Ministry of Environment

Soils

3.7 The Great Plains

A.J. Green and T.M. Lord

In British Columbia somewhat more than 10 percent of the province lies east of the Rocky Mountain Foothills and within the high plains division of the Great Plains of North America.

The term "Great Plains" brings to mind that part of Canada lying between the Rocky Mountain Foothills and the Canadian Shield, known as "the Prairies". However, no part of the British Columbia Great Plains is semiarid grassland, and only a relatively small portion can be described as subhumid aspen parkland. Most of the area is forested.

The region is drained by two major rivers and their tributaries, the Peace and the Liard. The Peace River is formed at the confluence of the Findlay and Parsnip Rivers which is now part of Lake Williston Reservoir. The Peace River leaves the lake near Hudson Hope, flowing to the east onto the plains, where it is joined by its tributaries the Halfway, Moberly, Pine, Beatton and Kiskatinaw Rivers. The Liard drains northwest with its major tributaries the Fort Nelson and Prophet Rivers. Most of these rivers originate in the Rocky Mountains and have cut deep postglacial valleys through the plains. Rivers such as the Chinchaga and Fontas which originate on the plains are not deeply entrenched and have meandering stream courses. There are very few large lakes in the region. The two largest are Moberly Lake in the south and Kotcho Lake in the north.

The Factors of Soil Formation

The geology, vegetation and climate combine to produce a soil forming environment that is different from the rest of the province in several ways.

The Cretaceous sedimentary bedrock has a great influence on the landscape. It is composed of sandstones and shales which are horizontally bedded or dip gently to give low asymmetrical ridges. The sandstone tends to form low rolling uplands. The more easily erodible shales usually underlie the flat lowlands. The sediments can be acid, neutral or alkaline and calcareous. Some marine shales of the Peace River Lowland are saline, others in the Fort Nelson Lowland are not.

The surficial deposits are closely related mineralogically to the bedrock, and this is reflected in the soils. The area was glaciated by Cordilleran ice from the west and Keewatin ice from the north east. The Cordilleran till is medium textured, stony, calcareous and often less than 3 m thick. The Keewatin till is fine to medium textured, has few stones and can be saline, slightly calcareous or acid. In the north around Fort Nelson it is quite thin and difficult to distinguish from the underlying bedrock. Lacustrine materials border many of the large river valleys. They are fine textured, have few stones and are generally saline and somewhat calcareous. The upper limit of these deposits in the Peace River lowland is at about 810 m, but they do occur in areas of local ponding up to 850 m. The large rivers flowing from the Rocky Mountains have carried coarse textured sediments out onto the plains. Sandy and gravelly fluvioglacial and fluvial deposits are found in and adjacent to the valleys in a broad zone bordering the Rocky Mountain Foothills. They have often been reworked by the wind into dunes and ridges. A rather different but important surficial material is the organic "muskeg" that blankets much of the plains in the northeast forming organic soils.

The important features of the climate that influence soil formation are the long cold winters that severely restrict many soil forming processes, the short warm summers and the moderate precipitation with a summer maximum, including many thunderstorms. Winter frost penetrates deeply into many soils and some organic bogs in the north have permafrost. Paradoxically, high rates of evapotranspiration lead to water deficits and very dry soils in late summer, especially in the lowlands.

The whole of this area lies within the Boreal White and Black Spruce biogeoclimatic zone. The lowlands have a mixedwood forest of trembling aspen, white spruce and black poplar. This adds a less acid litter to the soil surface than the coniferous forests of white spruce, lodgepole pine and subalpine fir of the uplands and the north. In the driest parts of the south there is an attractive open parkland of trembling aspen and grass much of which was probably burnt periodically. The very wet parts of the north have stunted black spruce, tamarack, willows, Labrador tea and mosses.

The shape or form of the land in the great plains is also important in determining soil quality. The influence of aspect is seen in many large river valleys. Dry, south facing slopes in the Peace River lowlands have dry soils with trembling aspen, open grassland and even plains prickly-pear. Cool, moist north facing slopes have white spruce. The combination of steep slopes and fine textured materials along many of the entrenched river valleys leads to very unstable soils and in extreme cases huge landslides, such as the Halfway slide that completely blocked the Peace River for 12 hours in May 1973. On the flatter plains and uplands the long smooth slopes of this plateau country are susceptible to sheet erosion and gullying. The combination of soft materials, slow percolation, long slopes and summer thundershowers makes any slope over 5% susceptible. Finally, there are peculiar topographic features, known as "humpies", bordering the main rivers and their tributaries in the Peace River lowlands (see Plate 3.7.1). They are earth mounds of variable composition and orientation up to 12 m high. They change rapidly in soil profile characteristics from their crest to their base.

