Ministry of Environment


3.6 The Northern And Central Plateaus And Mountains

G.K. Young and N.F. Alley

The Northern and Central Plateaus and Mountains region occupies the north half of the province and is bounded by the Coast Mountains on the west, and the Rocky Mountains on the east. The centre of this vast area comprises the westernmost portion of the continental divide, and the Omineca and Cassiar Mountains. Here the tributaries of the MacKenzie River rise, including the Dease, Liard, Finlay and Parsnip Rivers. To the west numerous large streams wind their way across the Yukon and Stikine Plateaus to the Pacific Ocean. Some of these include the Taku, Stikine, Iskut, Bell-Irving, Nass and Skeena Rivers, most of which have their headwaters in the Skeena Mountains.

Although the factors which generally affect soil formation anywhere in the province have been described in Part 1, some of these in the study area operate to produce a distinctive set of soil landscapes. These factors include the geology and climate which together produce a predominance of periglacial, geomorphic processes not found in other parts of British Columbia.

The Factors of Soil Formation

This region lies in the rain shadow of the Coast Mountains. As a consequence, precipitation is low, especially in the area adjacent to the mountains such as the Tagish and Tahltan highlands. Precipitation increases eastward across the Yukon-Stikine Plateaus towards the Cassiar and Omineca Mountains.

In the southern portion of the region, the large valley of the Nass River permits westerly winds to penetrate the Coast Mountain barrier, resulting in increased precipitation locally in the Skeena Mountains. Similar moderation of climate occurs along the Stikine River in the vicinity of Telegraph Creek although, in contrast to the Nass River area, the climate is warmer and drier. These two micro-environments are discussed later. With the exception of microclimates and edaphic situations, moisture deficiencies during the short growing season are rare in these dominantly humid to subhumid soil moisture regimes.

Generally, the northern plateau has a very cold, subarctic climate resulting from its elevation and the southward penetration of arctic air masses into the area. The west, south and east facing slopes of the NNW-SSE trending valleys of highlands and mountains peripheral to the plateaus range from cold cryoboreal to very cold subarctic. Thus, on the plateaus and adjacent uplands climate produces an environment in which frost plays an important part in geomorphic processes and soil formation.

Perhaps the most important factors influenced by the cold climate are the rate and operation of geomorphic processes. The periglacial processes which contribute most to the distribution and character of the soils in the area include frost shattering, cryoturbation, solifluction (and related processes), nivation, permafrost development and snow avalanches. Together these processes have produced large masses of colluvium which mantle the plateaus and mountain slopes, and an environment in which the materials are constantly in motion either downslope or by churning. Consequently, the dominant soil landscapes in the area are pedogenically youthful and have evidence of cryoturbation such as broken or buried horizons, and displaced materials.

The periglacial activity and turbic soils extend virtually from ridge and plateau tops to valley bottoms. Probably cold air, draining from the plateaus down into lower elevations, contributes strongly to periglacial activity in valley bottoms at an altitude where frost action is normally insignificant (see "Cold valley floors" in "Other soil landscapes").

The north-northwest orientation of major valleys and their asymmetry in transverse profile also influence the distribution and operation of periglacial processes in the plateau area. Gentler west-southwest-facing valley-sides are dominated by deep blankets of colluvium produced mainly by nivation and solifluction. In contrast, east-northeast-facing slopes are steeper and have snow avalanches and rockfalls. These slopes often consist of bare rock or thin colluvium in their upper reaches, where mass-wasting predominates, whereas blankets, fans and aprons of colluvium occur on the lower levels where deposition predominates.

Geology influences the soil landscapes of the region in a variety of ways. First, the geological materials affect the texture and chemistry of soils. Intrusive plutonic and low grade metamorphic rocks in the Omineca and Cassiar Mountains weather to produce generally medium to coarse grained acidic soils whereas the volcanic rocks in the western part of the region weather to fine and medium textured soils of higher base status. Sedimentary rocks in the Skeena Mountains and Stikine Plateau are variable, although the predominance of fine grained strata commonly produces fine textured soils which are normally acidic except in the vicinity of calcareous rocks.

