Course:EOSC270/2023/The Effects of Upstream Dams on River and Estuary Ecosystem Dynamics
Overview: Human Impacts on Estuary Ecosystems
Estuaries are highly productive land-sea interfaces where freshwater and seawater interact, and they act as a hub for tourism and transport while providing many essential ecosystem services. For example, in the U.S., estuaries account for more than 75% of commercial fish catch (Hunt, 2024). Estuaries are found on coastlines around the world, and they can vary in topography, salinity, and sediment type.
As societies progress, an unintended consequence is the alteration of natural landscapes and ecosystems. Notably, estuaries have both ecological and economic importance, as 60% of the world's population lives either directly on or adjacent to an estuary (Hunt, 2024). As a result, these ecosystems are at risk for human interference and exploitation.
Due to the significant human activity that occurs within and around estuaries, these ecosystems are highly stressed and are often artificially engineered to support societal needs. Major human impacts on estuaries include enrichment, toxins, overfishing, invasive species, and physical alteration (Kennish, 2005).
An Introduction to Dams
Dams are physical barriers constructed on almost one half of the world's rivers in order to control the amount of water that passes through (Adams et al., 2023). Approximately 48% of dams worldwide are used to store water for irrigation, 20% are hydroelectric dams used for electricity generation, and 8% are built for the purpose of flood control (Adams et al., 2023). While there are many human and societal benefits to dam construction, dams also impact water quality, sediment transport, nutrient flow, and habitat availability of the rivers they are built on. These alterations have adverse effects on the biodiversity and functionality of connecting estuary ecosystems, such as the migration of various anadromous (species that spawn in freshwater and spend adult stages in marine environments) or catadromous species (spawn in marine environments and spend adult stages in freshwater) (Hunt, 2024). The construction of dams also interrupts natural flow regimes, leading to significant ecological changes (Zhang et al., 2022; Wu et al., 2022).
To minimize these impacts, strategies such as community and mitigation planning, reservoir management, fishing bans, and the removal of obsolete or unnecessary dams are recommended for ecosystem restoration (Zhang et al., 2022; Wu et al., 2022).
Case Study: Seymour River
Located in North Vancouver, the Seymour Capilano Filtration Plant is Canada’s largest drinking water filtration plant, with a daily capacity of up to 1.8 billion litres (MetroVancouver, n.d.-c). Water from the Seymour Reservoir, constructed in 1981, and the Capilano Reservoir is diverted to the treatment plant through underground tunnels.
Seymour River is a prime local example that can be used to explore the abiotic and biotic impacts of dams, as well as potential restoration solutions.
Digging Deeper: What Are the Effects of Upstream Dams on River and Estuary Ecosystem Dynamics?
Sediment Retention
Dams significantly alter hydrological dynamics and sediment transport in river and estuarine environments, leading to notable ecological consequences. Hydrological alterations, as observed in the São Francisco River, demonstrate that dams can transform consistently flowing rivers into a series of disconnected reservoirs. This transformation leads to substantial reductions in water flow and changes in the natural seasonal flow patterns, which represent the periodic changes in water volume and speed throughout the year (Zhang et al., 2022). This modification disrupts the natural sediment transport mechanisms, with dams acting as barriers that trap sediment, reducing the sediment load reaching estuaries and coastal zones. For instance, the Three Gorges Dam on the Yangtze River retains around 151 million tons of sediment annually, severely reducing the sediment flux to the estuary and altering downstream geomorphology (Yang et al., 2007).
In addition to these observations, the effectiveness of check dams in the Yellow River's Hekou-Longmen section has been studied, revealing that these structures can significantly decrease the amount of coarse sediment entering the river system. When the coverage area of check dams in the catchments is 3%, sediment reduction ratios can reach 60%, highlighting their role in sediment management and the importance of strategic placement (Ran et al., 2008). Additionally, research in Maine estuaries has found that when river and seawater mix, the iron particles that form do not always get removed significantly from the water. Instead, a considerable amount of this iron settles into the estuarine bottom sediments. This process increases the iron concentration in these sediments, impacting the biogeochemical cycling within the coastal ecosystems (Mayer, 1982).
