Biodiversity in Canadian Lakes and Rivers
- Trends in Freshwater Fish of Special Interest
- Trends in Hydrological Regimes
- Trends in River and Lake Ice Break-Up/Freeze-Up
- Trends in Habitat Loss and Fragmentation
- Trends in Pollutants in Lake and River Systems
- Future Climate Impacts on Lakes and Rivers
- Synthesis of Data
- Appendix 1
Trends in Habitat Loss and Fragmentation
Habitat alteration is the most important threat facing freshwater fishes at risk in Canada (Dextrase and Mandrak, 2006). Habitat fragmentation in aquatic ecosystems occurs when river or lake habitat connectivity is disrupted through the addition or alteration of natural or human barriers to dispersal. Actively dispersing riverine species are particularly vulnerable because of their linear habitat use. The longitudinal and lateral fragmentation of lake and river systems is one of the most significant global threats to freshwater systems, often leading to habitat degradation and loss of biodiversity (Revenga et al., 2000; Jones and Bergey, 2007). Disruption of the natural habitat regime by longitudinal barriers (such as dams, weirs, and roads) and by degradation of the riparian zone (for example, gaps in riparian buffers) adversely affects aquatic communities, most notably by disrupting fish and wildlife passage (for example, Levesque, 2005; Reid et al., 2008a). For example, pipeline crossings within river and stream channels disrupt habitat continuity and negatively affect the physical and chemical nature of fish habitat (Levesque, 2005). Moreover, impoundments have been found to promote the spread of invasive alien species. Using data from the Laurentian Great Lakes region, a study by Johnson et al. (2008) demonstrated that invasive alien species were 2.4 to 300 times more likely to occur in impoundments than in natural lakes, with impoundments frequently supporting multiple invasive taxa. The authors suggest that anthropogenically-altered systems act as ‘stepping stone’ habitats for the continuing spread of invasive species because of the increased proximity of invaded water bodies with natural systems as numbers of impoundments expand (Johnson et al., 2008).
Globally, the rate of freshwater system degradation through the modification of waterways, draining of wetlands, construction of dams and irrigation networks, and by interbasin transfersFootnote 9 has been accelerating since the early 1900s (Figure 26) (Nilsson et al., 2005). In North America, interbasin transfers and diversions have irreversibly altered the hydrological regime (both water quantity and quality) for a number of large rivers (Figure 26). Created mainly for hydropower generation in Canada, these are predominantly concentrated in northern Saskatchewan and Quebec with additional diversions across Ontario, Newfoundland and Labrador, and British Columbia (Figure 26). These changes have disconnected river and lake systems from the normal hydrological cycle, with negative effects on habitat availability and biodiversity.
Modifications of freshwater systems worldwide have been extensive (Table 9). Globally, Revenga et al. (2000) reported that large dams have increased sevenfold since 1950 and now impound 14% of the world’s runoff. Of the world’s largest 227 rivers, 60% are strongly or moderately fragmented by dams, diversions, and canals (Revenga et al., 2000). At the same time, water withdrawals have increased six-fold between 1900 and 1995 despite 40% of the world’s population living in areas of high water stress. In addition, flow alteration from dams can lead to flow homogenization, particularly affecting the magnitude and timing of high and low flows (Poff et al., 2007). This can have a negative effect on aquatic communities, particularly locally adapted native species (Poff et al., 2007).
