Hydropower Reservoirs as Arbiters of Climate Change

Abstract

Hydropower is an important source of renewable energy worldwide. In 2022, hydropower was estimated to produce 15% of the world’s electricity with pump-storage an integral part of this production. Generating hydropower mitigates the use of fossil fuels thus reducing Green House Gas emissions from some of the most polluting industries such as Coal Fired Powerplants. However, reservoirs used for this type of energy production may be highly polluting themselves. Production of methane and CO₂ may be extensive from storage reservoirs. Current changes in precipitation patterns will bring in more organic material and nutrients to these reservoirs causing increases in GHG production as this material is broken down. And in the case of pump-storage reservoirs, artificial generation hydrology may be exacerbating this problem. In this chapter, I analyze current literature on the impact of reservoirs on GHG emissions. Further, I analyze my research on reservoir water quality looking at how this problem is worsening through time and how this may not be a sustainable energy when considering CO₂ and methane production from these reservoirs. Ideas related to the unique operation of hydropower reservoirs, changes in water quality, precipitation norms and weather patterns are discussed.

1. Introduction

The importance of energy driving world economic productivity and growth is universally accepted [1, 2]. Thus, how future energy needs are met coupled with the importance of sustainability is developing into a critical question. Continued reliance on fossil fuels, while efficient, generates levels of pollution the global climate may not further assimilate without change. Hence, with increasing climate change concerns and a growing need to meet energy demand sustainably, solutions using renewables are needed moving forward.

Currently, renewable energy sources account for about 20% of global energy production with hydropower accounting for 80% of that total or 15% of all renewables [3]. Hydropower is the generation of electricity by flowing water over a turbine. This may occur when a powerplant is constructed near a river diverting flow directly through that power plant (Figure 1). The drawback with this type of energy production is the one-way flow of water and reliance on river dynamics and precipitation patterns. During periods of drought and dry season the production of electricity may be limited.

Figure 1.

Hydropower plant located along Rio Aranjuez in Costa Rica. This power plant diverts water from the river above to drive turbines for electricity production. Power generation is completely dependent on adequate flow from the river.

By constructing a reservoir above the power plant, water is summarily stored for a more consistent production of energy. To provide even greater storage and efficiency of use, pump storage reservoir projects are built providing an upper reservoir to generate electricity and a lower reservoir for storage (Figure 2). In this system, the upper reservoir water is released over turbines to generate electricity during peak demand with water pumped back from the lower reservoir during low demand or off peak. This type of production is very attractive because of uneven precipitation distribution [1]. Reservoir storage has shown to be a very resilient power source even during drought [4].

Figure 2.

Smith Mountain Lake reservoir dam shown from the pump storage reservoir Leesville Lake located in Central Virginia. This pump storage operation pumps water from Leesville Lake during off peak to generate power during peak from Smith Mountain Lake.

Currently over 1 million dams are operating globally [5] with future hydropower development concentrated in developing and emerging economies [3]. In 2022, hydro power plant (HPP) production of electricity accounted for 28.7% of US renewables with pumped storage hydropower (PSH) representing 96% of all storage capacity [6]. Worldwide, hydroelectricity accounted for 15.6% of the world demand in 2019, ranking third behind coal and natural gas [7]. A 17% growth in hydropower by 2030 is expected [2]. As world energy demand increases and the need for renewables heightens to meet that demand it is believed hydropower will become the predominate renewable source of energy.

The construction of new hydropower facilities has been relatively dormant since the 1990s with most of the projects built in the 1970s [8]. It is expected however that hydropower project construction will rapidly increase into the future to meet demand with an expectation of lessening fossil fuel reliance. Large projects while incurring considerable capital costs still have a good breakeven point due to low operation and maintenance costs [9]. And pump storage reservoirs offer some of the best options for energy production and storage. Small hydropower is becoming an ideal energy source due to a cheaper price and ability to site in multiple locations [10].

So, while hydropower offers tremendous future growth potential and generates electricity without burning fossil fuels, the water source for generation must become a point of focus. The building of reservoirs can displace communities, consume a considerable amount of land, disrupt river flow and impact biodiversity and managed improperly can be significant emitters of Green House Gasses (GHGs). Thus, this energy source presents a dichotomy using a sustainable energy source of water to produce electricity while presenting environmental problems with how where the water is stored and managed. How storage water is used and managed makes these projects arbiters of climate change in the communities where they operate.

