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Threats to Water Availability in Canada

2. Dams, Reservoirs and Flow Regulation

Terry D. Prowse,1 Fred J. Wrona2 and Geoff Power3

1 Environment Canada, National Water Research Institute, Victoria, BC
2 Environment Canada, National Water Research Institute, Saskatoon, SK
3 University of Waterloo, Department of Biology, Waterloo, ON

 


Current Status

In Canada and throughout the world, dams have been constructed to reduce risks associated with flood hazards, to harness energy for industry and commerce, and to help secure a reliable source of water for domestic, industrial and/or agricultural use. While dams have been integral to agricultural/industrial development, they are also structures that transform river ecosystems over a range of spatial and temporal scales (e.g., Calow and Petts, 1994; Petts, 1984). Of special interest to Canada is the role of cold-regions processes in such transformations (e.g., Prowse and Conly, 1996; Rosenberg et al., 1997).

Through impoundment and increased residency times, dams alter water temperatures and chemistry, which in turn influences rates of biological and chemical processes. Dams create barriers to the upstream-downstream movement of nutrients and organisms, thereby affecting physical and biological exchange processes. They also alter the timing and magnitude of downstream fluxes of water, sediment, and ice, which modify biogeochemical cycles and the resulting structure and function of aquatic and riparian habitat. As dams occasionally collapse, they also present a risk to the built environment and downstream ecology.

Canada’s Current Dam Inventory

Although there is no national inventory of all sizes of dams in Canada, the Canadian Dam Association (CDA) periodically conducts an assessment and submits data to the International Commission on Large Dams (ICOLD). In 2000 (Canadian Dam Association, 2003), Canada had 849 large dams (≥10 m in crest height) in operation or under construction. This assessment does not include tailings-pond dams, which would add approximately 84 more dams to the total. The vast majority (70%) of large dams in Canada were constructed solely for hydroelectric production. Of the remainder, 7% were constructed primarily for water supply and 6% for irrigation, mainly in the Prairie Provinces. The rest serve a variety of purposes from flood control to navigation, recreation, or a combination of purposes.

Unfortunately, data concerning volume storage and flooded areas are not catalogued for all large dams. Canada does, however, have ten of the world’s largest 40 dams as measured by gross reservoir capacity (ICOLD, 2003). The storage capacity of the 849 largest reservoirs is sufficient to hold the equivalent of two years’ runoff from all of Canada, or approximately one quarter the volume of the five Laurentian Great Lakes. Although storage of water in reservoirs increases evaporation, the total effect cannot be estimated because information about reservoir areas and water depths is lacking. Greatest losses occur in shallow reservoirs and where dams have been constructed in hydro-climatic zones characterized by naturally high rates of lake evaporation (den Hartog and Ferguson, 1978), such as Lake Diefenbaker in the central Prairies (e.g., Canada-Saskatchewan, 1991).

No national inventory of small dams exists for Canada but they are estimated to be significantly more numerous than large dams. In British Columbia, for example, there are approximately 2500 small dams (Jolley, pers. comm.), but only 99 are classed as “large” (Canadian Dam Association, 2003). Similarly, Quebec has 5144 dams with heights ≥1 m in their database (Lavallée, pers. comm.) but only 333 large dams (Canadian Dam Association, 2003). Assuming that an average large-to-small dam ratio from these two regions applies to the entire country, Canada has at least 10,000 small to large dams. The effect on storage and evaporation by this larger total is even more difficult to estimate than for the large dams because of the complete lack of data concerning storage capacity and flooded area.

Regulation Effects

Water Levels and Flow

Two major hydraulic changes generally occur with the construction of a reservoir. First, the water area above the dam will change from lotic (i.e., running water) to lentic (i.e., standing water) in nature, with associated changes in hydrologic and ecological processes. Second, diurnal and seasonal variations in the demand for water or power will cause short- and long-term variations in discharge quite different from those seen in an undammed river.