|Waterways altered for navigation (km)||3,125||8,750||-||>500,000||-|
|# of large reservoirs (>0.1 km3)||41||581||1,105||2,768||2,836|
|Volume of large reservoirs (>0.1 km3) (km3)||14||533||1,686||5,879||6,385|
|# of large dams (>15 m high)||-||-||5,749||-||41,413|
|Installed hydro capacity (MW)||-||-||<290,000||542,000||~660,000|
|Hydro capacity under construction (MW)||-||-||-||-||~126,000|
|Water withdrawls (km3/year)||-||578||1,984||~3,200||~3,800|
|Wetlands drainage (km2)||-||-||-||160,600||-|
Source: Revenga et al. (2000) as adapted from Naiman et al. (1995)
Trends in dam completion in Canada
Habitat fragmentation from construction of dams has been monitored within Canada since the 1830s. Using data from the Canadian Dam Association (2003), Figure 27 summarises the number of dams greater than 10 m in height completed in Canada from 1895 to 2005. The number of dams rapidly increased from 1910, peaked between 1950 and the early 1980s, and has declined since then. Early dam construction was focused around the St. Lawrence/Great Lakes region and the Pacific coast (Figure 28). The majority of dams are within the southern regions of the country with the highest population densities (Figure 28). Recent dam developments have been concentrated in northern Quebec. Further examination of trends in dam construction grouped by ecozone+ demonstrates the majority of dams were completed in the Boreal Shield (n = 265) and the Taiga Shield (n = 177) (Figure 29). The timing of dam construction was variable across ecozones+ with dams completed predominantly in Mixedwood Plains and Pacific Maritime ecozones+ prior to 1920, while more recent construction was focused within Taiga Shield, Boreal Shield, and Prairies (Figure 29). Between 1930 and 1980, dams were constructed across most regions with a dominance of sites within the Taiga Shield towards the latter period (Figure 29). For some regions, the construction was fairly constant from the early 1900s onwards, for example in the Boreal Shield, Atlantic Maritime, Western Interior Basin, and Montane Cordillera ecozones+ (Figure 29).
Source: data from Canadian Dam Association (2003) updated to include data up to 2005
Long Description for Figure 27.
This bar graph shows the following information:
|Year||Number of |
Source: data from Canadian Dam Association (2003) updated to include data to 2005
Long Description for Figure 29.
This stacked bar graph shows the following information:
|Western Interior Basin||0||0||2||4||4||2||2||3||1||1||0||0|
Dams interrupt fish migration routes, destroy riparian habitat, increase sedimentation, affect habitat availability, and cause changes in water chemistry and water availability (McAllister et al., 2000). Fragmentation of river systems can also lead to a loss of genetic diversity and increase differentiation between isolated populations (e.g., Neraas and Spruell, 2001; Meldgaard et al., 2003). However, such effects depend on the nature of the dams and the ecological characteristics of individual species. For example, Reid et al. (2008b) found no evidence of population structure effects as a result of damming in black redhorse (Moxostoma duquesnei), a fish species found only in the Grand and Thames rivers in Ontario and assessed as Threatened by COSEWIC in 1998 (COSEWIC, 2005) This suggests dams on these rivers do not present substantial dispersal barriers to this species and that other factors, such as high nutrient levels, altered flow regimes, and physical habitat degradation, are more important factors contributing to its endangerment.
Examples of changing land use
Altering land use within watersheds directly affects lake and river ecosystems. For example, changing the proportion of urban or agricultural land will affect water quantity and quality through altered infiltration, transpiration, and runoff rates. Boyle et al. (1997) examined published data, aerial photos, and additional historical information to look at land use changes in the Lower Fraser Basin of southwestern British Columbia. The analysis quantified estimates of land cover for the years prior to 1827 (the start of European settlement) and for 1930 and 1990 (Boyle et al., 1997). The total area of wetlands (fen, swamp, bog, and marsh) declined from 831 km2 prior to 1827 to 163 km2 in 1930 to 121 km2 in 1990. These declines coincided with a dramatic increase in urban and agricultural area from none pre-1827 to 2,184 km2 in 1990 (Boyle et al., 1997). These changes resulted in a dramatic decline in waterfowl from numbers estimated to be in the billions in 1920 to 506,600 in 1995 (Boyle et al., 1997). Timoney and Argus (2006) explored trends in riparian vegetation cover in response to variability in water levels in the Peace–Athabasca delta using five common willow species. Between 1993 and 2001, overall willow cover declined but there was large variability between species, for example Salix bebbiana and Salix discolor appeared to be the most susceptible to flooding. There was a strong correlation between willow dieback and water depth, duration of flooding, and time since flooding. In addition, more rapid willow establishment coincided with a drying period in the delta, increased regional wildfire activity, a decline in river discharge, and a decline in the level of Lake Athabasca, during the early 1980s. An increase in willow cover followed and reached a peak around 1993 while flooding in the mid- to late 1990s resulted in a decline in willow cover (Timoney and Argus, 2006).
- Footnote 9
An interbasin diversion is the withdrawal of water, more or less continuously, over all or part of a year, by ditch, canal, or pipeline, from its basin of origin for use in another drainage basin.
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