The main feature of the Central Interior Ecoprovince is a wide plateau spread between the Coast Mountains and ranges of the Southern Interior Mountains Ecoprovince. Somewhat of an atypical feature for the Ecoprovince is the rugged landscape and large glacial lakes on the east slope of the coast mountains included in the Chilcotin Ranges Ecoregion. Despite the high relief, this area was included in the Central Interior because it is relatively dry.
There are eight Ecoregions in the Central Interior. Seven are headwater systems to the Fraser River and one is a headwater drainage of the Skeena River. Middle reaches of the Fraser mainstem are also included. In this area, the Fraser penetrates highly erodable glacial materials to form badland type terrain as it flows southward through the Central Interior Plateau and the Chilcotin ranges before dissecting through the Coast Mountains in the Coast and Mountains Ecoprovince. Major drainages and assigned Ecoregions in the Central Interior are as follows:
There are two large water storage reservoir systems in the Central Interior. The Nechako Reservoir is 160 km long and includes 95,000 ha of lakes, rivers and submerged forest that was not logged prior to flooding in 1954. The other is the Carpenter/Downton-Seton system located in the southern end of the Chilcotin Ranges. Flows in Carpenter-Seton are controlled by BC Hydro for power production at Shalalth located on the north shore of Seton Lake. Water surface elevation and flows out of the Nechako Reservoir are controlled by Alcan.
The Central Interior Plateau, Caribou Plateau, and Pothole Lakes, are gently rolling uplands at elevations of 800 - 1500 m. Glacial drift covers most areas creating an undulating surface having abundant small pothole lakes (<500 ha) connected by small streams. Under the glacial drift, bedrock is typically flat lava which have been eroded along river channels to form steep escarpments with flat caps and talus bases. The headwaters of the Dean River in the vicinity of Anahim Lake are unique from adjacent areas because of the presence of three high shield volcanoes. At the western end of the Nechako Plateau, close to Whitesail Lake (one of the lakes of the Nechako Reservoir), granite intrusives form large mountains. These parent materials are highly resistant to erosion, yielding extremely low concentrations of TP in surface water and lakes. In Whitesail Lake for example, TP concentrations are <0.003 mg·L-1 but at the eastern end of the reservoir in Knewstubb Lake which is less influenced granitic parent materials, TP concentrations are close to 0.007 mg·L-1 (Perrin et al. 1997).
Table 11 shows that the Central Interior Ecoprovince contains gradients of chemical characteristics.
In the Bulkley Basin, pH is slightly above neutrality, TDS is moderate (34 to 64 mg·L-1 ), alkalinity is low (<38 mg·L-1 ) and TP concentrations are 0.090 mg·L-1 to 0.025 mg·L-1 in streams and lakes. Locally where there is influence or has been influence from mine drainage (e.g. Equity Mine), TDS can increase to >300 mg·L-1 . Addition of dissolved solids is mainly due to high concentrations of sulfate (>100 mg·L-1 ) and components of lime where neutralisation of acid drainage is active (e.g. Ca up to 50 mg·L-1 from a background of 5 mg·L-1 and Mg up to 11 mg·L-1 from a background of 0.01 mg·L-1 ) (Perrin et al. 1992).
The prominent feature of the Nechako Plateau is the Nechako Reservoir which has a gradient of increasing dissolved solids and nutrient concentrations from west to east (Perrin et al. 1997). TDS increases from 19 mg·L-1 at the western end of Whitesail Lake and in nearby Eutsuk Lake (not part of the reservoir) to 30 mg·L-1 at the eastern end of Knewstubb Lake near the Kenney Dam. Alkalinity increases from 14 to 24 mg·L-1 over the same reach. TP concentrations increase from 0.002 mg·L-1 at the western end to reach more than 0.007 mg·L-1 at the eastern end. Throughout the gradient, pH remains slightly above neutrality (7.0 to 7.4).
In the Lower Nechako Ecoregion which includes Francois Lake and the Nechako River, nutrient and dissolved solids concentrations are higher than in the Nechako Reservoir. TP concentrations reach a median of 0.011 to 0.024 mg·L-1 in lakes and streams. Median TDS is up to 64 mg·L-1 in streams and 118 mg·L-1 in lakes.
The Dean Uplands are considered a separate Ecoregion because of the presence of shield volcanoes and abundant cattle grazing each of which produce high concentrations of TP in the Dean River. The volcanic parent materials have contributed to high concentrations of total phosphorus (0.060 mg·L-1 (Perrin 1997)) in the Dean River. Table 11 also shows TP concentrations up to 0.076 mg·L-1 in the Upper Dean River. These TP concentrations can be much higher (up to 0.150 mg·L-1) in the vicinity of cattle grazing areas and feed lot operations. These land uses contribute to mesotrophic or eutrophic conditions in numerous lakes of the area (e.g. Anahim Lake).