The Soil Landscapes

The arrangement and distribution of the main soil great groups in the great plains is shown in the soil map of British Columbia (Part 3.2). A single soil great group landscape may predominate in one area as does for example the Gray Luvisol. However, combinations of soil landscapes are common in areas where slopes, vegetation and parent materials change within short distances. This sort of situation can be seen in Plate 3.7.1 around Fort St. John, and it is also typical of the large area mapped as Organic Cryosol-Luvic Gleysol in the northeast. Other soil landscapes are quite limited in extent, as is for instance the Solod soil landscape within the trembling aspen parkland of the Peace River lowland. The general pattern of soil landscapes is shown as a diagrammatic cross section in Figure 3.7.1 which runs from Fort Nelson to Dawson Creek.

The Gray Luvisol Landscape

The Gray Luvisol landscape occupies about 40 percent of the Great Plains and extends in a broad zone east of the Rocky Mountain Foothills from the southern Peace River lowlands to the Yukon border. It occurs under pure or mixed stands of coniferous and deciduous trees with a humid to subhumid cryoboreal soil climate. It has gently to strongly rolling topography on the till uplands and long, smooth, gentle slopes on the lacustrine plains. The soils on the till are well to moderately well drained but on the lacustrine plains, with subdued topography and fine textured layered sediments, they are wet for part of the year. This is shown by mottling and dull soil colours in the upper horizons. Typically, the Gray Luvisols have a thin layer of decaying leaves, needles and grasses accumulated on the soil surface; a gray or grayish brown, distinctly platy Ae horizon, 10-30 cm thick; and light brownish gray or dark grayish brown B horizons, 50-100 cm thick. The C horizon is often similar in texture and colour to the B horizons but lacks a well developed structure. The peds in the B horizons frequently have strong subangular blocky structure and clay films can be detected on the surface of the peds. The Bt horizon of the Gray Luvisols in the Great Plains is often much more difficult to detect visually and texturally than the Bt horizon of the Gray Luvisols in the Interior Plateau. The latter are markedly different in texture and colour from the horizons above and below them, whereas in the Great Plains the difference is one of soil structure.

Plate 3.7.1

Plate 3.7.1
Soil Landscapes in the Peace River Lowland Around Fort St. John

  1. Steep sides of river valleys entrenched into the plateau. Disrupted Regosols on the slumping fine textured shales and lacustrine sediments. The Peace River is 240 m below the plateau rim.
  2. Solod soil landscape on the lacustrine clay plain, with "humpy" topography near the breaks to the river valleys.
  3. Solod soil landscape on the gently undulating lacustrine clay plain. The Solods are on the well drained sites with Gray Luvisols in lower positions and organic Mesisols in the depressions - 3(0).
  4. Solod soil landscape on the gently to moderately sloping ridges of clay till over sandstone with trembling aspen and grass vegetation.
  5. Gray Luvisol landscape on clay till over sandstone under a mixed forest of trembling aspen and white spruce. It is about 90 m higher than the Solod soil landscape on similar till (4).

Figure 3.7.1

Although the Gray Luvisols of the Great Plains have developed under much the same forest cover and climate (see Figure 3.7.1), they do vary structurally and chemically because of the nature of their parent materials. Those soils that have developed on till, lacustrine or fluvial deposits derived from slightly calcareous, somewhat saline sandstones and shales usually have strongly developed columnar and subangular blocky structure in the B horizons. Those soils developed on deposits derived from acid or calcareous materials usually lack this strongly developed soil structure.

Where climate and topography are suitable, Gray Luvisols have been used for agriculture, particularly forages and seed production. They require the addition of nitrogen and organic matter. They also have an inherently weak structure in the upper horizons that makes them susceptible to puddling and crusting and to wind and water erosion. About 25 percent of them are moderately to strongly acid in reaction and they may need liming to counteract aluminum and manganese toxicity, particularly for such crops as alfalfa and barley.

