Ground Water Resources of British Columbia
Chapter 10 — Ground Water Resources of the Plateaus and Highlands Ground Water Regions
10.1.2 NECHAKO PLATEAU
by
J.C. Foweraker and D. Johanson
GENERAL SETTING
The Nechako Plateau (Figure 8.7) is located within the Interior System of the Province to the north of the Fraser and Thompson Plateaus. West of the Nechako Plateau lie the Hazelton Mountains and to the north, the Omineca and Skeena Mountains. The southern boundary with the Fraser Plateau is in part defined by the West Road River. The plateau is bounded on the east by the Fraser Basin. Remnants of the Nechako Plateau also occur within the Fraser Basin, in areas where the plateau rocks were not destroyed by earlier erosion. On the eastern margin of the Fraser Basin there is also a small plateau area, the McGregor Plateau, which Holland (1964) included in his discussion of the Nechako Plateau.
Population density within the Nechako Plateau is to a considerable extent concentrated along the Endako and Bulkley River valleys, for example at Fraser Lake, Endako, Burns Lake, Decker Lake, Topley, Houston and Telkwa. There are additional settlements around such areas as Granisle near Babine Lake, Ootsa Lake and Francois Lake.
PHYSIOGRAPHY
The plateau is described by Holland (1964) as an area of low relief with large areas of flat or gently rolling land, sometimes almost undissected, but elsewhere incised deeply to the level of the Fraser River and its tributaries. The major valleys such as the Nechako, West Road, Ootsa Lake etc., are broad and uneven. The complex drainage pattern, according to Tipper (1963), forms a disjointed network of shallow V shaped valleys cut into the broad irregular valley bottoms. Much of the plateau surface is between 1220 and 1525 m in elevation. Several round topped mountain areas project above the general plateau surface in the southwestern area of the plateau where some more erosion resistant rock occurs.
GEOLOGY
Much of the plateau is overlain by flat or gently dipping Tertiary lava flows which are underlain by volcanic, sedimentary and intrusive rocks, (Holland 1964). Glacial drift covers much of the bedrock.
In Pleistocene times, ice moved across the plateau towards the northeast and east and marked the surface with thousands of grooves and drumlin like ridges.
After the ice retreated, depressions were left in the plateau surface which formed the many lakes now found on the plateau. Babine Lake for example is the second largest lake in the province.
Eskers and meltwater channels are a noticeable feature of the plateau and according to Tipper (1971), many meltwater channels, often bounded by stagnant ice, were eroded into the till and bedrock surfaces to provide drainage routes for the torrential streams coming from the rapidly melting ice sheet at that time.
In the Nechako and Bulkley River valleys and major tributaries located on the plateau, varying thicknesses of heterogeneous unconsolidated deposits containing both productive sand and gravel aquifers and impermeable deposits were laid down during the deglaciation periods. Present day rivers are now reworking these materials.
GROUND WATER RESOURCES
These are over 700 recorded wells located within the Nechako Plateau area. There may be many additional wells for which no records are presently available. Over 70% or over 500 of the records are for wells completed in unconsolidated water bearing deposits. Approximately 85% or 430 of these wells have either no known information on well yield or the well yield is reported to be less than 1 L/s. These wells are for the most part used for domestic requirements. The remaining 15% or 80 higher yield wells, completed within unconsolidated deposits, can be separated into two groups. In the first group approximately 75% or 60 wells have reported potential yields between 1 L/s and 4 L/s, while in the second group 25% or 20 wells have reported potential well yields between 7 L/s and 30 L/s. These higher yield wells, which include a number of test wells, are located along the Bulkley and Nechako River valleys.
Productive sand and gravel aquifers occur as deep as 100 m at Endako and at 46 m at Burns Lake, while at Houston most high capacity wells have been completed in sands and gravel less than 30 m deep. There are also records of higher capacity wells located to the west of the plateau boundary at Smithers, in the Bulkley Valley. However, heterogeneous deposits, aquifer boundary conditions which can limit well yield, and high static water levels can be expected at many locations in these valley fill deposits. There are also over 200 bedrock wells on the Nechako Plateau, 5% or 10 of these wells have estimated capacities in excess of 1 L/s, but less than 4 L/s. The remaining bedrock wells are reported to yield less than 1 L/s or the yield is unknown.
GROUND WATER QUALITY
Records of ground water quality analyses are mainly confined to the Nechako and Bulkley River valley areas. Based on these limited analyses, it would appear ground waters are generally moderately to very hard and the waters are most commonly of the calcium-magnesium bicarbonate type, or the calcium-magnesium- sodium bicarbonate type.
Concentration of dissolved iron exceeds the recommended limits in some areas of the Nechako Plateau. At Endako, 1981 analyses from two deep wells, one in unconsolidated deposits, one in bedrock, showed uranium concentrations ranging from 10 to 14 µg/L in the former and 53 to 68 µg/L in the latter. A high capacity well completed in a 46 m deep sand and gravel aquifer at Burns Lake, was reported to contain manganese concentrations in the ground water of 0.30 to 0.42 mg/L. According to the Guidelines for Canadian Drinking Water Quality (1989), the aesthetic objective for manganese is less than 0.05 mg/L concentration.
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