Ground Water Resources of British Columbia
Chapter 10 — Ground Water Resources of the Plateaus and Highlands Ground Water Regions
10.1.5 SHUSWAP HIGHLAND
by
R.A. Dakin
PHYSIOGRAPHY
The Shuswap Highland (see Figure 10.7) extends southward from Mahood and Murtle Lakes to the Coldstream Valley east of Vernon, and lies between the Thompson Plateau on the west and the Monashee Mountains on the east. It is 230 km long and 80 km wide. The western boundary of the highland is along a line from the east end of Canim Lake along Mann Creek to Barriere, and thence, to Vernon along a lineament coinciding with the Louis Creek fault zone. The eastern boundary is along a line from Blue River down the upper Adams River and across to the head of Anstey Arm of Shuswap Lake, thence, via Three Valley to Mabel Lake and Shuswap Lake to Sugar Lake.
Figure 10.7 Physiographic map of Quesnel and Shuswap Highlands
The Shuswap Highland consists of gentle to moderate sloping plateau areas rising from 1,500 to over 2,200 m, dissected by the Clearwater, North Thompson, Adams, and Shuswap Rivers and their tributaries into large polygonal upland tracts. The valley sides are commonly steep because of glacial erosion, and the total relief may be fairly great even though the local relief in the uplands is moderate.
The Shuswap Highland reaches heights of 2,726 m at Mount Mahood, 2,743 m at Trophy Mountain. The high points are progressively lower to the south. Most ridges and summits are rounded, and despite the height of much of the terrain the country lacks the jagged sawtooth profiles of the mountains to the east.
Numerous large lakes, such as Murtle, Adams, Shuswap and Mabel Lakes, occupy some of the major valleys of the area, and these, as well as the rather extraordinary pattern of the drainage, quite obviously diverted from its pre-glacial flow, are legacies of Pleistocene ice occupation.
CLIMATE
Precipitation is highest on the western flanks of the mountains and lowest in the southern valleys. Annual precipitation ranges from about 250 mm up to about 1,000 mm, and average monthly temperature from well below zero in January up to about 25° C in July.
GEOLOGY
Much of the area is underlain by mid-Palaeozoic sedimentary and volcanic bedrock. In the north, there are outliers of more recent basaltic flows and in the mid and south of the region, small scattered batholiths are present. The sedimentary rocks have poor sorting of grains (typical eugeosynclinal sediments), and hence, secondary permeability associated with structural discontinuities are important conduits controlling ground water movement in the bedrock.
The effects of glaciation in the region were largely to soften and reduce the upland relief while steepening and deepening the valleys. Cirqui glaciation on northeastern exposures was a minor feature. The typical "U" shaped valleys are known to have deep (up to 120 m) sequences of fluvioglacial and glacial detrital sediments (Fulton, 1975). Sediment size and gradation is very variable, and hence, permeability also varies significantly.
HYDROGEOLOGY
Bedrock
In general, the bedrock has a low permeability and well yields are low, especially in the more arid areas. Bedrock ground water typically also has a high total dissolved soils content (>800 mg/L). One of the reasons for the low number of bedrock wells in this region is that most areas of habitation are located in the valleys. One of the exceptions is the Silver Star Ski Resort, which has developed a ground water source using bedrock wells with yields in the order of 1.5 L/s.
Surficial Sediments
All of the valleys transecting the Shuswap Highland have been glaciated and backfilled with layers of glacial and post-glacial sediments, many of which are very permeable, the most extensive of which are the well populated North and South Thompson Valleys, and the North Okanagan and Shuswap Valleys, where there are numerous wells. Reported yields range up to 150 L/s.
Recharge to these aquifers is generally sufficient to maintain the present abstraction rates, and as many of the aquifers are hydraulically connected to nearby rivers, induced recharge ensure a significant supply. In one study for a large proposed well field (300 L/s) in the Clearwater area, it was found that the nearby Dutch Lake was being supplied by ground water leaking from the Clearwater River, through surficial sediments which underlie basalt lava, thereby ensuring adequate recharge (Piteau Associates, 1983).
A large well field has also been developed for the Shuswap Salmon Hatchery.
In the relatively arid North Okanagan area, where there typically is no well developed surface water flow, recharge is a major limitation to potential well yields. For example, wells drilled in the Armstrong-Falkland area have had high initial yields, but were generally not able to sustain this flow due to the limited areal extent of the aquifer and low recharge.
There are a number of artesian aquifers in the region. A test hole drilled in the Coldstream Valley south of Vernon encountered a very high pressure artesian zone that proved to be very difficult to control (Scott, 1968).
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