Ministry of Environment

SoilsSoils

3.5 The Columbia Mountains And The Southern Rockies

U. Wittneben and L. Lacelle

The Columbia and Southern Rocky mountains are in southeastern British Columbia, separated by the Rocky Mountain Trench. These highly faulted, folded and glaciated mountain ranges lie between the Interior Plateau to the west and the Great Plains to the east. The Columbia and Rocky Mountains trend in a northwest to southeast direction and have high relief, with rugged mountainous ranges separated by narrow valleys. The Columbia, Kootenay and Fraser Rivers and their tributaries, as well as several long, narrow lakes, occupy the floors of these valleys.

The Factors of Soil Formation

Since many of these mountain ranges rise to heights above 3000 m, they form barriers across the eastward movement of moist Pacific air masses. The forced rising of the air results in increased precipitation on western slopes and drier climates in the rainshadow of the mountains. Thus, major valleys such as the Rocky Mountain Trench are relatively dry, especially on their east-facing slopes. Abrupt changes in climate and relief mean that soil temperature and moisture regimes vary a great deal in this area. The drier main valleys have moderately cool boreal soil temperature classes while higher elevation valleys and mountain slopes have cold to very cold cryoboreal classes. Similarly, the dry main valleys have semiarid soil moisture regimes but higher elevation areas have humid to subhumid regimes. A variety of soil landscapes has developed in response to local climate effects.

The high relief, rugged topography and complex geology affects soil formation through the soil parent material found in differing topographic situations. Common parent materials include: rubbly deep and shallow colluvium, fine and coarse textured morainal materials (glacial till), and fine and coarse textured fluvial terraces, fans and floodplains. Remnants of silty lacustrine terraces occur in mountain valleys. Organic soil parent materials occur sparsely in localized depressional areas. In the higher mountain areas volcanic ash is an important component of the soil parent material. In the dry valleys thin cappings of wind-blown sands and silts are common on morainal and fluvial parent materials.

Soil and soil parent material textures bear a strong relationship to the types of bedrock from which they were derived. The common occurrence, especially in the Southern Rocky Mountains, of sedimentary limestones and dolomites results in the presence of large areas of soils developed on calcareous parent materials. Soft, readily weathered sedimentary and metamorphic rocks such as shales, siltstones and argillites result in the formation of fine textured soil parent materials, that are prone to mass wasting. Coarse textured, and more stable soils are derived from sandstone, quartzite and conglomerate bedrocks.

Figure 3.5.1

The variety in relief and climate in the Columbia and Southern Rocky Mountains results in the existence of a number of biogeoclimatic zones. The Interior Western Hemlock zone occurs on the mountain slopes receiving the heaviest precipitation. Acidic Dystric Brunisols and Podzolic soils develop in this zone. In the rainshadow of the mountains the semiarid Ponderosa Pine - Bunchgrass zone occurs. Grassland Chernozemic soils and Eutric and Dystric Brunisols commonly develop. At higher elevations the colder and wetter climate results in the occurrence of the Subalpine Engelmann spruce - Subalpine fir zone. Humo-Ferric Podzols develop under the dense forest canopy and the humid to subhumid soil moisture regimes in this zone. The highest mountain peaks reach the Alpine Tundra zone where severe climate plus variable relief result in the development of Regosols, Melanic and Sombric Brunisols, and Gleysols.

The Soil Landscapes

The soil map of British Columbia shows the distribution of many of the soil great groups in the area. Because the map is very general, some soil landscapes found in this region are not specifically identified, but are grouped with other soil great groups.

The Grassland Soil Landscapes

The Chernozemic soils of these landscapes occur in the dry valleys of the Grand Forks area, Creston Flats and the southern Rocky Mountain Trench (Figures 3.51, 3.52 and Plate 3.5.1). Because of their relatively gentle topography and surface horizons enriched with dark organic matter, these soils are often farmed. In addition, because they are open with grass cover, they are an important range area for cattle and big game animals.

Soil moisture classes are generally semiarid and the soil temperature regimes moderately cool boreal. The vegetation is an open grassland with scattered ponderosa pines.

The Dark Brown Chernozemic soil landscape is most common at lower valley elevations, on west and south aspects and on coarse textured fluvial and morainal deposits. Soil parent materials are often highly calcareous. The moderately high temperatures and low precipitation results in incomplete soil leaching and a somewhat hardened carbonate layer develops at a shallow depth. The pH of the soil is mostly alkaline and base saturation is high. The Ah horizon capping these soils is enriched with organic matter and well humified.