Second, the western portion of the area is composed of flat-lying or gently folded rocks which form broad plateaus high enough to be in the alpine-subalpine zone. This has created extensive areas in which periglacial processes dominate, producing thick mantles of colluvial debris which form solifluction terraces and lobes, that often lead up to nivation hollows, patterned ground, block-fields, stone streams and rock glaciers in the valleys. The soils are obviously youthful and dominantly of the Cryosolic order or have been influenced by cryoturbation.

Third, glacial deposits exposed at the surface were formed largely during the Fraser Glaciation; only small remnants of ice now remain either as cirque or short valley glaciers. Advances of these glaciers during the Neoglaciation formed the moraines now found in close proximity to the ice. Regosols or Cryosols have formed on these moraines. More important, however, is that the colder climatic conditions which produced the Neoglacial advances also led to an expansion of the periglacial zone well into what is now the subalpine. The features, such as talus, patterned ground, solifluction lobes, nivation hollows, block streams and even rock glaciers, which formed as a consequence of the colder climate are now relict.

The Soil Landscapes

The soil map of British Columbia (Part 3.2) shows the distribution of the main soil great groups in region. However, the map is very general and does not show some of the soil landscapes which may cover only small areas. An idealized diagram illustrating the vertical sequence of soil landscapes is presented in Figure 3.6.1 and the main features of each soil landscape are described in the following pages.

Gray Luvisol Landscape

This soil landscape occurs on well drained morainal deposits below 1200 m in broad valleys of the Sub-Boreal Spruce and Boreal White and Black Spruce zones. Parent material texture, colour and mineralogy are closely related to bedrock. Texture varies from medium to coarse and colours vary from dark brown to dark gray on moraine derived from basalt and sedimentary rocks, to gray or light gray on moraine derived from granitic and plutonic rocks. Parent materials formed from limestones are calcareous. Basalts and other sedimentary rocks produce mildly calcareous or neutral materials, and granitic materials are acidic. The soil solum, however, is affected by leaching resulting from the humid and subhumid soil moisture regime and organic acids formed by decomposition of vegetation. Consequently the soils are neutral to slightly acid on limestones, basalts and sedimentary rocks, and acidic on granites and plutonic rocks.

The moraine has a rolling, ridged or hummocky topography with a repetitive pattern of soil drainage conditions. Slope position, gradient, and slow permeability of morainal parent materials result in a range from well drained to very poorly drained soils. Organic soils occupy enclosed depressions, whereas Humic Gleysols occur where seepage accumulates along extensive gentle slopes, or at the base of steep slopes.

Dystric and Eutric Brunisol Landscapes

Broad valleys within the Sub-Boreal Spruce and Boreal White and Black Spruce biogeoclimatic zones often contain extensive deposits of fluvioglacial deposits, which range in texture from coarse cobbles and stones to gravels and fine sands. As a result of coarse texture the soils are characterized by rapid percolation and iron accumulation, especially on acidic parent materials. The preponderance of lodgepole pine on these soils, the lack of minor vegetation elements, and the presence of charcoal in the litter and upper mineral soil indicates a relatively high frequency of fires. These edaphically dry acidic landscapes result in Dystric Brunisol soils with highly coloured B horizons.

Brown coloured soils of Eutric Brunisol landscapes also occur here on calcareous, coarse textured parent materials, especially on well drained fluvioglacial and colluvial deposits. In other areas they are commonly restricted to climatically or edaphically drier sites such as southern exposures and along the lower terraces of the Stikine River, in the extreme rain shadow of the Coast Mountains. As a consequence of frequent burning in the rain shadow, vegetation is commonly reduced to trembling aspen, lodgepole pine, shrubs and grasses, representing early stages of secondary plant succession.

Humo-Ferric Podzol Soil Landscape

This landscape is common between elevations of 1200 to 1500 m on steep, moist slopes in the Boreal White and Black Spruce biogeoclimatic zone. Surficial deposits are mainly colluvium and moraine covered by a veneer of coarse colluvial debris. Humo-Ferric Podzols are the main soils on these sites, but Dystric Brunisols are associated with them due to locally dry environments produced by the combined influence of aspect and shallow colluvium. Other associated soils receiving considerable seepage water along the base of slopes are Humic Gleysols.