Typically, the reservoir has two purposes: to increase the hydraulic head or difference in water level across the plant, and to provide storage for periods of low inflow from upstream. Hydroelectric operations are referred to as "run-of-river" when only the first of these is important. Such plants are common additions downstream of large reservoirs (e.g., Peace Canyon Dam below Williston Reservoir) or lakes (e.g., power plants on the Nelson River system, with storage provided by Lake Winnipeg and other natural lakes). They require only sufficient upstream storage to balance flows and to develop the necessary head across the plant.

The ability to store significant amounts of water is a common design feature of larger hydroelectric operations. Demands in Canada are typically at a maximum during winter and at a minimum during summer, a direct contrast to the natural seasonal availability of water over most regions, which is characterized by spring/summer maxima in runoff and winter minima (except in west coastal systems). To counter this imbalance between power demand and the seasonality of the natural hydrologic cycle, a significant portion of the flow is stored during summer and released during winter. Overall, the regulated regime results in a flattening of the annual hydrograph including a dampening of peak flows, particularly where reservoir storage is large relative to runoff volume.

Although the above scenario describes the most common form of seasonal redistribution of flows by regulation, other regime changes can occur depending on the interrelated design of the hydrologic and hydroelectric networks. Some rivers, for example, can experience a decrease in flows throughout the year because a portion of their flow is diverted to feed hydroelectric production in another system. This latter system then experiences an annual increase in discharge, as is the case with the diversion of the Churchill River into the Nelson River in Manitoba, and the Nechako and Kemano river systems in British Columbia (see discussion of flow diversion in Chapter 1).

On large rivers, the physical and ecological effects of flow regulation can be experienced several hundreds of kilometres downstream, with compounding effects occurring on systems with series of dams. For example, significant, far-reaching, downstream hydrological, biogeochemical, and ecological effects have been observed in the St. Lawrence estuary that have been attributed directly to cumulative effects arising from upstream hydro impoundments (Neu, 1982a,b). Similarly, major environmental concerns have been raised about the compounding effects of multiple dam and causeway construction on a majority of rivers entering the Bay of Fundy (Wells, 1999). By and large, however, quantification of far-reaching cumulative effects (e.g., habitat degradation in estuaries, related offshore nutrient disruptions) has been largely neglected (Rosenberg et al., 1997, 2000).

Ice and Sediment Regimes

River ice is an integral part of most of Canada's rivers, many of which remain ice-covered for six months a year or longer. It also governs a number of processes controlling timing, duration, and magnitude of flow and water levels (Prowse, 1994). Related discharge and water-level hydrographs vary significantly from those of more temperate regions, although the full significance of this to winter aquatic ecology is only now gaining recognition (Prowse, 2001a,b). Any modification of the winter flow regime, as by reservoir regulation, will have concomitant impacts on the ice regime and related winter ecology of a river. These can be highly significant, given that ice is responsible for many of the annual extremes in hydrologic events, such as floods and low flows (e.g., Prowse, 1994).

Regulation can significantly modify duration of an ice cover or even the presence of ice through the release of warm hypolimnetic water. Increased or fluctuating discharge during the winter can lead to changes in integrity of ice cover as well as in location and number of lodgement sites for initial cover development. Moreover, higher flows can lead to covers of greater thickness and rougher surfaces: features that combine to elevate water levels above those occurring under unregulated conditions and greater than those expected under high-flow events. Once an ice cover has been established, regulated-flow conditions can further control its growth and even induce mid-winter breakups and associated flooding.

River regulation can modify the sediment regime of a river through retention of material within the reservoir and through modifications of downstream erosion and deposition processes. Short reservoir life expectancies are associated with small-scale dams that impound rivers with high levels of sediment influx. Continued reduction in storage capacity of such reservoirs through sediment accumulation results in a decreased water-retention capacity, and may lead to an inability to retard the passage of floodwater downstream.

Scouring of a river channel immediately downstream of a reservoir commonly occurs but patterns of morphological change become more complex downstream. Changes in the flow and flood regime have implications relative to the competence of the channel to carry sediment and to the ability of the system to flush sediment deposited during low-flow events. On large alluvial rivers, degradation processes are constrained to the first few or tens of kilometres downstream of the point of regulation, and a one- to three-metre depth of degradation typically occurs within a decade or two of regulation (Church, 1995).