The reduction in sediment transport has cascading effects on riverbank and estuarine morphologies, leading to enhanced erosion downstream and significant changes in underwater delta formations. This geomorphic transformation can shift the turbidity maximum zone upstream, impacting benthic environments and altering aquatic habitats (Zhang et al., 2022). In the Yangtze River, these changes have led to a transition from delta expansion to recession, emphasizing the significant impacts of sediment retention on ecosystem structure and function (Yang et al., 2007).
Globally, dams have led to a decreased sediment delivery to the oceans, affecting both riverine and marine ecosystems. The alteration in sediment dynamics extends beyond the physical landscape, influencing biogeochemical processes within these environments. Sediment retention in reservoirs alters the natural flow and composition of sediments, impacting the downstream sediment load and consequently affecting the riverine and estuarine ecology. For instance, typical sediment distribution in reservoirs shows a marked reduction in sand and silt, with an increased proportion of clay, demonstrating significant shifts in sediment composition across the reservoir (Wang et al., 2018).
Seymour River - Relevance
The Seymour Falls Dam on the Seymour River serves as a prime example of the complex relationship between dam infrastructure and sediment dynamics. Like the São Francisco and Yangtze rivers, the Seymour River has undergone significant hydrological and sediment transport alterations due to damming. The dam has transformed the river's flow regime, creating segmented reservoirs and altering the natural sediment load that reaches the estuarine and coastal zones. Such changes mirror the substantial flow reductions and altered seasonal river rhythms noted in global case studies (Zhang et al., 2022; Yang et al., 2007).
The existing Seymour Falls Dam highlights the need for a deeper understanding of local sediment management. The effects observed in other river systems, such as reduced coarse sediment transport in the Yellow River and altered iron concentrations in Maine estuaries, suggest that the Seymour River's sediment and ecological dynamics may have shifted similarly (Ran et al., 2008; Mayer, 1982). These global insights highlight the potential impacts on the Seymour River, indicating that the dam has influenced riverbank and estuarine morphologies, led to changes in sediment composition, and affected the broader ecological framework of the river system.
Nutrient Flow Alteration
Dams affect the biogeochemical dynamics within river ecosystems, through the retention of essential nutrients such as nitrogen and phosphorus in their reservoirs. This retention impacts the downstream flow of nutrients, leading to the alteration of aquatic productivity and the ecological balance of downstream waters (Graf, 1999; Stanley & Doyle, 2002). The large outpour of nutrients, while accumulating in the reservoir, can lead to the growth of algal blooms, which, upon release, can cause episodes of hypoxia, impacting aquatic life and water quality (Carpenter et al., 2011).
The interruption of sediment-bound nutrients transport is another effect of damming, affecting both the structure of riverine habitats and the bioavailability of sediment-bound nutrients. Sediments has many roles in the aquatic nutrient cycle, one of them being supporting benthic communities that are important to the ecosystem's overall productivity. The decrease sediment-bound nutrient flow due to dams leads to degradation in habitat quality for these organisms, leading to effects in the aquatic food web (Vörösmarty et al., 2003). Additionally, the reduced delivery nutrients to river deltas (rivers carrying sediment reach of body of water) can contribute to the erosion of coastal wetlands, areas that are important for fishery nurseries and as natural protective barriers against storms (Syvitski et al., 2005).
Altered hydrology resulting from dam operations disrupts the natural flood of rivers, which is crucial for the functioning of floodplain ecosystems nutrient exchange. This alteration affects not just the hydrological dynamics but also endangers the ecological nutrient connectivity between the river channel and its floodplain, leading to decline in habitat diversity and lowered biological productivity. Alterations to river flow patterns have implications for species that depend on the nutrients in floodplain ecosystems for part of their life cycle, impacting their reproduction and feeding behaviours (Poff et al., 2007; Bunn & Arthington, 2002).
The disruption in nutrient flows due to dam retention affects aquatic life, specifically the anadromous fish species. These fish depend on the chemical cues from nutrient-enriched waters to navigate and complete their life cycles, this includes migration and spawning. Alterations in nutrient availability can also shift the distribution of primary producers such as phytoplankton, setting off a series of negative effects through the food web, which can alter the abundance and diversity of higher trophic levels, including species of commercial fisheries (Pringle et al., 2000; McCluney et al., 2014; Higgins & Vander Zanden, 2010).