The Luvic Gleysol Landscape

This soil landscape occurs on the wetter margins of the Gray Luvisol landscape. The moisture regime is humid for most of the year, but there are short periods of complete saturation (subaquic regime). The landscape is found most commonly on the northern border of the lowlands where fine textured, nearly impervious materials occupy poorly drained level to depressional areas with sedge, willow and other moisture loving forms of vegetation.

A typical Luvic Gleysol has a thin layer of raw organic litter on the soil surface; a bleached, gray, mottled, platy Aeg horizon, 5 to 10 cm thick; a dark gray, mottled Btg horizon with rounded or subangular blocky structure (when dry), 10 to 15 cm thick; and a mottled light olive brown C horizon.

Although saturated for short periods they are well enough aerated for clay translocation to have occurred. Since they occupy areas that receive runoff from surrounding higher areas, they usually have more bases than the Gray Luvisols. However, like the Gray Luvisols they need the addition of nitrogen and organic matter for crop production. Poor natural drainage and the promotion of poor physical condition if worked when too wet are also important factors limiting the agricultural productivity of Luvic Gleysols.

The Organic Soil Landscapes

The Fibrisols and Mesisols that make up these landscapes occupy extensive tracts of level or depressional ground in the Peace River and Fort Nelson lowlands as wetter associates of the Luvic Gleysols. A typical portion is illustrated in Plate 3.7.2. They are saturated for moderately long periods of time (aquic regime), and organic matter accumulates on the surface of the mineral soil. These landscapes (with the Organic Cryosol discussed below) are the "muskeg" that is such a fascinating but perplexing part of northern Canada. It has been perplexing mainly to engineers who have had to build roads, railways or pipelines through this very wet, very cold and often very unstable land. The organic surface is usually quite thin, commonly less than 1 m. But it acts as a huge sponge that regulates stream flow by holding and slowly transmitting vast quantities of water. If the sponge is disturbed by route construction the water may be released quickly to cause extensive erosion.

The peat is moderately (Mesisol) or poorly (Fibrisol) decomposed plant residues, mainly mosses and sedges. It is usually reddish to yellowish brown in colour and below it there will be strongly gleyed sands, silts or clays. It is moderately to very strongly acid and very low in nutrients. This is because most of the soil water is derived from precipitation; it has not accumulated as runoff from surrounding uplands from which it would have carried dissolved nutrients, as is the case with the organic soils of the Interior Plateau. Mesisols have a vegetation cover of sedges, hypnum mosses, blueberries and bog glandular birch. Fibrisols have sphagnum mosses, common Labrador tea and stunted black spruce and tamarack.

The Organic Cryosol Landscape

This soil landscape which occurs in the Fort Nelson Lowland consists of organic soils that have permafrost within 1 m of the surface. The southern limit of discontinuous permafrost is just south of the -1ºC mean annual isotherm which crosses the region at about 58ºN latitude. From here to the 60th parallel parts of the organic terrain will have layers that do not thaw in the summer, because they are effectively insulated by the surface peat. Not all the peatland will be frozen, neither will all of it have a thick organic cover. Therefore, there is a very complicated pattern of Organic Cryosol, Organic and Luvic Gleysol landscapes in the Fort Nelson lowland.

The surface of these soils is poorly decomposed reddish or yellowish brown fibric peat. Below this the frozen peat looks very similar. It just feels hard. There are no large ice lenses only ice coatings on all the organic fibres that bind them together. The vegetation cover is also similar to the Fibrisol landscape with sphagnum mosses, Labrador tea, stunted black spruce and tamarack.

With the unfrozen Organic soil landscape this forms part of the muskeg terrain of the north. It presents all the same problems of use as the unfrozen soils with the added difficulty of a frozen layer whose upper surface fluctuates in depth throughout the year.

The Eutric and Dystric Brunisol Landscapes

The principal difference between these two landscapes is the nature of the fluvioglacial parent material. The Eutric Brunisol has developed on calcareous materials, the Dystric on acid materials.

The Eutric Brunisol landscape commonly occurs on the coarse textured calcareous fluvioglacial deposits on the terraces of major rivers such as the Peace and the Liard. The vegetation is a mixed forest cover of trembling aspen and white spruce. Because of the coarse texture, low water holding capacity and rapid drainage characteristics of the materials many sites are very dry and subject to forest fires. In these areas the forest trees are lodgepole pine and trembling aspen. The profile characteristics are similar to the typical soils that were described in Part 2.4, apart from very weak structure.

