Ministry of Environment


3.4 The Interior Plateau

K.W.G. Valentine and A.B. Dawson

The interior plateau occupies the whole of the southern and central part of the province lying between the coast mountains to the west and the Columbia and Rocky Mountains to the east. It is a very large area approximately 900 km in length and 376 km at its maximum width. Almost all of it is drained by the Fraser River or its tributaries.

The features of the climate, geology and vegetation have been described in Parts 1.2, 1.3 and 1.4. These "factors of soil formation" have combined to give a number of highly distinctive soil landscapes, whose principal features are well known. The terms Okanagan, Cariboo, Chilcotin or Blackwater evoke an immediate picture of distinctive types of country. The main aspects of the environment that have produced the soils of these landscapes are discussed in the next few paragraphs.

The Factors of Soil Formation

The interior plateau lies in the rain shadow of the coast mountains. Precipitation is therefore low in the west and south and increases eastward towards the Quesnel, Shuswap and Okanagan highlands and northward to Prince George. Effective precipitation is influenced by temperature through evaporation rates. Mean annual temperature decreases from south to north and with any increase in elevation. Most of the soils are frozen in the winter so it is the temperature and precipitation regimes through the spring, summer and fall that influence soil development. Soil temperature classes vary a great deal. Southern valleys around Osoyoos or Ashcroft are classed as moderately cool boreal with hot summers, whereas the plateau northwest of Prince George falls in the cold cryoboreal class. The same areas have soil moisture regimes that range from subarid with a severe moisture deficit to humid with only a slight moisture deficit, respectively.

Most of the plateau lies between 1200 and 1500 m above sea level. The topography is rolling with few high peaks. The main types of soil are homogeneous over large areas. The rugged topography in the east and the steep sides of some large valleys produce colluvial soil parent materials and sharp changes in soil types due either to the influence of elevation or aspect.

The basic basalt lavas of the centre and north contribute to the high base status of many soils. In the south and east the rocks are much more varied. The volcanics and limestones provide base-rich parent materials in parts of the Quesnel Highlands, while metamorphic rocks such as quartzites and gneiss give more acidic parent materials in the Shuswap Highlands.

Most of the parent materials are unconsolidated materials which were deposited during the last phase of glaciation and the subsequent wasting of the ice, or during post glacial time. Most of the plateau surface and the mountain slopes are covered with glacial till which usually bears a close association mineralogically with the underlying bedrock. Drumlins and glacial grooves are common and there are large areas of hummocky till where the ice stagnated and melted. The plateau surface therefore is rolling with many enclosed depressions. These depressions contain organic soils in the cooler and wetter north and often saline soils in the hotter and drier south. As the ice melted a large amount of water was released and often accumulated as huge lakes impounded by temporary ice dams. Throughout the interior plateau there are the remnants of lacustrine basins with stratified silty and clayey soils. The largest are around Fort St. James, Vanderhoof, Prince George, Kamloops and in the Okanagan valley. Very coarse materials were deposited by the meltwaters before they reached the lakes. So in many valleys or on the lower parts of the plateau there are gravelly and sandy terraces, plains, esker complexes and occasionally large deltas. All these deposits have thin droughty soils. Lastly, over much of the land, the wind deposited a thin layer of silt and fine sand. This material probably originated from the exposed shores of lakes or river floodplains, but some is volcanic in origin being derived from the Mazama, St. Helens Y and Bridge River ash falls. This volcanic ash can become quite thick close to the source (i.e. Bridge River).

There are a number of quite different biogeoclimatic zones in this large and varied plateau country. At the lowest elevations of the southern valleys are the open grasslands and sparse trees of the Ponderosa Pine-Bunchgrass zone. At higher elevations and further north is the Interior Douglas-fir zone. The parkland of the Cariboo Aspen - Lodgepole Pine zone is found from Clinton to the Bulkley River, and north of this is the Sub-Boreal Spruce zone. Throughout the region the Engelmann Spruce - Subalpine Fir zone occurs above about 1200 m and on the high mountain tops is the Alpine Tundra zone.