Seymour River - Relevance
The Seymour River Dam is a local example of how dams impact on nutrient flow and ecosystem dynamics. By retaining essential nutrients such as nitrogen and phosphorus, the dam disrupts the natural biogeochemical processes within the river ecosystem, affecting everything from algal growth within the reservoir to the aquatic productivity and ecological balance of downstream waters. Additionally, the dam alters sediment-bound nutrient transport, thereby affecting the habitat for benthic communities and affecting the river's contribution to coastal wetland productivity and structure. There are also potential challenges for anadromous fish species and other aquatic life, which rely on nutrient-enriched waters of Seymour River for navigation and life cycle completion.
Reshaping of Water Flow
For a given watershed, flow regime is characterized based on the magnitude, duration, rate of change, and frequency of the water being discharged (Clarke et al., 2008). The construction of dams disrupts natural flow regimes, which can be detrimental to downstream river and estuary ecosystems.
Zhang et al. (2022) concluded that dam construction reduces river flow velocity and the average water flow entering downstream river estuaries, and this modified freshwater input has been shown to impact downstream estuary salinity. In a case study conducted on the Çine River, Turkey, it was shown that the operation of the Çine Dam decreased the electrical conductivity of the water downstream from the dam, which is indicative of a decrease in the water’s salinity (Bor & Elçi, 2022). In general, periods of high discharge from dams will decrease downstream salinity, while periods of low discharge will increase downstream salinity. Changes in salinity can have consequences for salinity-sensitive estuarine organisms; for example, Loveridge et al. (2021) demonstrated that the scyphozoan jellyfish in the Fitzroy River are unable to survive at the low salinity levels they are exposed to when the Fitzroy Barrage is fully opened.
Changes in water flow due to dam operation can also lead to changes in water temperature. Reduced water flow has been shown to result in decreased temperatures in the winter and increased temperatures in the summer, creating a more extreme thermal environment downstream of dams (Clarke et al., 2008). In rivers with hydropower dams, hydropeaking (adjusting the water flow through a dam to match electricity demand) can cause daily fluctuations in water temperature. Changes in water temperature are known to impact growth, metabolism, and other physiological processes (Clarke et al., 2008), threatening the stability and composition of communities in downstream riverine and estuarine ecosystems.
Beyond this, the operation of dams can result in a significant reduction in downstream flood flows. Dam operation not only reduces the amount of downstream flooding, but also shifts the timing of annual maximum and minimum flow (Graf, 2006). Without high flows to flush the riverbed, excess sediment is deposited, harming organisms in the river downstream of the dam (Clarke et al., 2008). Natural flooding in estuaries is important for the maintenance of water quality, nutrients, and salinity levels (Fitzhugh & Vogel, 2011). An added benefit of natural flood regimes is that they support healthy floodplain habitats, which transport sediment to help form structures such as river bars and riffles (Poff et al., 1997). A study by Graf (2006) showed that regulated river reaches have 79% less active floodplain area and 3.6 x more inactive floodplain area when compared to similar unregulated reaches.
For hydropower dams in particular, one way to minimize water flow alteration is to use strategies that mitigate hydropeaking. The unnatural fluctuations in water flow due to hydropeaking can be reduced using either direct or indirect measures. Direct hydropeaking mitigation measures include making changes to power plant operation (which may sacrifice energy production), constructing retention basins that release the water more gradually over time, or diverting the water into a larger body of water that will be less impacted by peaks in water flow (Greimel et al., 2018). Indirect measures focus instead on making rivers more resilient to hydropeaking, either through channel widening or habitat improvement (Greimel et al., 2018). For effective hydropeaking mitigation, a combination of strategies must be employed on a case-by-case basis.
Seymour River - Relevance
Water from the Seymour River is diverted by the Seymour Falls Dam for use as drinking water for the Greater Vancouver Water District. Rainfall is also captured in its reservoir for use during dry periods, with 100-290 BL of water stored between the Seymour, Capilano, and Coquitlam reservoirs depending on the time of year (MetroVancouver, n.d.-a). As a result of this water diversion and storage, the Seymour Falls Dam disrupts the natural flow regime of the Seymour River and reduces the total volume of water being received by its downstream river and estuary ecosystems.