Further downstream, where tributaries add more material to the river, aggradation may be more common than degradation. Lower regulated flows, especially without the natural freshet peaks, simply do not have the conveyance power to carry material produced by upstream degradation as well as that contributed by the tributary flow. Where aggradation occurs, the nature of the morphological response depends on the character of the alluvial deposits. Typical responses may include lateral scour, channel widening, braiding, and a reduced mean flow depth. Successive species advance of vegetation down the banks onto abandoned floodplains, however, can lead to an adjustment in the overall flow pattern and, ultimately, to a narrower channel.

One critical aspect of changes to a river-sediment regime is time scale. Although some dramatic changes can be observed in the first few years after regulation, the time required for a system to achieve a new equilibrium depends on manner of regulation, form and composition of the channel, and rate at which vegetation becomes established. Because of the huge volumes of sediment involved on large northern rivers and the associated slow rate of vegetation change, the time scale for adjustments can be in the order of centuries. As yet, however, no system in Canada has been studied systematically for more than a few decades (Church, 1995).

Water Quality

Water quality can be significantly affected by impoundment. Physical, biogeochemical, and biological processes occurring within a reservoir can affect the temperature and chemical composition of the water leaving the system to an extent that its quality upon release no longer resembles that of the inflows. The degree to which water quality is affected on a diel, seasonal and/or annual basis depends on factors such as surface to volume ratio and depth of the reservoir; geology and soil geochemistry of the surrounding catchment; latitude of the reservoir; rates and magnitude of sedimentation; magnitude and timing of incoming flows and their residency time; and level of biological productivity in the reservoir.

Temperature changes relate to the reservoir’s thermal mass and surface area for radiant exchange, retention time, thermocline development and whether release water originates from the surface or at depth. For instance, in hypolimnetic discharge reservoirs, water is released at depth and is cooler in summer and warmer in winter than unregulated flow at the same location prior to reservoir formation. In contrast, epilimnetic discharge reservoirs (which tend to be shallower) produce elevated downstream water temperatures due to the thermal warming of surface waters. Consequently, these types of alterations in thermal regimes can have profound consequences on the type and complexity of biological communities that can be sustained downstream (Baxter, 1977; Baxter and Glaude, 1980).

Chemical changes in water quality are less predictable due to the complexity of interrelated physical, biological, and chemical processes occurring in the reservoir, both in the open-water season as well as under-ice in the winter. Chemical changes include altered nutrient levels and dynamics, modified water-column and sediment oxygen regimes, nitrogen supersaturation in downstream waters, and increased mobilization of certain metals. In newly formed reservoirs, water quality is often also affected by a trophic upsurge due to release of materials from the newly flooded area, which can be of short duration or last several years in the largest impoundments. One of the more predictable water-quality effects of impoundment is release of mercury from flooded sediments (Rosenberg et al., 1997). Mercury in its methylated form enters the food chain and is bioconcentrated, with highest concentrations occurring in piscivorous fish and birds. These elevated tissue levels can often exceed those recommended for human consumption (particularly in older biota), thereby creating associated human and environmental health risks.

Largely because of the climate-change driven pursuit of “clean” energy sources, attention has also focussed on the role of water storage in affecting production and emission of greenhouse gases (GHG). In contrast to the widespread assumption (e.g., in Intergovernmental Panel on Climate Change scenarios) that GHGs emitted from reservoirs are negligible, measurements made in boreal and tropical regions indicate they can be substantial (St. Louis et al., 2000; World Commission on Dams, 2000). Although Canada is attempting to complete an inventory of the Canadian situation, comprehensive data concerning flooded areas are difficult to access (St. Louis et al., 2000) and accuracy of methods to estimate gas fluxes from reservoir surfaces remains uncertain (Rosa and dos Santos, 2000).

Reservoir and Downstream Ecology

Changes in the physical and chemical characteristics of water from impoundment inevitably affect distribution and abundance of aquatic biota and resulting community structure. Within new reservoirs, fish populations are often quite large during the first few years, largely because of increased nutrients leached from flooded soils and vegetation, enhanced productivity throughout the food chain, and provision of secure sites for spawning and predator protection (e.g., Baxter and Glaude, 1980). Once established, the new physical/chemical characteristics of a reservoir can pose challenges to biota, primarily because they are not in synchrony with natural cycles. Disturbance to spawning resulting from the drawdown/raising of water levels, changes in seasonal temperature cycles, and blocked migration for fish are some major examples. See Baxter and Glaude (1980) and Rosenberg et al. (1997) for more detailed treatment of in-reservoir effects.