Since the vegetation consists primarily of white spruce the resulting litter is acidic and tends to accumulate as thin, distinct L and F horizons but thicker H horizons, as a result of the cold climate and slow rates of decomposition. With the exception of some soil creep and avalanche tracks on steep slopes, periglacial processes are virtually absent. However, relict periglacial features are common but have been covered by vegetation and are not readily visible.

The Subalpine Soil Landscapes

Subalpine vegetation occurs between elevations of 1350 m and 1800 m. There are extensive areas of subalpine fir in mountain valleys (notably on northern exposures) and willow and glandular birch on plateaus. The Humo-Ferric Podzol soil landscape is the most common throughout this subalpine area. Relict periglacial materials, including solifluction lobes, and other well to moderately well drained sites on colluvial and morainal parent materials, allow subalpine fir to grow in pure stands. Here, the brightly reddish brown podzolic B horizons occur under thin litter layers.

At higher elevations in the krummholz subzone, solifluction and nivation become active in the moist openings between the clumps of trees. On gentle slopes shallow nivation hollows have permanently frozen subsoils and shallow organic surfaces. Thus, in the upper subalpine area Turbic Cryosol landscapes occur with the Humo-Ferric Podzols.

Drier, more exposed plateau areas, such as the Spatsizi, are dominated by subalpine willow and glandular birch with occasional white spruce trees, and small openings occupied by alpine vegetation. Fire appears to be frequent here, and in combination with a cold relatively dry climate, it retards plant succession. The most common soil landscape is again that of the Humo-Ferric Podzol. However, as a result of slow decomposition and humification of litter from the deciduous shrub vegetation, a significant depth of organic matter accumulates at the soil surface. The Podzolic soils are best developed along well drained portions of glacial and relict periglacial features where shrub vegetation is present. In open depressions along gentle slopes, on the leeward sides of ridges, the vegetation ranges from stable alpine plants to an increasingly sparse cover as periglacial activity increases. Soil development is determined by the severity of the dominant geomorphic processes and other soil landscapes include Humic Regosols and Melanic Brunisols.

The Alpine Soil Landscapes

The alpine tundra occurs above 1500-1800 m. It has a very cold subarctic soil climate with a subhumid moisture regime. Within this environment windswept ridge-tops are often desiccated, because there is a lack of snow cover in winter and relatively dry conditions during the short growing season.

Cryoturbation is very active, churning the soil materials and developing patterned ground features including block fields, sorted and unsorted circles, and stone stripes. Regosol landscapes are the most common with disrupted horizons resulting from this strong periglacial activity. Constant churning of surface and subsoil particles results in poor horizon differentiation and the prevalence of buried and fractured bands of organic-rich materials resembling former A horizons. Parent material, commonly derived from local bedrock, is mixed directly with the A horizon resulting in an AC horizon sequence.

Turbic Cryosol landscapes with permanently frozen horizons are found on fine textured materials, such as thin moraine, where subsurface drainage is impeded by topographic location. Patterned ground features associated with these soils are prevalent but not as conspicuous as they are with adjacent shallow colluvial soils. However, on sloping landscapes, features associated with solifluction and nivational activity are enhanced by the better water holding capacity of the fine textured parent materials. Such sites form a Melanic Brunisol landscape marked by disrupted and buried horizons. Ah horizons attain greatest thickness along the downslope edges of solifluction lobes with buried organic horizons often occurring under the solifluction material. The surface horizon thins from the front of one lobe to the leading edge of the next lobe upslope. Permanent and intermittent snow patches occupying nivation hollows provide a continuous supply of melt-water to solifluction lobes lying immediately downslope. Sometimes melting snow patches reveal that the bottom of nivation hollows are bare of vegetation, creating an environment in which patterned ground is readily formed. Regosols with disrupted horizons predominate in these hollows. Gleying is usually absent since aerated seepage water percolates through soil parent materials throughout all or most of the growing season. The topographic position of these alpine soil landscapes in addition to some of the others is shown in Plate 3.6.1.

In addition to the generalized vertical zonation of soil landscapes described in the previous paragraphs there are numerous other variations. A few of the more important are described here.