2. Reservoir ecology

Reservoirs have unique characteristics that impact their functionality. Water stored in these reservoirs have physical, chemical and biological characteristics impacting overall water chemistry and quality. By understanding and managing the ecology of reservoirs, greater sustainability can be achieved in the overall process of energy production.

2.1 Stratification and spatial structure

Stratification occurs as the reservoir surface water warms creating a less dense mass of water that floats to the surface. Cooler and denser water sinks lower creating layers. These stratified layers have differing water qualities and characteristics. The epilimnion is a term used to describe the upper layer and hypolimnion the lower layer. The term metalimnion is used to describe the layer of changing conditions between the other two layers. Temperature is the most common measure used to define these layers but oxygen concentration is a very good delineator as the metalimnion becomes hypoxic and the hypolimnion becomes anoxic when eutrophic reservoirs stratify (Figure 3).

Figure 3.

Stratification due to temperature (degrees C) and resultant oxygen (mg/L) concentrations in Leesville Lake - a pump storage reservoir in Central Virginia USA. Note the rapid loss of oxygen through the metalimnion (2–6 meters) developing into anoxic conditions at depths greater than 6 meters in the hypolimnion during the 3 summer months (July-august). This pattern typically occurs from may – October each year with turnover soon after causing low oxygen conditions throughout the reservoir.

Reservoirs are also structured spatially from dam to headwaters. This gradient begins where river inputs generate patterns similar to a river. In this section, high concentrations of nutrients, bacteria and sediment enter from river transport. As water travels further into the reservoir, these riverine conditions begin to lessen and more lake qualities (defined as lacustrine) influence water quality. This middle portion of the reservoir is considered a transition zone as the riverine and lacustrine portions of the reservoir mix. This area may have the highest overall productivity in the reservoir as sediments associated with river flow settle from the water column yet nutrient concentrations remain plentiful. The final sections of a reservoir are considered lacustrine and resemble lake qualities. This area often is lower in productivity due to settling of particulates and lower in nutrient concentrations. If stratification is continuous, upper layers become very isolated from lower portions of the reservoir further isolating nutrients and other pollutants. The best water quality for the reservoir is located near the dam.

In any reservoir, water quality is best evaluated along both gradients. Headwater often mix due to storm inputs eliminating stratification with resultant poor water quality due to heavy river impacts. The remaining portions of the reservoir develop extensive areas of oxygen loss in the hypolimnion from the transition zone through the dam. The epilimnion becomes productive with algae growth (phytoplankton that are floating forms of algae) with increases in pH and supersaturation of oxygen due to photosynthesis. These reservoirs become two water masses between the productive and highly oxygenated epilimnion and the isolated hypoxic or even anoxic hypolimnion.

Pump storage reservoirs can be an exception to this pattern. These reservoirs receive water input from the upper reservoir that mixes often in an unpredictable way with river inputs from various sources (Figure 4). Upper dam release is often good water quality but may have very low oxygen content if release is from the upper reservoir hypolimnion. Secondary sources from river inputs can bring in high concentrations of nutrients, sediment and bacteria during stormwater flow. The mix of these water sources create both a unique hydrology and water quality in these reservoirs.

Figure 4.

Unique inflow characteristics in the headwater region of a pump-storage reservoir. Here, turbid river water inflow mixes with dam release from the upper reservoir. Water becomes fully mixed several kilometers below this area adopting water quality resembling these river inputs.

One additional pattern occurs in pump storage reservoirs. Water is periodically pumped back into the upper reservoir. This generates two distinct patterns. Primarily it provides water for the next round of energy production in the upper reservoir. Unfortunately, when considering ecology of the upper reservoir it will degrade water quality. Such decisions to pump water from lower to upper are almost exclusively energy demand driven. Secondarily, it creates reverse flow that can move water throughout the lower reservoir headwater region in the opposite direction. This causes turbid river water to predominate lower reservoir water quality. It also disturbs the river bottom causing even greater turbidity to flow. This water may also move back and forth as a slug – a patch of water along a spatial gradient moving back and forth due to pump-storage operations. These scenarios create unique hydrology and water quality in both reservoirs.

2.2 Eutrophication

Eutrophication is the excessive loading of nutrients into a body of water. When this occurs, algae growth is stimulated and a multitude of problems occur that are well documented in the literature occur [11]. Reservoirs are prone to eutrophication. There are multiple reasons for this. High catchment to surface area ratios generates high loading rates from rivers of both organic material and nutrients [12, 13]. Reservoirs unlike lakes may behave similar to rivers during certain times of the year with water input moving rapidly through. This reality renders the watershed extremely important toward resultant water quality. Activities such as farming, urban development, deforestation or other land disturbing activities directly influence the river and hence the reservoir water quality [14, 15].