Similarly, downstream biota are exposed to a new disturbance regime (e.g., diel and/or seasonal alterations in discharge and thermal regimes), the degree of disturbance depending on the severity of the change and the distance downstream of the dam. For instance, lotic fish species select their preferred habitats by depth, water velocity, and type of substrate. If these change rapidly, as they would immediately downstream from a peaking hydroelectric station, the area would likely be abandoned by these species.

Intuitively, biotic communities should exhibit a dynamic response to opportunities presented by their environment, although the role of physical and biotic factors in structuring aquatic ecosystems is not always clear (Power et al., 1988; Rosenberg et al., 1997). When physical characteristics change too rapidly and in unpredictable frequencies, a stable equilibrium may never be achieved. In general, damaged communities of colonizers, tolerant species and temporary residents established nearest to the dams are replaced by more natural communities downstream as conditions ameliorate and tributaries and groundwater exchanges return the river to a more natural regime (e.g., Ward and Stanford, 1989; Curry et al., 1994).

Dams, designed to meet daily to weekly hydroelectric demands, have more variable water levels and flow regimes than large storage reservoirs. Consequently, they can produce higher disturbance effects on in-channel and riparian processes and related biota (Nilsson et al., 1997; Jansson et al., 2000). Hence, regulated discharges are often directly responsible for reduced habitat diversity and biodiversity in downstream reaches (Jansson, 2002). Although most responses to flow regulation are site-specific, general patterns of large-scale downstream effects are being observed worldwide and a synthesis of these is emerging (e.g., Dynesius and Nilsson, 1994; Nilsson et al., 1997; Rosenberg et al., 1997, 2000).

Manic 2A Gravity Dam | Photo: Hydro-Québec

Trends

Dam Construction

Figure 1 depicts the growth in the number of large dams in Canada from the start of the century to the present. Prior to the 1940s, the majority of large dam construction occurred in southern Ontario and Quebec. Since then, major dam construction has occurred in all provinces and territories. Within the U.S., the most active period of dam building occurred between 1950 and 1970, and has been called “the golden age of dam building” (Doyle et al., 2003). The same comment is frequently made about the situation in Canada. As shown in Fig. 1, however, the 1970s experienced almost equivalent intensity of dam construction as the 1950s. The 1970s peak is primarily due to extensive dam construction in Newfoundland and northern Quebec. Since the 1970s construction has steadily decreased with the vast majority of development (i.e., >70%) continuing in Quebec. However, given the number of large dams currently under construction and proposals for further expansion, for example, in northern Quebec (Holzinger, 1998; Hydro-Québec, 2002), Manitoba (Lett and Samyn, 2003) and the Northwest Territories (Howatt, 2001), it is truly debatable whether Canada has yet passed its major period of large dam building.

Number of new Canadian dams constructed by decade

Fig. 1. Number of new Canadian dams constructed by decade (not including tailings-pond dams). Line shows percentage of dams older than x years. Data from Canadian Dam Association (2003).

Although northern rivers hold the most remaining potential for large-scale hydroelectric development in Canada, there is also a trend toward construction of small-scale hydroelectric facilities. Numerous small-scale plants were in operation earlier this century, but there followed a gradual move by public utilities to larger generating units to achieve economies of scale. In fact, many small-scale plants were decommissioned following World War II because they became progressively less economical to maintain and operate. More recently, however, because of the size, cost and negative environmental impacts of large dam projects, hydro development has been increasingly focussed on small-scale projects, i.e., those with less than 10 MW of generating capacity. Many of these are run-of-the-river projects. There are currently more than 300 plants in Canada with a capacity of 15 MW or less (Industry Canada, 2003) and numerous others under consideration, particularly for remote communities that rely on high-cost diesel generation. Approximately 5500 sites in Canada are technically feasible for small-scale hydroelectric production (Natural Resources Canada, 2000).