The Soil Landscapes

The soil map of British Columbia (Part 3.2) shows the distribution of the main soil great groups in the interior plateau. The features of each great group landscape are described on the following pages. The map is very general and cannot show some of the soil landscapes which may cover small areas in total but are nonetheless important members of the whole soil picture. The organic, saline and alpine soils are examples. The geographical relationships of all the soil landscapes are shown by two east-west cross sections; one across the humid northern portion of the plateau (Figure 3.4.1) and one across the south (Figure 3.4.2).

Figure 3.4.1

The soil landscapes of the northern Interior Plateau

The Gray Luvisol Landscape

This is by far the most extensive of all the soil landscapes. In the south Gray Luvisols occur on the medium to moderately fine textured glacial till of the plateau slopes. Most of the till is calcareous being derived from base rich volcanics or limestone, but some in the east is more acid. The vegetation is a moderately dense Rocky Mountain Douglas-fir forest with bluebunch wheat grass and pine grass. The elevation range of these soils in the south can extend from about 1000 m to over 1800 m above sea level. Further north the elevation range becomes lower because the cooler temperatures and higher precipitation result in greater moisture effectiveness at lower elevations. Thus in the Williams Lake area Gray Luvisols are found on the calcareous till of the plateau and on the calcareous lacustrine silts in the valleys down to 800 m above sea level. In the Fort St. James and Vanderhoof area Gray Luvisols occur down to about 600 m on the lacustrine silts and do not extend much above 1100 m on the till of the plateau. Here the trees are mainly white spruce and lodgepole pine. Gray Luvisols rarely occur on colluvium, possibly because the material is too unstable for a definite horizon of clay accumulation to be formed.

The vegetation cover does not add a very large amount of organic matter to the soil so that the surface litter layer of needles and grass (L, F horizons) is thin and poorly decomposed. The uppermost mineral horizon is typically 10-25 cm of grayish brown silt and fine sand (Ae horizon), much of which is wind blown loess and volcanic ash. Below this there is a much finer textured layer 15-30 cm thick (Bt horizon) which has strong angular blocky structure. This horizon, in which clay has accumulated, tends to restrict water percolation, so that these soils can become extremely wet in the spring. The carbonates of the original parent material have been washed out of the A and B horizons, but the intensity of leaching is not great enough to wash them through the C horizon and away via the groundwater. Therefore, there is often a white powdery layer of carbonate accumulation at depth to give a Cca horizon. In the semiarid south this accumulation will be less than one metre below the surface. In the humid north it may be as deep as two metres.

These are the soil landscapes that support much of the extensive logging industry of the interior plateau. Timber productivity is especially high in humid areas bordering the Humo-Ferric Podzol landscapes. In the subhumid and semiarid areas associated with the Brunisolic landscapes the forest is more open and provides important summer grazing for the interior cattle ranching industry.

Figure 3.4.2

The Humo-Ferric Podzol Soil Landscapes

In broad terms this soil landscape borders the Gray Luvisols on the higher elevations of the Nechako, McGregor, Fraser and Thompson plateaus and the Quesnel, Shuswap and Okanagan highlands. The soil temperature class is mainly cold cryoboreal and the moisture regimes are humid to subhumid. The soil parent materials are usually coarse to moderately coarse textured very permeable glacial till or colluvium derived mainly from igneous and metamorphic rocks such as granodiorites, gneisses and schists. In the more humid northern portion similar soils also occur at much lower elevations on very coarse textured fluvioglacial materials. The vegetation is usually a dense subalpine fir and Engelmann spruce forest with moderately dense shrub, forb and moss layers. It is the basis for the logging industry of the central interior. In the south this soil landscape occurs above about 1200 m but lower on northerly aspects. In the north its elevation range is approximately from 750 m to 1500 m above sea level. Plate 3.4.1 illustrates the occurrence of this soil landscape in the Quesnel Highlands.

The dense vegetation cover provides a thick organic surface litter of needles, twigs and moss (L-F-H horizons) which is acid and poorly decomposed. Below this usually occurs a bleached sandy Ae horizon and then a reddish brown Bf horizon in which iron, aluminum and some amorphous organic matter has accumulated. Leaching of bases is intense in these soils and there is no trace of carbonates throughout the profile.

