
recharge zone areas. Glacial rivers generally have extensive hyporheic
zones. These may be very deep and wide: effectively, they may
include all of the material which has been deposited by the glacier.
The deposited materials tend to be relatively coarse, to have
relatively low surface area to volume ratios, and to be non-reactive
(to chemical degradation/dissolution). Depositional features tend
to be in lens' of poor- to well sorted material such as sands,
gravels, or cobbles. Water enters the hyporheic zones (or aquifers:
there is no clear defining line between the two concepts) easily;
travels quickly; and discharges at multiple points. Most water
enters the hyporheic zone during high water periods. As it travels
through the zone, heat is absorbed by the substrate. The heat
is released during the cold water periods. Water discharging from
the zone tends to have very stable temperatures throughout the
year. Reduced volumes of ground water flow will result in less
thermal energy entering the zone; less water, possibly at a lower
temperature, discharging from the zone; and an increase in TDS
and a decrease in dissolved oxygen (DO) due to the water remaining
in the aquifer for a longer period.
-
Altered physical features and vegetative communities
at ground water discharge areas. There will be a succession of
vegetative communities that will encroach on the discharge areas.
The discharge area may change from a "channel" which is open to
fish bearing waters to a "spring" type area which is not.
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5.3.2
Watercourses without input from melting surface ice
This category of watercourses has a much wider range of physical
characteristics than does that of glacial streams. The characteristics
ultimately depend on the structure of the lands that are drained
and through which they flow. The quantity, seasonality and type
of precipitation which falls in their drainage basins, the short
term weather and the longer term climate are also determinants.
As noted in the introduction, this paper assumes that the quantity,
seasonality and type of precipitation will remain constant and that
the climate will warm. Classification will therefore be made by
the dominant characteristics of the lands. In the Upper Yukon, the
dominant land forming characteristic was that of glaciation.
5.3.2.1 Streams
draining glaciated terrain
Maps showing the extent of glaciation in the Upper Yukon River
basin are usually presented as 8X11.5 inch pages in reports, etc.
A smooth line is shown for the limit of ice advance. The actual
limit of advance was highly complex, with unglaciated upland areas
separated by glaciers or by large lakes associated with the glaciers.
Streams and rivers flowed under, through, or over the ice, carrying
sediments which varied in size from clay to boulder. When deposited
by flowing water, this is collectively known as glacio-fluvial material.
It tends to be sorted by particle size (i.e. homogenous layers of
cobbles, gravels, sands etc.). It is generally non-reactive, with
larger particles being rounded in form and presenting a high volume:surface
area ratio. The particles therefore dissolve slowly and tend not
to release toxic substances. Water travels through these materials
relatively easily, and tends to discharge with relatively low TDS
and relatively high DO.
Where the sediment laden streams and rivers flowed into lakes,
finer grained sediments were given the opportunity to settle out.
These materials are collectively known as glacio-lacustrine. There
are predominately silt and finer sands. The present characteristics
depend on the environment in which they were deposited, and vary
from well sorted and evenly bedded to a heterogeneous mix of fine
sediments. Due to the size of the particles, they have a low volume:surface
area ratio. Water tends to travel through the deposits slowly, and
is in contact with the material for an extended period. Ground water
discharges from glacio-lacustrine deposits tends to be high in TDS
and low in DO.
The glaciers also pushed sediments ahead of them. This material
tends to be un- or poorly sorted and is known as glacial till. It
may be reactive, and the particles are often only roughly rounded.
Water travels through these materials slowly, and is in contact
with the material for extended periods. Ground water discharging
from till tends to be high in TDS and low in DO. In some areas,
the glaciers rode over previously deposited till, compacting it
almost to the consistency of rock. This material effectively does
not allow the passage of water, and serves as an aquiclude. The
underlying material of the outlets of many of the lakes in the Yukon
are composed of in part or entirely of till.
The structure of the landscape is often complex in a vertical sense.
Glacial till may be overlain by glacio-fluvial deposits, which are
in turn overlain by glacio-lacustrine materials.
This depositional landscape lies above an erosional landscape.
The glaciers flowed over the pre-existing landscape, eroding complex
channels. The glaciers met, split and sometimes rejoined. Great
trenches were eroded into the surface of the Yukon Plateau. The
trenches tend to have a "U" shape in cross-section. Functionally,
the result was that many of the lateral tributaries from the lightly-
or unglaciated upland surface were perched above the valley bottoms.
Most streams have since formed graded channels. These streams typically
include a canyon section cut into bedrock at the rim (or valley
wall) of the deeper valley leading to a fan extending into- and
often across the valley below.
In profile, the tributary stream channels are generally convex
or stepped. There is a low gradient upper section and a higher gradient
mid section. The gradient increases until the larger valley is entered.
The profile of the water course flowing down a glacier cut valley
tends to be stepped. A low gradient section upstream of each of
the tributaries' alluvial fan is followed by a higher gradient section
downstream.
Possible effects on the streams in the glaciated area of the Yukon
may include:
-
Decreased volumes of flow due to increased evapotranspiration
and sublimation in the drainage basin, and resulting reduction
of input from surface flow, snowmelt or ground water discharge.
-
Increased lateral channel stability. This will
be related to reduced available stream energy.
-
Reduced erosion of the bed and banks, less transport
of sediment (both in regard to total volume/weight moved and to
the particle size(s) which can be moved) and less deposition of
sediment into downstream water bodies and courses.