Cold Valley Floors

The most extensive of the variations that occur throughout the region is the phenomenon that can best be described as "the inverted alpine-subalpine valley". Narrow and broad upland valleys with limited air drainage form collecting basins for extremely cold air flowing southward across the plateaus and mountains from the arctic or merely flowing down from the cold adjacent uplands. The cold air trapped in the lowlands forms a strong atmospheric inversion and thus induces periglacial activity, growth of permafrost and delays snowmelt along the valley bottoms. The floors of the valleys are occupied by vegetation that is normally considered alpine or subalpine, although the normal vertical zonation from subalpine fir to willow and glandular birch to alpine tundra occurs on the slopes above.

Plate 3.6.1

Plate 3.6.1
Soil Landscapes on the Klastline Plateau Looking Southwest From the Stikine River

  1. Alpine Tundra soil landscape. Regosols with disrupted horizons. Unit comprised of cirques with active alpine glaciation and nivation patches. A large percentage of this mountainous upland unit is dominated by bare rock, talus and other products of active periglacial processes.
  2. Alpine Tundra soil landscape. Melanic Brunisol forms the most common landscape on morainal blankets or morainal veneer. Cryosols may occur in depressions.
  3. Subalpine soil landscapes (willow and glandular birch). Humo-Ferric Podzol is most common soil landscape. Moist depressions are either Humic Gleysol or Cryosols.
  4. Humo-Ferric Podzol soil landscape on coarse textured colluvial (steep slopes) and fluvial materials. Humic Gleysols occur in depressions.
  5. Aspect as factor of soil formation. Shallow colluvial materials accentuate the effects of aspect. Soils range from dry Dystric Brunisol to moister Humo-Ferric Podzol.
  6. Humo-Ferric Podzol and Gray Luvisol Landscapes. Deep post-Pleistocene colluvial fan material overlying and dissecting the morainal blanket. Within the Boreal White and Black Spruce biogeoclimatic zone Humo-Ferric Podzols occur on coarse parent materials and Gray Luvisols on fine textured substrate.
  7. Humo-Ferric Podzol soil landscape occurring within the Boreal White and Black Spruce biogeoclimatic zone on fluvial and fluvioglacial deposits.
  8. Subalpine soil landscape. Gently sloping areas of moraine extend from subalpine conditions to lower elevations. Soil landscapes are a complex of Humo-Ferric Podzols, Humic Gleysols and Organic Fibrisols. Sporadic permafrost probably occurs.

Soil landscapes are varied and reflect the harsh characteristics of the alpine micro-environment. On well to rapidly drained coarse textured fluvioglacial deposits, deep Ah horizons develop under a dominantly grass vegetation comprised of alpine fescue producing Black Chernozemic soil landscapes. On finer textured colluvial and fluvial parent materials, Melanic and Sombric Brunisol landscapes predominate but on well drained medium textured moraine deposits under the subalpine vegetation in the broader upland valleys, Humo-Ferric Podzol soil landscapes occur.

Valley Asymmetry

As discussed above, the north-northwest orientation of many major valleys and their asymmetrical transverse profiles profoundly influence the distribution and operation of the geomorphic processes. The asymmetry and the strong orientation of the valleys is a result of glacially modified bedrock structure. The gentler west-southwest-facing valleysides are commonly dip-slopes and, since the slopes are gentle, nivation, solifluction and deepseated rockslides are dominant subaerial processes. The steeper east-northeast-facing slopes are escarpments eroded through the bedding planes; on these slopes snow-avalanches and rock-falls are common.

Vegetation is also affected by the orientation of the valleysides. The vertical zonation from subalpine fir through willow and glandular birch to alpine tundra is most common on north-facing slopes. These zones are often dissected by avalanche tracks. However, south-facing slopes may have only a sparsely treed subalpine fir zone, or the subalpine and alpine vegetation of the cold valley bottoms may continue up the valleyside to the mountain tops.