On its drier and warmer margins the Humo-Ferric Podzol soil landscape borders the Gray Luvisol and Dystric Brunisol landscapes. At higher elevations it extends into the subalpine from continuous tree line to the limit of tree growth. Here there is an accumulation of amorphous organic matter in the surface mineral horizons, a significant accumulation of volcanic ash in the topsoil and a shrub cover of grouseberry. In the extreme south its upper limit is about 2100 m above sea level.

The Dystric Brunizol and Eutric Brunisol Landscapes

These soil landscapes occur in a wide variety of environments. They may be found in areas of low precipitation, high temperatures and coarse textured acidic or calcareous parent materials. Whatever the particular conditions are in one place they lead to soils whose main feature is in situ weathering with little translocation of soil materials. Morphologically, Dystric and Eutric Brunisols are quite similar. Their difference lies in precipitation effectiveness. The Eutric Brunisol landscape has the lower effective precipitation; there is less leaching of bases than in the Dystric Brunisol landscape and it has higher pH values and higher base saturation.

The soil map (Part 3.2) shows that the Dystric Brunisol is the most common. It occurs mainly in the southern portion of the interior plateau. The parent materials are acid till and colluvium on the plateau and mountain slopes, with a forest cover of Rocky Mountain Douglas-fir or Engelmann spruce. There are some expanses in the northern portion either because the parent material is not calcareous or fine textured enough for Gray Luvisols to develop, or because leaching is too efficient for Eutric Brunisols to develop. Thus, north and west of Prince George, Dystric Brunisols occur on fluvioglacial gravels or on neutral and shallow glacial till and colluvium over bedrock, and southwest of Prince George they occur in the humid Blackwater River country.

The Eutric Brunisol landscape is mainly in the southern valleys. The most common parent materials are slightly alkaline fluvioglacial sands and gravels. The vegetation is an open forest of ponderosa pine or Rocky Mountain Douglas-fir with bluebunch wheat grass or lodgepole pine and pine grass.

The soils of both landscapes have relatively thin, poorly decomposed organic surface layers, little incorporation of the organic matter in the mineral soil and brown Bm horizons. The Eutric Brunisols have pH values above 5.5, high base saturation and may have white powdery calcium carbonate accumulations less than one metre below the surface. They are associated with Chernozemic and Gray Luvisol landscapes. The Dystric Brunisols have pH values below 5.5 and calcium carbonate accumulations usually do not occur within one metre from the surface. They are associated with Gray Luvisol and Humo-Ferric Podzol soil landscapes.

The Grassland Soil Landscapes

The grasslands of the central interior have Brown, Dark Brown, Black and Dark Gray Chernozemic soils and some small exposures of Solonetzic soils. They are a very well known part of the British Columbia scene. The cattle ranching industry depends on them for much of its grazing land. Most of the orchards of the southern valleys have been developed on them under irrigation. They also form an integral part of the parkland scenery with lakes, trembling aspen and coniferous trees that attracts thousands of campers, fishermen and hunters each year.

The Brown, Dark Brown and Black Chernozemic soil landscapes form a sequence from drier to moister grassland environments. The Dark Gray Chernozemic soil landscape is a transition zone between the grassland Dark Brown or Black Chernozemic soils and the forest Luvisolic or Brunisolic soils. Solonetzic soils can be associated with any of the Chernozemic soils wherever restricted drainage and high evapotranspiration rates lead to the accumulation of soluble salts.