-
Altered location and structure of riparian and
floodplain vegetative communities. Emergent vegetation will colonise
the banks and upper bed of the streams. A modified successional
phase will follow, leading to a general spruce climax forest interspersed
with more hydrophilic species in or near areas of ground water
discharge.
-
Increased numbers of beaver dams. Beaver will
respond to more favourable conditions for dam construction (less
chance of wash-out due to high water, more attractive riparian
vegetation). This may increase recharge of in- or out-of-channel
aquifers.
-
Reduced thermal energy stored in aquifers. Less
water entering aquifers will reduce the amount of thermal energy
stored, and of thermal energy discharged.
-
Altered quality of ground water discharges. Lower
volumes of water entering certain aquifers will reduce the head
(that is, the pressure under which the water is forced through
and out of the aquifer) and the rate at which water flows through
them. There will be an increase in TDS and a decrease in DO.
-
Altered characteristics of ice formation in the
autumn, overflow ice conditions in the winter and of ice-off in
the spring. The type and degree of effect will vary as a result
of basin characteristics. In a gross form, though, a trend to
ice/substrate contact for a greater portion of the cross-sectional
area of a stream for a longer period of the winter is possible.
This could result in greater icing, as ground water discharges
would flow over the ice surface rather than within channels under
it. Ice on the streams would melt later both due to the increased
depth of ice and to the lack of warm(er) water to melt it from
beneath
-
Altered characteristics of spring freshet. This
could flow over the ice rather than in existing channels. Results
could include new channels being eroded around the frozen stream
channel, with resulting sediment release and stream destabilisation
downstream. Alluvial fans will be particularly vulnerable.
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5.3.2.2
Streams draining unglaciated terrain.
These streams flow through valleys which have been formed through
the action of flowing water. The valleys have a "V", or modified
"V" shape in cross-section. They have a concave profile. There is
a steep headwall basin and a long, moderate- to low- gradient section
to the confluence with a larger river or stream. There may be a
section of locally increased gradient near the mouth.
With few exceptions, there is limited deposited materials in the
upper drainage (a notable exception being the "White Channel" gravels
in the Dawson area). The land surface is, for the most part, fractured
bedrock. The degree of fracturing, or shattering, of the bedrock
may be intense. Resulting particles are often tabular and have low
volume:surface areas ratios. Reactive surfaces are continually being
exposed due to the shattering of the substrate. Rates of flow of
water through this zone of fractured rock depends on the characteristics
of the bedrock. The chemical composition of the bedrock also affects
the type of dissolved solids in the water. Highly reactive bedrock
may cause significant depressions in the pH of ground water and
result in the uptake of heavy metals. If the ground water then flows
through shattered limestone, the pH will rebound and most of the
metals will be deposited.
Most of the unglaciated area of the Upper Yukon basin is located
in the zone of discontinuous permafrost. North-facing slopes tend
to have near-surface permafrost. The frozen soil appears to provide
a degree of control the surface stability of certain of the slopes.
South facing slopes have a deeper active layer (i.e. depth to which
surficial material thaws each summer)
Possible effects of climate change may include:
- Increased deposition of organic or inorganic sediments into streams.
This may be due to the mass wasting of significant areas of ice-rich
upper landscape, resulting in liquefaction of soils and downslope
flow of sediment. It may also result from degradation of permafrost
on steep north facing slopes. This may reduce the strength of the
vegetative veneer to such an extent that it will tear and slide
downslope. The composition of the underlying substances (i.e. rock,
frozen organic material, etc.) will determine whether gullying,
karst or other effects may occur.
- Changes to water quality. Inorganic sediments will generally be
tabular in shape, have low volume:surface area ratios and may have
a high chemical oxygen demand. Organic sediments will have a high
biological oxygen demand. Either mechanism may result in depressed
dissolved oxygen levels in streams. Thawing of deep permafrost may
release water which has travelled very slowly through shattered
or decomposed bedrock. The water may be high in TDS and, potentially,
in heavy metals.
- Increased icing/glaciation/aufeis formation in valley bottoms.
Bodies of ice located deep below the surface will melt slowly and
release water to the valley bottoms. During the winter, the ground
water will discharge and freeze along the base of the valley and
in the channel. Spring freshet may flow over the ice surface rather
than within the channel. This could result in new channels being
eroded around the frozen stream channel, with resulting sediment
release and channel destabilisation downstream.
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Appendix
A: Fish Species Utilising the Upper Yukon River Basin
Native:
- chinook salmon (Onchorynchus tshawytcha)
- chum salmon (Onchorynchus tshawytcha)
- coho salmon (Onchorynchus kisutch)
- lake trout (Salvelinus namaycush)
- Dolly Varden charr (Salvelinus malma)
- lake (humpback) whitefish (Coregonus clupeaformis)
- round whitefish (Prosopium cylindraceum)
- broad whitefish (Coregonus nasus)
- pygmy whitefish (Prosopium coulteri)
- least cisco (Coregonus sardinella)
- Bering cisco (Coregonus laurettae)
- Arctic cisco (Coregonus autumnalis)
- Arctic grayling (Thymallus arcticus)
- inconnu (Stenodus leucichthys)
- northern pike (Esox lucius)
- burbot (Lota lota)
- longnose sucker (Catostomus catostomus)
- slimy sculpin (Cottus cognatus)
- lake chub (Couesius plumbeus)
- Arctic lamprey (Lampetra japonica)
Introduced:
- rainbow trout (Onchorynchus mykiss)
- Arctic charr (Salvelinus alpinus)
- threespine stickleback (Gasterosteus aculeatus)
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