Secondly, storm events play a critical role. Storms are responsible for the bulk (up to 90%) of nutrient loading to river systems and into these reservoirs [16, 17, 18]. And more importantly, the watershed attributes (land use) and not necessarily the strength of the storm cause the greatest impacts [19]. Additionally, reservoir orientation into prevailing winds creates fetch that impacts mixing and how nutrients are ultimately incorporated into lake productivity. Reservoirs tend to have a greater fetch (length exposed to an air mass) because they are elongate being built into stream channels. As fetch increases, mixing becomes more pronounced and storm impact intensifies.

Ultimately, the excessive inputs of nutrients stimulate algal growth to the point of generating harmful algal blooms (HABs) and loss of oxygen in the hypolimnion [20]. As these blooms grow, they are very resistant to grazing or other losses generating very problematic conditions [21]. As this biomass builds up, it generates organic matter sinking into the lower levels of the reservoir. The amount of organic material is proportional to oxygen loss in the lower layers during stratification.

2.3 Loss of oxygen

A consequence of both stratification and eutrophication, hypoxia (low oxygen) and anoxia (no oxygen) develop throughout the lower stratified layers [22]. Oxygen cannot be replenished due to isolation from the upper oxygenated layers and lack of light allowing photosynthesis. This phenomenon occurs during the summer and fall periods of stratification. And during fall turnover as air temperature cools, the hypolimnion and epilimnion mix due to equilibration of temperature with resultant low oxygen concentrations throughout the entire reservoir.

There are multiple ramifications from oxygen loss such as habitat reduction and environmentally adverse chemical reactions. Most importantly, a reducing environment is created. This decouples complexes formed with sediment such as iron and phosphorus releasing available phosphorus. Phosphorus is understood to be a limiting nutrient for algae growth and implicated as the greatest contributor to eutrophication of reservoirs [22]. Release of available phosphorus can stimulate algal growth further complicating the problem of oxygen loss. Other compounds also develop in this environment such as sulfide and manganese that can be toxic to aquatic life.

Predicting oxygen loss as it is related to other variables is more difficult. Nürnberg [23, 24] suggest it is best predicted by phosphorus concentration and shape of the lake or reservoir. For hydropower reservoirs, this is more difficult due to the nature of reservoir operation considering both the release though the dam and inputs from rivers. Ultimately, oxygen loss is the greatest problem limiting their sustainable operation.

2.4 Methane and carbon dioxide production

Both carbon dioxide (CO₂) and methane (CH₄) are produced by reservoirs. Carbon dioxide is a water-soluble gas that quickly dissolves in water. It is a byproduct from the breakdown of organic material in the hypolimnion or from gas exchange with the atmosphere. It is mediated through photosynthesis where CO₂ is consumed through the growth of algae. It can also build-up in the hypolimnion as it is produced through the breakdown of organics [22]. Methane is generally insoluble in water and is formed in the hypolimnion as a byproduct of organic material breakdown. Some of the methane is oxidized to CO₂ but the majority of production occurs as bubbles in the sediment water interface.

Whether reservoirs act as sinks for CO₂ or are net emitters to the atmosphere is variable [23, 24]. Phytoplankton uptake consumes CO₂ but decomposition of algal biomass in the hypolimnion generates it. During summer peaks and stratification, the warm epilimnion acts as a CO₂ sink and all exchange is almost exclusively diffusion. During turnover however, release of CO₂ can be significant [25]. Warmer water and mixing facilitates this CO₂ release. The synergistic effects of warming, nutrient driven eutrophication and development of algal blooms in turn increases CO₂ production. The difference between algal uptake and CO₂ accumulation released during turnover will account for the net emission or consumption of CO₂ in the reservoir.

Methane production operates differently. As generally insoluble, a majority of it is produced along the sediment – water interface as bubbles. This can happen very rapidly as high nutrient loading supports primary production, favorable (hypoxic) conditions develop and the organic substrate necessary is available [26]. Trophic status or increased eutrophication also influences the rates of production as eutrophic reservoirs produce greater amounts of methane [27]. Hence, chlorophyll a (the measure of algal biomass) may be the master variable regulating methane production in reservoirs [28, 29]. This methane, produced as CH₄ bubbles rises directly from the sediments and into the atmosphere [30]. Heat waves, algal blooms, fall overturn and strong stratification all impact the effluxes of CH₄ into the atmosphere. In some instances, dissolved methane may accumulate in the hypolimnion [31] but this is a small fraction of all methane production.