Plate 3.4.1

Plate 3.4.1
Soil Landscapes in the Quesnel Highlands

  1. Humo-Ferric Podzol soil landscape on gravelly moderately coarse glacial till derived from quartz-mica schist and quartzite. Bedrock is close to the surface in some areas. The vegetation is the interior western hemlock forest.
  2. Humo-Ferric Podzol soil landscape on gravelly moderately coarse textured colluvium and glacial till derived from quartz-mica schist and quartzite. Bedrock is close to the surface in many areas. The vegetation is the Engelmann spruce - subalpine fir forest.
  3. Alpine soil landscapes on acid metamorphic bedrock (quartz-mica schist and quartzite). Mainly Dystric and Sombric Brunisol and cryoturbated Humic Regosol landscapes. Treeless alpine tundra vegetation.
  4. Alpine soil landscapes on massive marble bedrock. Mainly Melanic Brunisol and cryoturbated Humic Regosol landscapes. Treeless alpine tundra vegetation.

There are three main areas of the Brown Chernozemic soil landscape: the southern Okanagan valley, the valley of the Thompson near Kamloops and the Fraser valley near Lillooet. All have semiarid to subarid moisture regimes and the soil temperature class is moderately cool boreal. This is the driest and warmest environment occupied by any of the soil landscapes. It occurs at the low elevations of the terraced river valleys (below about 600 m at Kamloops) and especially on south facing aspects where evapotranspiration is highest. The soil parent materials are typically lacustrine silts, fluvial sands and gravels and glacial tills that occupy the valley bottoms. In such a dry environment the vegetation is sparse; mainly big basin sagebrush, brittle prickly pear cactus and bluebunch wheat grass.

There is little or no surface litter layer on these soils. The uppermost horizon is usually a light brown friable sandy loam 15 cm deep with quite a high organic matter content and granular structure (Ah horizon). Below this will be about 25 cm of a brown Bm horizon. Carbonates are precipitated at a shallow depth in such a dry soil to give a Cca horizon. Soil reaction will be alkaline with pH values approaching or exceeding 8. Base saturation will be at or near 100%, and the organic matter is well humified giving C/N ratios of 10 or less.

The Dark Brown soil landscape is found in a slightly cooler and more moist environment than the Brown soil landscape. Hence it is either slightly further north or at a higher elevation. The three areas shown on the soil map are: the central Okanagan valley; Nicola Lake to around Kamloops; and near Clinton. The soil temperature classes are moderately cool to cool boreal and the moisture regime is semiarid. The range of soil parent materials is similar to that on which the Brown soils have formed. However, as the Dark Brown soil landscape occurs at higher elevations (from 600 m to 825 m near Kamloops) a greater proportion of these soils occur on the glacial till of the higher valley and lower plateau slopes. The vegetation is commonly a continuous grass cover dominated by needle-and-thread grass and Sandberg's blue grass.

These soils will have a similar horizon sequence to that of the Brown soils but the Ah horizon is a darker brown and all the horizons tend to be thicker. Their chemical characteristics are similar too, except for a higher organic carbon content in the Ah horizon.

The Black soil landscape lies at the upper elevations of the grasslands (from 825 m to 975 m near Kamloops). At this height on the plateau most of the parent material is glacial till. The soil temperature classes are moderately cool to cool boreal and the moisture regime is subhumid to semiarid. The Black soil landscapes have many more plant species, especially forbs, than the other two lower landscapes. The principal grass is rough fescue and clumps of trembling aspen are common. The horizon sequence is similar to the Brown and Dark Brown soils but the Ah horizon will be thicker with a darker colour and a greater organic carbon content, and carbonates will have been precipitated at a greater depth in the C horizon.

The grassland-forest transition of the Dark Gray soil landscape is the furthest north or highest of all the Chernozemic landscapes. It is found in the cool boreal and subhumid to semiarid zone around the junction of the Chilcotin and Fraser rivers and in the Nicola, Kamloops, Okanagan and Kettle valley areas. Soil parent materials are mainly medium to moderately fine textured glacial till, but they also include sandy and gravelly fluvioglacial deposits on many of the lower valley terraces. The vegetation is typically a continuous grass cover dominated by Kentucky blue grass with many tree islands of trembling aspen, lodgepole pine and Rocky Mountain Douglas-fir.

The morphology of the soils shows the transition to Luvisolic soils, since below the dark gray Ah horizon there is a horizon from which clay and organic matter has been leached (Ahe or Ae). Below this is a brown, blocky, finer textured horizon (Bm or Bt). Greater leaching is also evident in the greater depth to carbonate accumulation which is often more than one metre from the surface. Similarly pH values and base saturation percentages will be lower and C:N ratios higher than in the other Chernozemic soils.

