Ground Water Resources of British Columbia
Chapter 11 — Ground Water Resources of the Mountain Ground Water Regions
11.2 ROCKY MOUNTAINS AND FOOTHILLS
by
R.A. Dakin
11.2.1 ROCKY MOUNTAINS
PHYSIOGRAPHY
The Rocky Mountains extend in a northwesterly direction along the eastern side of the province for 1,360 km between the 49th parallel and the Liard River. Their western boundary is the Rocky Mountain Trench, which, except for the interruption at the McGregror River, exists as a continuous valley for the entire distance. On the east the Rocky Mountains are flanked by the Rocky Mountain Foothills, which are not everywhere inferior in height to the main mountains.
There are four divisions in the Canadian Rocky Mountains (Holland 1975). These are the Border Ranges between the International Boundary and a line between Elko and North Kootenay Pass; the Continental Ranges extending northwestward from a line between Elko and North Kootenay Pass to Jarvis Creek just north of Mount Sir Alexander; the Hart Ranges between Jarvis Creek and the Peace River; and the Muskwa Ranges between the Peace River and the Liard River (see Figures 11.1 and 11.2 ). The highest peaks are in the Continental Ranges section, with many peaks exceeding 3,100 m, and Mount Robson (3,954 m) being the highest.
CLIMATE
Temperatures are below freezing for much of the year and hence, over 50 percent of the annual precipitation (1,000 mm) falls as snow. Precipitation is higher on the west side of the mountains than on the east side, and is greatest at higher elevations. Many of the intermountain valleys are relatively dry.
GEOLOGY
The Rocky Mountains are underlain mostly by sedimentary and metamorphic rocks, which range from Proterozoic to Cretaceous in age. The youngest rocks are exposed in the foothills, and progressively older rocks lie to the west. The predominant rocks of the Rocky Mountains are Palaeozoic and Proterozoic limestones, quartzites, schists and slates. In contrast, the foothills contain a predominance of Mesozoic, especially of Cretaceous formations. Two formations of volcanic origin are known to occur, one of possible Mississippian age and the other Lower Cretaceous. The intrusive rocks are so few that they have no regional geomorphic significance.
During the Pleistocene the ranges were covered to heights of 2,100 to 2,500 mm by continental ice, the erosional effects of which are slight compared to the modifications wrought by alpine and valley glaciers. In the southern Rocky Mountains there is an area where the Cordilleran ice reached a height of only 2,000 m; hence, there are extensive areas that were not glaciated.
Figure 11.1 Physiographic map of the southern portion
of the Rocky Mountain Region
GROUND WATER RESOURCES
Bedrock
Permeability of bedrock is very variable, with more permeable zones occurring in zones of discontinuity, such as along cross and/or thrust faults and in the nappe of tight folds. As there are not many people living high up on the side of mountains, there are not many bedrock wells. However, in some of the mountain coal areas, deep dewatering wells have been drilled to control in-pit seepage, and a few wells have been developed for mine use. Typical well yields range from 0.8 to 4 L/s in fractured shale and/or coal aquifers.
In areas where the bedrock is predominantly limestone, permeability is much higher. Hot springs are common in areas where either deep seated thrust faults or transect faults, which allow regional ground water flow systems to flow rapidly to the surface. Most of the hot springs are in national parks and only a few occurring in British Columbia (Van Everdingen, 1972, and Souther and Halstead, 1973).
Surficial Sediments
As indicated earlier, the north and south intermountain valleys have all been glaciated, and have a characteristic "U" shape. These valleys have been backfilled with a wide range of sediments ranging from dense fill, up to very permeable sands and gravels. Because of the relatively inhomogeneous nature of these sediments, ground water potential is difficult to predict with any certainty.
Some examples of wells in these sediments are three wells operated by the Village of Elkford, two developed in lateral moraine and one in river alluvium (Piteau Associates, 1979). The Fording Coal Wash Plant utilizes a ground water source and so does the Greenhills Mine (Hardy Associates, 1981) located farther down the same valley.
Ground Water Quality
Due to the calcareous nature of the sediments, ground water is typically moderate to very hard. Total dissolved soils typically range from 300 to 500 mg/L in some of the slow moving ground water flow systems. In some of the coal mine areas, high barium concentrations of up to 80 mg/L have been encountered.