Dam Safety

Of all civil engineering works, dam failure poses some of the highest potential risk of damage to life and property. Moreover, it can lead to loss of drinking and irrigation water supplies, and of hydroelectric generating capacity. Although there is no historical cataloguing of dam failures in Canada, fortunately there has been no major dam failure resulting in loss of life (Bechai and Christl, 2001). There have been, however, numerous incidents involving smaller impounding structures, often designed to mitigate floods of short return period (Watt, 1989). Moreover, as noted by the International Joint Commission with respect to the safety of transboundary dams, concern has been expressed about the lack of a federal dam safety program and regular government inspections. They further point out that although current guidelines developed by the Canadian Dam Association are influential with dam owners and governments, they are only voluntary and fall short of being actual standards or specifications (Legault et al., 1998).

In general, three types of initiating events are considered in failure initiation: static, seismic and hydrologic--overtopping by extreme events being the major failure concern. The most significant overtopping event in Canada was associated with the enormous 1996 Saguenay flood (Bechai and Christl, 2001). To avoid flood-generated dam failures, most dam safety studies begin with a hydrologic analysis to derive an Inflow Design Flood (IDF: volume, peak, duration, shape and timing), commonly defined as the most severe inflow flood for which a dam and its associated facilities are designed. For major dams or for those whose failure may cause significant economic losses or loss of life, the IDF is often defined as the Probable Maximum Flood (PMF) (Zielinski, 2001). Unfortunately, in most situations, available data are insufficient to define precisely the probability of large floods and estimates. Hence, estimates must be made beyond the temporal range of the existing data.

Static failure modes of particular concern include erosion, increased seepage, ice effects and, more recently, terrorism (e.g., Martin, 2001). Ice effects are of particular importance to a cold country like Canada, including how changing water levels influence ice loads (Comfort et al., 2000). Seismic impacts on dams can be a concern and some older small structures have been removed because they do not meet current engineering standards. Dam construction is also known to induce seismic activity, water depth being the most important determining factor. One well-established instance of induced seismicity (shock magnitude of 4.3) occurred during the filling of the Manicougan 3 Reservoir in 1975 (Baxter and Glaude, 1980).

The general ageing of dams is another major safety concern. This is especially true for regions where development in the watershed and urbanization below dams have increased the risk to loss of life and property damage. Figure 1 shows the median age of large dams in Canada, which currently stands at 40 years. Based on extensive U.S. experience, the life span of typically unmaintained dams is conservatively estimated at 75 years, refuting the common misconception that the average life of a dam is 50 years (Donnelly et al., 2002). Almost 80% of dam removals in the U.S. have occurred because of concern for dam safety or spillway capacity, but there is an emerging trend toward removal for environmental concerns, as discussed below regarding Canada.

Downstream Biotic Impacts

Over the past three decades, a significant scientific effort has been invested in improving the predictive understanding of relationships between streamflow and aquatic habitat quantity and quality. Collectively these approaches have been referred to as Instream Flow Needs or Requirements (IFN or IFR) (Bovee et al., 1998; Stalnaker et al., 1995). In the 1970s, instream flow determination focussed primarily on methods that attempted to predict: “what is the minimum flow which must be released from the dam in order for the downstream aquatic ecosystem to survive?” Approaches often focussed on maximizing microhabitat for a single life stage of a high-profile fish species (most often salmonids) at a few isolated locations in a river system.

These methods were generally developed for use in small stream and river systems assuming that if flow needs were met, the rest of the aquatic ecosystem would be protected. Examples include the Tennant or “Montana” method (Tennant, 1976) and the Minimum Ecological Flow approach (Stalnaker et al., 1995). Further advancements led to development of the Instream Flow Incremental Methodology (IFIM) and related physical habitat simulation modelling (PHABSIM), which provide more rigorous methods of quantifying effects of stream flow on fish habitat.