Wherever high rates of evapotranspiration and restricted drainage occur together soluble salts will accumulate to produce a saline Solodized Solonetz or Solodic soil landscape. This can occur in association with any of the described Chernozemic soil landscapes. Sodium and magnesium salts dominate the soil chemistry.

If the soil drainage is restricted by an enclosed depression there may be a muddy saline slough in the centre. Surrounding this there is an area bare of vegetation which usually has a white salt efflorescence on the surface. Towards higher ground salt tolerant vegetation such as desert salt grass, alkali grass, or common silverweed gradually covers the surface, until the normal grass vegetation of the particular Chernozemic soil typical of the area is reached.

The sequence of all these grassland soil landscapes and their bordering forest soils is shown in Plate 3.4.2 which illustrates the country around Merritt.

Plate 3.4.2

Plate 3.4.2
Soil Landscapes on the Southern Plateau Near Merritt

  1. Humic Regosol and Humic Gleysol landscapes on the present river floodplain, with medium textured parent materials. The vegetation consists of sedges, and willows.
  2. Dark Brown soil landscape on lacustrine silts with grassland vegetation. There are some pockets of the Solodized Solonetz soil landscape with salt tolerant vegetation.
  3. Dark Brown soil landscape on gravelly medium textured glacial grassland vegetation.
  4. Black soil landscape on gravelly medium textured glacial till. The vegetation is mainly grassland with tree islands of trembling aspen and Rocky Mountain Douglas-fir.
  5. Eutric Brunisol landscape on gravelly medium textured glacial till and colluvium with an open Rocky Mountain Douglas-fir - pinegrass vegetation cover.
  6. Gray Luvisol landscape on gravelly medium textured glacial till. There is a relatively dense Rocky Mountain Douglas-fir - pinegrass vegetation cover.

The Organic Mesisol and Fibrisol Landscapes

Organic soils occur wherever soil drainage is very restricted in depressions or on floodplains throughout the humid to subhumid and cold to moderately cold cryoboreal portions of the interior plateau. In the south they are limited to the higher elevations of the plateau and to the low floodplains. They are not found in intermediate positions. Towards the north they are found at progressively lower elevations on the plateau. Thus they can be associated with all types of terrain and have been mapped on floodplains, on terraces, in lacustrine basins, on the plateau and into the alpine country.

The nature of these organic soils depends to a large extent on the quality of the water draining into them. On the calcareous till plateau the water is neutral to slightly alkaline and rich in nutrients. Sedges, bog glandular birch, willows and sometimes black spruce and lodgepole pine grow on them. The organic material is relatively well decomposed. The soil is a Mesisol. In some of the southeastern highlands and on the plateau northwest of Prince George the surrounding mineral landscape is more acid. There the organic soils have a lower pH, a lower nutrient status, support more mosses and fewer shrubs and trees and the organic materials are poorly decomposed. They are Fibrisols.

The Alpine Soil Landscapes

The alpine soil landscapes cover only a minor portion of the central interior. They often have large amounts of volcanic ash in their surface horizons and in this respect they are quite different from most of the other alpine soil landscapes of the province. The pattern of individual soils is exceedingly complicated. The topography, vegetation and meso- and macroclimates change drastically over short distances. In addition the geomorphological processes typical of cold environments often disrupt the soil forming processes. Only a few studies of these soils have been carried out but there seem to be two main soil landscapes depending primarily upon elevation and aspect. Above the limit of tree growth to about 2400 m in the south, acid Dystric and Sombric Brunisol landscapes are found with deep turfy topsoils which contain much volcanic ash. The vegetation is an alpine tundra plant community of grasses, sedges, willows and lupins. Between here and the highest rock peaks are the poorly developed stony soils that show extreme disruption due to cryoturbation and solifluction. They have patchy alpine tundra vegetation communities and are mainly Regosol and Humic Regosol landscapes.