Migratory and Stationary Species
Migratory species rely on the connectivity of ecosystems, spending part of their lifecycle in freshwater and part of their lifecycle in seawater. Dams, typically found in estuaries where salt and freshwater converge, lead to the fragmentation of these ecosystems (Ferguson et al., 2011). Longitudinal fragmentation, or the lengthwise disruption of ecosystems, is a leading cause of freshwater fish habitat degradation, responsible for reducing fish populations and species diversity. This fragmentation can isolate populations, reduce access to feeding areas, and disrupt access to spawning sites (Barbarossa et al., 2020). In North America, salmon species typically synchronize juvenile entry into marine ecosystems (seawater) and adult entry into freshwater ecosystems (rivers/streams) to maximize population productivity and decrease their chances of predation. Thus, the initiation of this migration is largely dependent on the hydrological cycle and seasonal variations in flow, which are evident in natural ecosystems and lost as a result of dams (Ferguson et al., 2011).
Ohm et al. (2022) found that outmigration was hindered for both juvenile migrants and steelhead kelts, largely due to the creation of ecological traps. Partial restoration leads to organisms making maladaptive habitat choices, and often dams provide upstream passage (for organisms to enter the dam) but are not paired with adequate downstream passage. For this study on the central California coast, migrants preferentially used the spillway, the structure that allows the controlled release of water, for downstream passage, leading to migrational delay and increased mortality due to the spillway's water depths, reservoir loss, and avoidance of the bypass. Ultimately, juveniles migrated through the reservoir 4-8 times slower, as reduced water currents also lead to increased swimming effort to travel through these engineered waterways (Ohm et al., 2022). The freshwater environment and the first estuarine inlet are already two regions of particularly low survival, and the creation of dams in these areas, further reduces these chances for species such as Oncorhynchus mykiss (Healy, 2017).
Evidently, dams have significant direct effects on species diversity and habitual patterns. However, they also have indirect upstream effects. The construction of dams leads to a loss of native fauna upstream, often creating reservoir-associated riparian or wetland habitats. Consequently, there is a decrease in organic matter and nutrient availability for benthic invertebrates that are needed to support higher trophic levels, largely due to a change in sediment composition (Greathouse et al., n.d.). Altered sediment composition, alongside changes in water velocity, has created a shift in species. The shift from actively moving to slow moving water after dam construction, favours generalist over specialist species, lowering both diversity and species richness for these river and estuarine ecosystems (Liermann et al., 2012). Anthropogenic impacts, and the disruption of pre-existing trophic cascades and ecosystem dynamics, can lead to an increase in parasites and diseases. In addition, since dams block the movement of aquatic organisms, in addition to structurally changing ecosystems, there is an increase in invasive species and a greater chance for them to adapt to this new ecosystem (Greathouse et al., n.d.). Both pathogens and invasive species are more likely to be both introduced and able to establish themselves in degraded and modified environments.
Seymour River - Relevance
When the Seymour Falls Dam was constructed, the flow of Seymour River was altered and sediment delivery was reduced, removing the salt marsh habitat at its mouth. Instead, the majority of the water is diverted and used for fresh drinking water and electricity generation to supply the city. Salt marsh habitats are crucial ecosystem components, providing climate regulation, flood protection, storm buffering, important nursery grounds for various fish, and a habitat for countless other species (Government of Canada, 2018).
The dam has also impacted native salmon species such as Oncorhynchus kisutch (coho salmon) and Oncorhynchus mykiss (steelhead trout) (Healy, 2017) which use the river as historic spawning grounds. Salmon migration to Burrard Inlet and through Seymour River is harmed by both changes in the hydrological cycle, the change in flow rate that is largely responsible for mass migration in North American species, and physical restrictions due to dam construction.
Restoration Efforts and Potential Solutions
Dam Removal
Due to the significant disturbance dams cause, most restoration efforts focus on the removal of dams where possible. Removing dams can set off a chain of positive feedback loops, slowly returning the river and estuary to a state closer to its original health (Figure 3) (Bellmore et al., 2019).