11.2.2 ROCKY MOUNTAIN FOOTHILLS
PHYSIOGRAPHY
The Rocky Mountain Foothills (see Figure 11.2) lie along the eastern margin of the Rocky Mountains in a continuous belt from the 49th parallel to the Liard River. In the south the foothills are in Alberta, but the belt enters British Columbia just north of latitude 54 degrees, at the headwaters of the Narraway and Wapiti Rivers and continues northwestward for about 640 km to their northern termination at the Liard River. The belt of foothills ranges from a width of 17 km where crossed by the Muskwa River to 72 km at the head of the Pine and Sekunka Rivers.
Elevations of summits and high longitudinal ridges mostly range from 1,830 m to 2,130 m, the highest being Mount Laurier (2,350 m) near the head of the Halfway River. Summits increase in height southward from the Liard River to the area between the heads of the Prophet and Pine Rivers and then diminish southward where the Monteith quartzite is thinner.
Figure 11.2 Physiographic map of the Rocky Mountain Foothills and
northern portion of the Rocky Mountain Region
CLIMATE
With temperatures well below freezing for much of the year, much of the annual precipitation falls as snow which feeds the many mountain glaciers. Annual precipitation ranges from 500 to 1,000 mm, with more than 60 percent falling as snow. Mean monthly temperatures range from - 15° C in the winter up to 15° C in July.
GEOLOGY
The western boundary of the foothills for the most part is along a structural line which follows the trace of the easternmost major fault that thrusts Palaeozoic over Mesozoic formations. This fault normally brings thick cliff-forming Palaeozoic limestones into position to form typical grey limestone Front Ranges, which stand out prominently on topographic maps and are easily distinguished on the ground. The eastern boundary of the foothills, especially between the Narraway and Peace Rivers, is a series of southwesterly dipping echelon thrust faults which separate strongly folded and faulted sedimentary formations of the foothills from flat-lying or gently dipping formations of the plains.
The foothills are entirely underlain by sedimentary rocks that in northeastern British Columbia are largely of Mesozoic age, but throughout their length range from Precambrian to Tertiary. The rocks are folded about northerly to northwesterly trending axes and are cut by southwesterly dipping thrust faults. Bedrock has a strongly developed structural grain which is closely reflected by the character of the topography.
The structural grain of the bedrock has resulted in prominently developed northwesterly trending longitudinal ridges and a trellis pattern of drainage. Valleys are eroded along belts of soft rock and along fault zones and are generally wide and flaring. They lie at 760 to 915 m elevation, producing a variable and moderate relief.
The foothills were occupied by continental ice during the Pleistocene and easterly trending valleys were glaciated by ice moving outward from the mountains. The northwesterly trending valleys generally were not eroded, but they, like the others, received a mantle of drift when the ice disappeared. By and large the foothills display landforms only slightly modified by glaciation.
GROUND WATER RESOURCES
The Rocky Mountain Foothills of British Columbia are not well populated; however, many of the communities with a municipal water supply system have chosen a ground water source. For example, Tumbler Ridge has a well field tapping Quatenary sediments in a river valley. Ground water development potential is generally good, from either surficial sediments in the valleys or deep bedrock holes in the hillsides.
Bedrock
In areas where surficial sediments are thin to absent, a number of deep holes in sedimentary bedrock have encountered artesian zones with modest flows. Hydrogeological studies for coal mine development have shown that typical hydraulic conductivity for fractured sedimentary rocks is variable, ranging from 10-10 m/s for a tight shale to 10-3 m/s for a vuggy limestone. Hence, it is not uncommon to have yields from bedrock holes ranging up to 13 L/s.
Surficial Sediments
In areas where there is thick sequence of permeable alluvium or glacial outwash sediments, individual well yields of 100 L/s or more are possible, provided that the sediments are hydraulically connected to a significant source of recharge, such as a river.
For example, a well field was designed for the proposed Monkman Open Pit in the Kinuseco Creek Valley and projected steady state pumping from the well field was between 200 and 300 L/s.
GROUND WATER QUALITY AND TEMPERATURES
Because of the calcareous nature of the local bedrock and sediments, ground water is generally moderately hard, and in some areas elevated concentrations of barium, iron and manganese are a problem.
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