Applying these approaches requires detailed knowledge of habitat selected by target fish to be protected--usually a valued game fish. By collecting data on the depth, nose water velocity, and substrate where target fish occur, habitat preference and suitability relationships are generated. Coupled hydraulic modelling is then used to produce detailed maps of the stream showing depths, velocities and substrate, and to calculate species-specific usable habitat area. By simulating different discharges, the model predicts the numbers of target species likely to be present in the area modelled and, by extrapolation, the numbers of fish in a longer reach of river.

IFIM has been widely applied as a tool for predicting potential aquatic habitat effects of water abstraction and flow regulation, in spite of increasing criticisms (see Armour and Taylor, 1991; Bovee et al., 1998; Mathur et al., 1985 for a full discussion). Although IFIM approaches are complex and their results must be interpreted with caution, the method continues to evolve as a more refined decision-support system/framework to examine possible benefits/impacts of flow regulation on various components of the aquatic ecosystem and related socio-economic issues (Bovee et al., 1998; Stalnaker et al., 1995; Walder, 1996).

To develop techniques that can be applied on very large Canadian rivers, recent attempts have been made to use remote-sensing based evaluations of habitat under changing flow regimes (e.g., Courtney et al., 1996), and to broaden the scope of assessments to include all aquatic biota and hydrograph components (e.g., Milburn et al., 1999). Such techniques remain in developmental stages but are much needed since most large dams affect the larger river systems. A recent review of the state-of-the-art in aquatic habitat modelling and conservation flows is provided by St. Hilaire and Leclerc (2003).

Emerging Issues

Decommissioning/Removal of Dams

With changing societal needs in developed watersheds and a growing recognition that dams impair the structure and function of river ecosystems, interest has increased in removal and decommissioning of dams (Babbit, 2002; Poff and Hart, 2002; Pohl, 2002). Typically, dams have been removed because the cost of rehabilitation measures to satisfy dam safety concerns is considered to be higher than the value offered by continued operation of the structure, not because of a dam failure (Donnelly et al., 2002).

There is, however, a great deal of social, economic, and scientific uncertainty about the short-, medium- and long-term environmental benefits of dam removal. While it is known that dams alter the geomorphic and hydrologic processes of riverine systems, our current scientific understanding of how their removal directly affects the downstream flow of water, patterns of sediment movement, and overall channel morphology is limited (e.g., Zhou and Donnelly, 2002a). This is further complicated by ever increasing rural and urban development on downstream floodplains that produce additional constraints on dam removal.

Sediments accumulate behind dams during their operations and the potential for accumulated sediments to have elevated levels of specific chemical contaminants (e.g., metals, petroleum hydrocarbons, pesticides) is high (e.g., Warren and Zimmerman, 1993). Little is known about how the concentrations, fate and distribution of contaminants in the sediments will change with dam removal and affect downstream biological communities, both spatially and temporally. Equally, dams usually alter the residence times of water, thereby affecting various chemical properties. However, uncertainty remains as to how and to what extent removal restores riverine biogeochemical processes to states similar to those before dam construction. As physical barriers, dams often break “natural” food web linkages and interactions that exist between species in riverine communities (e.g., species are not allowed to move freely upstream/downstream). The effects of dam removal on recovery of biological communities have not been well studied.

Climate Change

As noted by the World Commission on Dams (2000), it is generally agreed that the future can no longer be assumed to be the same as the past with respect to design and maintenance of dams. It is becoming increasingly recognized that climate change is likely to have significant impact on the safety of existing dams that were designed based on past records. Although some detailed evaluations of the effects of continuing present-day trends in hydrologic conditions on future IDFs have already been made (e.g., Zhou and Donnelly, 2002b), predictions based on future climate scenarios modelled by GCMs (Global Climate Models) are, for the most part, yet to be conducted. Most future flood predictions are fairly nebulous and rely on generalized predictions about future precipitation and runoff conditions, such as the likelihood of advanced spring melt or shifts from nival (snowmelt dominated) to pluvial (rainfall) regimes (IPCC, 2001). This is understandable given the difficulty of using current GCMs to provide good regional estimates of precipitation. As the quality of such predictions increase and as more detailed RCMs (Regional Climate Models) are developed, it will be possible to conduct regionally focussed assessments of the effects of climate change on specific flood characteristics--information that subsequently should be incorporated into dam-safety assessments.