Soil landscapes follow the pattern described earlier for these environments with the addition of soils characteristic of the snow and debris avalanche tracks. The latter are most commonly Melanic Brunisols with deep Ah horizons developed under lush vegetation. These landscapes are associated with the Humo-Ferric Podzol soil landscapes where they extend in forested areas,

Warm Coast Valleys

In the western part of the region where major river systems breach the Coast Mountain barrier, a small but significant micro-environment exists. Warm, moist air from the Pacific Ocean fingers up the Stikine and Nass Rivers moderating the predominantly continental climate of the region. The extent of this influence diminishes rapidly eastwards and with increasing elevation.

At Telegraph Creek (300 m a.s.l.) on the Stikine River, other factors such as steep south-facing slopes, coarse textured, basic colluvial and fluvial soil parent materials, and copious amounts of coarse grained volcanic ash deposits from Mt. Edziza combine with the warmer climate to produce a Eutric Brunisol landscape with minor inclusions of Dark Gray Chernozemic soil landscapes. As a result of the microclimate and the soils a variety of agricultural crops can be grown.

Further south, the Nass River and its major tributary, the Bell-Irving River provide a similar opportunity for warm, moisture-laden air flow to penetrate the Coast Mountains. Here, however, the Skeena Mountains form a northern and eastern barrier to the air-flow thus intensifying the orographic effect and resulting in heavy annual precipitation including deep snow accumulations. The mountain systems also block out arctic air from the north, to produce a markedly different environment from that along the Stikine River. Indeed, vegetation is lush and dominated by conifers, including western hemlock, Sitka spruce and balsam fir in the lower zone with subalpine fir in the upper zone, giving way to extensive alpine tundra on the mountain tops. Ferro-Humic Podzol and Humo-Ferric Podzol soil landscapes dominate the lower forest zones with the varied soils common to the alpine occurring above tree-line.

The Soil Landscapes of Volcanic Terrain

Volcanic features ranging in age from 15 million years right up to 180 years dot the landscape in an area extending from the Mt. Edziza - Spectrum Range uplands to the western Spatsizi Plateau. Although most of the volcanic deposits have been modified by glaciation, some post Pleistocene volcanic activity has added volcanic ash and cinders, and subaerial lava flows to the original surface.

The Mt. Edziza plateau is flanked by recent volcanic ash deposits, fluvio-aeolian material, recent moraine, lava flows and cinder cones. Soil landscapes on these youthful materials are disrupted Regosols and Turbic Cryosols. In contrast, Level Mountain to the north of Mt. Edziza, takes the form of a large inverted saucer. A series of shallow ridges across the slope of the mountain obstructs lateral surface and subsurface drainage. This, along with the cold environment, results in poorly drained soil landscapes, mainly Organic and Humic Gleysols. At higher elevations where periglacial conditions prevail, soil landscapes include Organic Cryosols and Fibrisols. Turfy, moderately well drained solifluction lobes and hummocks permit development of Humo-Ferric Podzol soil landscapes within the unusually extensive subalpine area.

Land Use

Traditional resource activities such as hunter and guide outfitting and fur trapping are the most common. This pattern of use extends over the broad upland subalpine and alpine terrain where many species of wildlife, including caribou, mountain sheep, mountain goat, moose, grizzly and black bear and many smaller fur bearers, abound. Because of the remoteness of much of this region and because there are no alternative land uses over large areas, the importance of the wildlife resource cannot be overstated. However, regardless of the vastness of the area involved, the sensitivity of the terrain and its inherently low productivity of food and cover impose constraints on animal populations not encountered in southern regions thus demanding careful management.

Within the Gray Luvisol and Humo-Ferric Podzol soil landscapes there is a limited potential for forest harvesting. It is estimated that forest capabilities range from classes 4 to 7. The Nass River Basin, in the extreme southwestern corner of the region, is an exception and contains moderate and high capability forests especially at lower elevations in the Ferro-Humic Podzol soil landscape of the coastal environment.

Mineral potential in the region remains relatively unexploited with some notable exceptions such as the asbestos at Cassiar. Exploration activity is flourishing in areas of mineralization. Within the Skeena Mountains in the Ground Hog Range an extensive area of coal has been found but as yet has not been developed.

Transportation is provided by air, and by road. The Stewart-Cassiar Highway is currently being upgraded as an alternative route to the Yukon. It leaves Highway 16 at New Hazelton in the south and joins the Alaska Highway just west of Watson Lake in the Yukon Territory.