Dark Gray Chernozemic soil landscapes occur in the grassland - forest transition area and often grade into the Eutric Brunisol soil landscape. Black Chernozemic soil landscapes occur sporadically at higher elevations wherever soil moisture effectiveness is greater and the thick black Ah horizons characteristic of this landscape can develop.

Figure 3.5.2

The Eutric Brunisol Landscape

Eutric Brunisols are usually found in areas of greater precipitation and cooler temperatures than those of the grassland soil landscapes. Soil moisture classes are generally semiarid to subhumid and the soil temperature regime varies from moderately cool boreal to cold cryoboreal. In this soil climate Eutric Brunisols develop under fairly open stands of Rocky Mountain Douglas-fir and ponderosa pine. At higher elevations they are also found under more dense stands that include lodgepole pine and Engelmann spruce. Thus they occur within three biogeoclimatic zones; the Ponderosa Pine - Bunchgrass, the Interior Douglas-fir and the lower part of the Subalpine Engelmann Spruce - Subalpine Fir.

The Eutric Brunisol landscape is common in the relatively dry rainshadow valleys such as the Rocky Mountain Trench. It occurs in the valley bottoms and on lower mountain slopes in areas with calcareous parent materials (Figures 3.5.1, 3.5.2 and Plate 3.5.1). It is common on coarse textured parent materials and may grade into the Gray Luvisol landscape in areas where fine textured materials occur. Coarse textured fluvial, morainal and colluvial deposits derived from limestones and dolomites commonly have Eutric Brunisol soil developments. Sandy or silty eolian cappings are frequent on morainal and fluvial materials. Volcanic ash may make up part of the solum.

Eutric Brunisols in the region are shallow with a hardened carbonate layer, having developed from calcareous parent materials. Surface organic layers (L-F) are thin and poorly incorporated with the mineral soil. Leached Ae horizons or organic Ah horizons are usually thin or absent and the Bm horizons are pale brown and usually very shallow. The pH of the soil is generally above 6.0, and that of the parent material is usually greater than 7.0. These soils are either too coarse, or the soil moisture regime is too dry, for significant leaching of clays to have taken place.

Plate 3.5.1

Plate 3.5.1
Soil Landscapes in the Southern Rocky Mountain Trench

  1. Floodplain soil landscapes with Regosols and Gleysols on active silty to gravelly floodplain areas. Vegetation varies from aspen groves to grassland.
  2. Grassland soil landscapes with Dark Brown Chernozemic soils on open grassy areas and Eutric Brunisols in area- with open forests of ponderosa pine and Rocky Mountain Douglas-fir. Parent materials are mostly fluvioglacial gravels and morainal deposits.
  3. Eutric Brunisol landscape with an open canopy stand of ponderosa pine and Rocky Mountain Douglas-fir. Dark brown Chernozemic soils may occur on shallow coarse textured deposits or on south and west aspects. Parent materials include morainal, fluvioglacial and colluvial deposits.
  4. Eutric Brunisol landscape on the lower mountain slope- with a Rocky Mountain Douglas-fir forest and colluvial parent materials.
  5. Humo-Ferric Podzol soil landscape with closed canopy stands of Engelmann spruce and subalpine fir. Shallow and deep colluvium are the most common parent materials.
  6. Alpine soil landscapes with steep bedrock and Regosolic soils on shallow colluvial materials. These mountain peaks have little or no vegetation cover.

The Dystric Brunisol Landscape

Dystric Brunisols may be found in the same topographic position as Eutric Brunisols but where the parent materials are non-calcareous. More commonly, however, they occur in an elevational sequence between the Eutric Brunisol landscape in dry valley bottoms and the Humo-Ferric Podzol soil landscape at higher elevations (Figure 3.5.2). Dystric Brunisols are better for forestry than are Eutric Brunisols because of their non-calcareous parent materials and greater soil moisture. Being at middle elevations and on steep terrains, they are not often used for agriculture. They are common in the Columbia Mountains in areas with acidic parent materials or where precipitation is sufficient to leach carbonates out of the soil.