Knowledge and Program Needs

Overall, the water-quantity related threats associated with dams, reservoirs, and flow regulation fall into three broad categories:

  • threats to the aquatic environment created by the installation of such systems
  • emerging related threats linked to their removal, and
  • the threat posed by climate variability and change.

Although new knowledge is required to address all these threats, Canada first needs to undertake a major program to quantify the degree to which hydrologic systems are impounded and regulated. This would provide, for example, a basis for assessing the cumulative impact of flow regulation on freshwater ecosystems, evaluating future impacts of climate change, and quantifying greenhouse-gas emissions. Canada needs a broader inventory of dams and impoundments beyond the large-dam summary compiled by CDA or Dynesius and Nilsson (1994). Many provinces also inventory small-scale developments and these need to be integrated with the CDA inventory. Furthermore, all such inventories should include data on reservoir surface area, a critical variable for many scientific assessments. Such a variable would permit, for instance, calculation of water losses due to enhanced reservoir evaporation and its significance relative to other water-balance components--information critical for proper planning and managing of basin water resources.

So that the relative GHG merits of different energy-producing sectors can be evaluated, Canada needs to complete an inventory of GHG emissions from Canadian reservoirs. Specifically, net GHG emissions should be calculated for pre- and post-flooding conditions. Given the paucity of such data, calculations will probably have to be based on emissions from flooded and unflooded areas of similar landscapes, and related estimates of the carbon budget for the respective contributing watersheds.

Through its strong effect on the hydrologic cycle, climate change poses a threat to the current network of dams and reservoirs. To minimize risk, more research is required to define new IDFs that can be used to gauge the safety of existing structures and to guide future constructions. There is a related need to quantify the new downstream flow and ice regimes under which dams will have to be operated to minimize downstream flood risks and disruptions to aquatic ecosystems. The potential effects on dam safety posed by climate change also raises further questions about the often-cited need to introduce a more formalized dam-safety inspection program.

Although effects of impoundment on stored water and related aquatic habitats are relatively well studied, significant gaps remain in our understanding of downstream effects. For small systems, there is a dearth of data to satisfy complex IFN-style approaches for evaluating biotic impacts. Moreover, evaluation of regulated-flow impacts on medium to large rivers is in a rudimentary state of development. Since such knowledge is crucial to designing proper flow-management strategies for regulated rivers, IFN advancements need to be made. One logical course of action is refinement of remote-sensing techniques, particularly to quantification of critical habitat. Developing such tools would be particularly beneficial for evaluating near-shore and/or shallow-habitat zones, where productivity tends to be highest and most susceptible to changes in flow.

Since effects of flow regulation have not been systematically studied for more than a few decades, a program to continue long-term assessments of such changes on Canadian rivers is needed. Moreover, to evaluate the range of flow-regulation sensitivity such a program should include sites located in different hydro-morphological regimes. Similarly, effects of flow regulations on downstream ice conditions have received limited study and only in selected hydro-climatic regimes. This should be a special focus for Canada considering most rivers are ice-covered for a significant portion of the year, and ice is a major source of extreme events (low flows and floods) and a significant modifier of hydro-ecological processes. Such a program should also focus on assessing far-downstream effects and cumulative impacts on systems containing multiple dams/reservoirs.

Given the age and shift in requirements for dams/reservoirs in Canada, it is likely that dam removal will become increasingly popular--following a trend already established in other countries, particularly the United States, where attempts are in progress to integrate it into policy and decision making (e.g., The Aspen Institute, 2002). To be better positioned to evaluate costs and benefits of removing dams, it is essential for Canadian science to develop a comprehensive understanding of dam-removal methods and effects applicable to this country’s broad range of regulated rivers.

Acknowledgements

The authors are indebted to Ms. Chloe Faught and Mr. Laurent de Rham who tirelessly pursued much of the background data and information employed in this report. Thanks are also due to the various agencies and their representatives that supplied additional up-to-date information about their activities and/or data catalogues. Reviews and comments provided by A.T. Bielak, J.J. Gibson, M. Healey, R. Kallio and D.M. Rosenberg are also gratefully acknowledged.

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