Moving southward to the Central Interior Plateau and the Pothole Lakes area, there is increasing nutrient enrichment in streams and lakes (Table 11). Much of this enrichment is derived from the weathering of the flat lava bedrock. Lava has high concentrations of phosphorus which can be eroded from the bedrock in a highly soluble and bio-available form. This weathering also contributes high concentrations of cations to solution. The phosphorus contributes to high productivity in all aquatic ecosystems of the area and eutrophic conditions in many of the small lakes and streams. Where there is little surface drainage and long water residence times, many of the pothole lakes can be hypereutrophic. Large amounts of organic matter in lake sediments that results from the high productivity, induces oxygen demand under ice in many of the lakes leading to periodic fish kills in winter (Lirette and Chapman 1993). Anoxia can also cause the release of large amounts of bio-available phosphorus back into the water column each year. This annual return of phosphorus maintains long term eutrophication of these lakes.
In lakes and streams of both the Central Interior Plateau and the Pothole Lakes, TDS concentration is commonly >100 mg·L-1 (Table 11). In 6 out of the 17 Watershed Groups in the Central Interior Plateau, TDS is >200 mg·L-1 . The same is true in 5 out of the 11 Watershed Groups of the Pothole Lakes. Highest concentrations can reach more than 600 mg·L-1 (e.g. lakes of the Deadman River Watershed Group). In a survey of many lakes of the Ecoprovince, Lirette and Chapman (1993) found that TDS values reaching 600 mg·L-1 are common (Lirette and Chapman 1993). In many pothole lakes and similar sized lakes of the Central Plateau where there is little or no surface drainage, TDS can reach several thousand mg·L-1 . Accompanying the high TDS concentrations is high alkalinity (most values >100 mg·L-1 and many are 200 to 600 mg·L-1 ). Median pH is >7.6 throughout the Central Interior Plateau and the Pothole Lakes.
There are also some relatively low TDS and alkalinity concentrations found in these Ecoregions. For example, the Euchiniko Watershed Groups have median TDS concentrations between 60 and 96 mg·L-1 and a single alkalinity measurement from a stream site in the Upper Chilcotin River is only 4.5 mg·L-1 .
TP concentrations are generally high throughout the Central Interior Plateau and the Pothole Lakes (mostly 0.020 to 0.080 mg·L-1 ). In lakes of the Deadman River watershed, TP concentrations reach more than 0.100 mg·L-1 . In contrast, median TP concentration is only 0.007 mg·L-1 in Green Lake.
Anther important feature of the Central Interior Plateau and the Pothole Lakes is the Fraser River mainstem. Downstream of the Quesnel River, the Fraser River has TDS concentrations of 80-150 mg·L-1 , slightly alkaline pH and TP concentrations of 0.031-0.050 mg·L-1 (Hall et al. 1991). Some of this TP may originate from pulp mills that are located in Prince George. The Quesnel River also introduces a high sediment load which produces a median turbidity of 21 NTU downstream in the Fraser River.
In the Chilcotin Ranges there are more variable concentrations of most chemical parameters. In Chilko Lake, TDS concentrations are 40-60 mg·L-1 , alkalinity is close to 10 mg·L-1 and TP concentrations are <10 µg·L-1 (J.G. Stockner, Ecologic, Pers Comm.). These values are low compared to data from small lakes which have median TDS concentrations of 100 to 148 mg·L-1 , alkalinity up to 32 mg·L-1 , and TP concentrations up to 0.017 mg·L-1 . Streams have median TDS concentrations of 38 to 68 mg·L-1 which is lower than TDS in the small lakes. If much of the data from lakes were collected at times of mixing, the higher lake values may be related to internal nutrient return from sediments. High sediment loads have generally not been typical in streams of the Chilcotin Ranges. Median TSS concentrations are 5 or 6 mg·L-1 .
In the southern part of Chilcotin Ranges, there is a strong influence of localised volcanic parent materials and extensive glacial flour in streams. In the Downton-Carpenter Reservoir system, for example, TP concentrations can reach 0.300 mg·L-1 in some streams during the spring freshet but the levels drop to 0.010-0.020 mg·L-1 at lower flows (Perrin and Macdonald 1997). TDS can be up to 300 mg·L-1 in small streams at low flows but in larger rivers it is typically 20-60 mg·L-1 , except during the spring freshet when high concentrations of TDS and TSS are found in the large rivers (e.g. the Bridge River). Glacial turbidity can be up to 70 NTU in spring, declining to <20 NTU at lower flows in winter. The pH is typically close to 8 throughout the year.