The Dystric Brunisol landscape has developed on fluvioglacial deposits that are acid in reaction and low in bases. Such deposits are found along streams such as the Doig River and Milligan Creek that have their sources in the acid peatlands of the Great Plains, or they are found bordering the sections of the Rocky Mountain Foothills that have less calcareous bedrock. The forest is composed of white spruce and trembling aspen. The soil profile is similar to the Eutric Brunisol except that the horizons are less calcareaus.

Under suitable climatic conditions the soils of these two landscapes can be used to grow forage crops. Moisture deficiencies in the growing season are the main limiting factor.

The Grassland Soil Landscapes

The Black Chernozemic, Solodized Solonetz and Solod soil landscapes that form the grasslands of the Peace River lowlands are of limited extent. But they form a part of the larger aspen parkland of the Prairies. It was this landscape that first attracted white settlers to the Peace River lowland and formed the basis of the agricultural economy that sustained them there.

The limited Black Chernozemic soil landscapes occur mainly on the gently to strongly sloping fans along the south facing wall of the Peace River valley between Hudson Hope and the Alberta border. They are young shallow soils (often having only an Ah, C horizon sequence) developed on young geological deposits. Other small patches occur on the lacustrine deposits of the shale lowlands. The vegetation is grasses with occasional lumps of trembling aspen.

Plate 3.7.2

Plate 3.7.2
Soil Landscapes Near Boudreau Lake in the Peace River Lowlands Looking South

  1. Fibrisol landscape. Deep fibrous sphagnum peat occurring around the periphery of lakes and along drainage ways.
  2. Gray Luvisol landscape on the moderately well drained low ridges on the till plain. There are some Luvic Gleysols in depressions.
  3. Luvic Gleysol and Gray Luvisol landscapes on the "fluted" till plain. Luvic Gleysols occur in the depressions and nearly level topography. Gray Luvisols are limited to the moderately well drained slopes of the low ridges, which have a forest cover of white spruce, trembling aspen and white birch with a ground cover of reindeer moss.
  4. Luvic Gleysol and Mesisol landscapes on nearly level topography. Luvic Gleysols occur in imperfectly and poorly drained sites with Mesisols in the slight depressions that are very poorly drained. The vegetation cover is sphagnum moss, common Labrador tea, willows, tamarack and black spruce.

The saline soil landscapes contain two soil great groups; the Solodized Solonetz and Solod. They are found in the subhumid aspen parkland of the Peace River lowland where the parent materials contain considerable quantities of residual soluble salts. They occur mainly on the lacustrine deposits overlying the marine shales in the lowlands. But they are also found on the adjacent till deposits.

Both types of soil have B horizons that contain relatively large amounts of Na and Mg salts, and a strong columnar structure that breaks to subangular blocks. There are large accumulations of calcium carbonate at depth and below this accumulations of gypsum.

The Regosol Landscapes of the Floodplains

The soils of the recent fluvial deposits on the floodplains and terraces of the Peace River, the Liard and their tributary streams are Regosols. They are formed from sediments deposited with each flood. They do not have distinct horizons, although they may have an organic surface litter or a weakly developed Ah horizon. Distinct layers may be found in Regosolic soils but they are due to variation in sediments rather than to soil horizon development. Sometimes buried Ah or L-F horizons are encountered in the soil profile. They result from repeated burial by flooding.

The frequency of flooding is variable since it is often caused by unpredictable ice jamming in the spring. Depth to water table is also variable both from place to place and through the year. In the spring the soils can be saturated, later in the summer they are liable to moisture deficits. The vegetation cover is a dense forest of black poplar and white spruce with mountain alder, willows and red-osier dogwood forming a dense understory.

Regosols also occur on the steep valleysides above the floodplains. These slopes are extremely unstable. The soft fine textured materials are constantly moving downslope. What soil horizons do occur are disrupted. The soils are well drained but unstable.

These floodplain soils are very productive if not flooded. With the suitable climate of the Peace River valley good crops of potatoes, corn and other vegetables may be grown on many suitable sites in the floodplains. Large populations of deer, moose and domestic cattle graze the vegetation on the terrace and adjacent slope soils.