Soil moisture classes are subhumid to humid and soil temperature regimes are moderately cold to cold cryoboreal. Characteristic tree species in this environment include: Rocky Mountain Douglas-fir, western red cedar, western hemlock and Engelmann spruce. Dystric Brunisols occur in the Interior Douglas-fir, the Interior Western Hemlock and the Subalpine Engelmann Spruce - Subalpine Fir biogeoclimatic zones.

Soil parent materials are medium to coarse colluvial and fluvial deposits derived from non-calcareous sandstones, quartzites and argillites. Volcanic ash is often a component of the topsoil. In areas of fine textured materials the Dystric Brunisol landscape grades into the Gray Luvisol landscape. At high elevations or in wet areas, it is associated with the Humo-Ferric Podzol soil landscape.

These soils have brown Bm horizons that are more reddish than the Bm horizons of Eutric Brunisols. The whole soil is much deeper and the carbonate layer is absent. Surface litter layers (L-F-H) are shallow and poorly incorporated in the mineral soil. An eluviated Ae horizon may or may not be present. The pH of the soil is acidic (4.5-5.5) and compared with Eutric Brunisols and Chernozemic soils, the base status is low.

The Gray Luvisol Landscape

This soil landscape frequently occurs on finer textured parent materials than the Eutric or Dystric Brunisol landscapes. In this region it is found in a broad elevational range from valley bottoms to high on the mountain slopes (see Figure 3.5.1). It is one of the most common soil landscapes in the region. The fine texture of the parent materials and their instability when wet must be considered when planning logging, road construction, agricultural or urban land uses.

Soil moisture conditions vary from subhumid to humid and soil temperature regimes from moderately cool boreal to very cold cryoboreal. At low elevations this soil landscape is associated with the Eutric Brunisol landscape and at high elevations with the Dystric Brunisol or the Humo-Ferric Podzol soil landscape Because it occurs over a broad elevational range, its vegetation is variable Tree species range from ponderosa pine at low elevations, through Rocky Mountain Douglas-fir, western red cedar, western hemlock and Engelmann spruce at middle elevations, to subalpine fir at high elevations. Therefore, this soil landscape can occur in the Interior Douglas-fir, the interior Western Hemlock and the Subalpine Engelmann Spruce - Subalpine Fir biogeoclimatic zones (Figure 3.5.1).

The soil parent materials are fine textured morainal, fluvial and lacustrine deposits, generally derived from readily weathered siltstones, shales and argillites.

In the Southern Rocky Mountains Gray Luvisols develop on calcareous parent materials. They have a relatively shallow clay-enriched Bt horizon, in which carbonates are often still present. The blocky structure and clay skins that are characteristic of Bt horizons are not always strongly developed. In fact, the structure of the parent Ck horizon often closely resembles that of the Bt horizon.

In the Columbia Mountains Gray Luvisols develop on non-calcareous parent materials which often contain wind blown silt or volcanic ash. They have deeper profiles than those developed from calcareous parent material. However, even these non-calcareous soils often do not have strongly structured Bt horizons with prominent clay skins. Base saturation, pH and cation exchange capacity are lower in the Columbia Mountain Gray Luvisols.

The Humo-Ferric Podzol Soil Landscape

This soil landscape is found at high elevations (1000-2300 m). In an elevational sequence, they are found above the Dystric Brunisol landscape and below the alpine soil landscapes (Figure 3.5.1 and Plate 3.5.2). They occupy large areas in the Columbia Mountains, especially in the "interior wet belt". With their increased moisture and cooler climate these soils are among the most productive in the whole region for tree growth.

Soil moisture classes are humid to subhumid while soil temperature regimes range from cold to very cold cryoboreal. In general, the climate of this soil landscape is much colder and wetter than that of the Eutric Brunisol landscapes.

Plate 3.5.2

Plate 3.5.2
Soil Landscapes in the Columbia Mountains

  1. Floodplain soil landscapes with Regosols and Gleysols on active, gravelly floodplain and fan materials. The vegetation cover includes western red cedar, western hemlock and white spruce.
  2. Humo-Ferric Podzol is the dominant soil landscape from valley bottom to alpine in this part of the "interior wet belt" in the Columbia Mountains. Dense stands of western hemlock, western red cedar, Engelmann spruce and subalpine fir on colluvial, morainal and fluvial parent materials are characteristic.
  3. Alpine soil landscapes with steep bedrock, shallow Regosols, Brunisols, Gleysols, and Podzolic soils on colluvial materials. Vegetation cover is limited to grasses, sedges, forbs and dwarf shrubs.