An example of a biotic feedback loop is that by removing the physical barrier the dam creates, organisms gain freedom of movement across the river and are better able to colonize upstream and downstream regions. More species means an increase in nutrient cycling and organic matter, which also increases resource availability, allowing for more colonization (Bellmore et al., 2019). Specifically, the increase in river connectivity after dam removal and the decrease in water depth due to an increase in sediment will increase the light available to benthic producers. Increases in producers allows for higher biomass of benthic consumers and other higher trophic levels (Bellmore et al., 2019). Overall, this leads to an increase in species richness and diversity across the whole ecosystem. Further, studies of river ecosystems where the dam had been removed found an increase in juvenile salmon populations, possibly because there is a greater estuary habitat for them to use due to increased sediment deposits or because they are spawning further upstream where they could not migrate to before (Quinn et al., 2014).
Increases in sediment and nutrient delivery to estuaries from restored rivers can also help to restore important riparian and coastal vegetation, such as salt marshes (Perry et al., 2023). This is because the additional sediment moved downstream by the river after dam removal is often deposited in estuaries and deltas, where it becomes a suitable substrate for estuarine plants and increases the area of vegetated habitats. A study of the Elwha River and estuary after two dams were removed found that the river delta increased by around 26.8 hectares, almost half of which was colonised by early successional vegetation before transitioning into salt marsh or willow-alder communities (Perry et al., 2023).
Overall, removal of dams has been shown to restore river and estuary ecosystem functioning, increasing habitat and biodiversity from the mouth of the river and the greater estuary to the upstream of the dam site. When river connectivity is regained and nutrient transport, sediment delivery, and water flow is restored, biodiversity is greatly benefitted.
Seymour River - Other Restoration Strategies
While removing dams is undeniably the best way to restore dammed ecosystems and the connecting estuary from an ecological standpoint, it is often not feasible due to the myriad of functions the dams provide to the surrounding human populations. In the case of Seymour Falls Dam, its removal would severely impact the residents City of Vancouver who use the water from the dam as a primary source of fresh water. In these cases, other restoration strategies can be employed to mitigate the ecological impacts of dams.
Water Releases
Estuaries rely on freshwater input to maintain salinity gradients and to deliver nutrients and sediments to maintain the ecosystem (Hunt, 2024). When dams are constructed on the rivers that input freshwater to estuaries, the amount of freshwater reaching the estuary may be reduced, decreasing mixing levels in the estuary and endangering the many migratory species that require connectivity between fresh and marine ecosystems as water levels at river deltas decrease (Adams et al., 2023). The use of dam releases, where a controlled amount of water is released from a dam is seen as a technique to maintain freshwater flow through the river and into the estuary. Particularly, pulse releases of water during the spawning seasons of fish that make use of the estuary has been shown to stimulate fish spawning and migrations, increasing the health and survival of species (Adams et al., 2023). With the salmon population of Seymour River heavily reliant on the Seymour River Hatchery after the dam's construction blocked fish from reaching 90% of their spawning grounds, stimulating spawning in the remaining habitat through water flow management will be highly beneficial to the ecosystem (Ackerman et al., 2007).
Gravel Augmentation
Dams block physical sediment transport downstream, reducing the amount of sediment in the river and estuary below the river. Gravel augmentation, where sediment is added to this lower stream portion, is a common restoration technique (Sellheim et al., 2015). The introduction of additional sediment raises water levels in the river, creating a higher floodplain in the river and river delta, promoting sediment erosion from banks during flooding and increasing natural sediment and nutrients in the system. Adding sediment also slows the river’s flow as the river becomes wider and shallower. One study conducted in the American River in California, USA, found that adding gravel increased the area of the floodplain by up to 20%, decreased the water velocity by 80%, and increased riparian vegetation by up to 22% (Sellheim et al., 2015).
By restoring the sediment and water velocity to that of a healthy ecosystem, gravel augmentation creates habitat suitable for juvenile salmon (Sellheim et al., 2015). Further, due to increased water levels, augmented rivers have better connectivity with the estuaries and marine environment, easing migrations of fish species between river and marine environments, benefiting populations of salmon in rivers such as Seymour River.
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