Humo-Ferric Podzols develop under dense tree stands that include Engelmann spruce, subalpine fir, western red cedar and western hemlock in the Interior Western Hemlock and Subalpine Engelmann Spruce - Subalpine Fir biogeoclimatic zones (Figure 3.5.1 and Plate 3.5.2).

They develop on medium to coarse textured morainal, colluvial and fluvial parent materials derived from non-calcareous bedrock such as quartzite, sandstone and argillite. Even on calcareous parent materials, the soil leaching in this environment is sufficient to allow shallow podzols to develop. The topsoil often contains considerable volcanic ash.

The dense coniferous forest cover, plus the colder climate at this elevation result in the build-up of thick organic surface horizons. This material is acidic, relatively undecomposed and poorly incorporated with the mineral soil. Eluviated Ae horizons are present and podzolic Bf horizons are readily evident, but these are not so red as those on the coast. Heavy leaching ensures that the soils are carbonate free even on calcareous parent materials. By comparison with the lower elevation Eutric Brunisols, the pH, base status and cation exchange capacities are low.

At higher elevations, or in wetter areas, considerable amounts of organic matter can accumulate in the B horizon of podzols, giving the horizon a darker colour (Bhf). These soils are called Ferro-Humic Podzols and are found in the transitional krummholz area between the Humo-Ferric Podzol soil landscape and the alpine soil landscapes.

The Alpine Soil Landscape

Found at the highest elevations (2300-3600 m) in the Columbia and Southern Rocky Mountains (Figure 3.5.1 and Plates 3.5.1 and 3.5.2), these soil landscapes are portrayed on the soil map of British Columbia as containing dominantly lithic soils. Since they are rugged and have panoramic scenic views, the alpine soil landscapes have important potential for recreational uses.

Soil moisture classes at alpine elevations are generally humid while soil temperature regimes are very cold cryoboreal. In general the climate at these elevations is very severe and growing seasons very short. The harsh climate limits vegetation cover to non-forest species including alpine grasses, sedges, lupine and willows. Trees do not occur in the Alpine Tundra biogeoclimatic zone.

A variety of soils occur as a result of variations in topography, local climate, parent materials and drainage. Because of active processes such as colluviation, cryoturbation and solifluction, Regosols with disrupted horizons are common. Areas with impeded drainage have Gleysolic soils while under alpine grass vegetation Melanic and Sombric Brunisols with turfy Ah horizons develop. Because of the steepness of the topography, exposed bedrock or unconsolidated rock slopes make up much of the land surface.

Soil parent materials are rubbly colluvium of varying depth, and youthful morainal deposits associated with recent deglaciation. Volcanic ash is commonly found in the alpine soils surface horizons. Soil developments in the alpine soil landscape are often obscured because of the activity of geological processes in the landscape. Surface and subsurface soil horizons are often churned and intermixed due to cryoturbation and solifluction.

The Floodplain Soil Landscapes

The largest areas of floodplain soil landscapes occurs in the major intermountain valley bottoms such as the upper Columbia River and the Creston flats (Figure 3.5.1 and Plates 3.5.1 and 3.5.2). Regosolic, Gleysolic and Organic soils are common. These floodplain soils are often used for agriculture because they are flat and accessible. Marshy areas on the floodplains are important habitat for waterfowl.

Soil moisture classes are generally subaquic to peraquic on floodplains while soil temperature regimes resemble those of other soil landscapes in the valley. On these wet soils the vegetation consists primarily of water tolerant species such as grasses, sedges, trembling aspen, black cottonwood and Engelmann spruce.

Because of regular flooding, floodplain soils are commonly Regosols with some mottling in lower horizons. Gleysols, Humic Gleysols and Organics are common in areas that are more or less continuously wet. In the major valleys floodplain soil textures are silty. In the narrow, steep, high elevation valleys, floodplain soil textures are often gravelly and sandy. The texture of floodplain soils depends on the velocity of the river and the type of bedrock in the upper drainage basin.

In the Rocky Mountains and in major valleys floodplain soils are generally calcareous. Soil development is often limited to surface organic matter accumulation, gleying and mottling.