3. Greenhouse gas emissions

Conventional hydropower production seeks to optimize energy and producer-economic benefits while operating within ecological and reservoir management constraints [32]. While reservoirs and even more so pump-storage reservoirs provide an abundant supply of energy in the form of stored water, it is the storage and movement of that water that is most concerning. If the reservoirs that store the water emit enough GHGs to negate the benefit of this type of energy, then the industry is ultimately not sustainable and loses its renewable appeal.

The concern with the building of reservoirs and storage of water for energy production is allowing the buildup and ultimate release of GHGs into the atmosphere. The essential question lies in the understanding of how much of these gasses are released into the atmosphere during use or simply as a by-product of the storage and whether it is a significant contribution to overall global carbon budget and climate change. To gain some insight on how these processes impact the atmosphere it is best to understand processes in reservoirs and subsequent release.

3.1 Diffusion

Diffusion occurs when gas is exchanged between the reservoir surface and atmosphere due to concentration differences. Carbon dioxide is very soluble in water and thus diffusion accounts for 99% of CO₂ emissions from reservoirs [33]. Yet, as the epilimnion functions as a CO₂ sink it can offset the overall impact to the atmosphere. As methane is rather insoluble in water, very little diffusion occurs directly to the atmosphere. Some diffusion occurs between sediments where methane is produced and overlying water in the hypolimnion through oxidation. Diffusion of CO₂ and CH₄ accounts for a very small fraction of all GHGs emitted from reservoirs into the atmosphere with the exception of fall turnover [34].

Off-gassing of GHGs during fall overturn presents the greatest diffusive flux to the atmosphere [35]. Methane and CO₂ buildup that has occurred in the hypolimnion during stratification is released via diffusion. Up to 46% of stored methane can be released during overturn which is likely 80% of the diffusive release. Overturn represents a very large GHG diffusive release event in eutrophic lakes.

3.2 Ebullition (bubbling)

As methane gas is generated in the sediment bubbles form. These bubbles may remain in place due to hydrostatic pressure or be oxidized into CO₂. However, as conditions change these methane bubbles may release moving up through the water column and into the atmosphere via ebullition (the physical bubbling of methane from lake and reservoir sediments). Methane flux, and in particular ebullition is recognized as a highly significant source of global GHG [36]. And it is important to recognize that ebullition dominates most of the CH₄ emissions [28, 37]. The generation of methane in the sediment is temperature and trophic state dependent [38] as more methane is produced in eutrophic and warmer reservoirs.

Multiple reservoir conditions can cause methane to release from the sediments and bubble to the surface. Strong patterns in stratification (a well-developed hypolimnion) generates high levels of ebullitive emission [39]. Eutrophic nutrient rich reservoirs tend emit more CH₄ than less productive ones. Shallow reservoirs and those with a long fetch generating wind disturbance and multiple mixing events will emit greater levels of methane [19].

Further, the greatest ebullition rates occur when we drawdown reservoirs [27, 40]. Water movement creates turbulence releasing methane bubbles. Estimates suggest a 1.4–77% increase in ebullitive methane release in just a 24-hour period during draw down [40]. Reservoirs with higher epilimnion chlorophyll a experienced larger increases in CH₄ emission in response to drawdown [27].

Pump storage reservoir are very prone to this type of emission. These reservoirs are often shallow making them prone to excessive methane release rather than deep and stable water systems [41]. Lowering of water level decreases hydrostatic pressure releasing bubbles from confinement in the soils. Harrison et al. [27] observed pulses of CH₄ emission via ebullition, via diffusion, and total emission (ebullition plus diffusion) associated with water level drawdown with a 3.6-fold increase in emissions during drawdown. Reservoir drawdown and in particular a shallow system with repeated water movement and drawdown represents a very high potential source of methane release (Figure 5).

Figure 5.

Changes in water level for storage reservoir Leesville Lake (blue line) (storage reservoir) and upper reservoir Smith Mountain tail water (orange line) over an 8-day period. Data illustrates the fluctuation in water levels (187–185 meters) of the reservoirs in a typical week. The pump storage reservoir is in a constant state of flux as it may fluctuate 1–1.5 meters in a 24-hour period and over 2 meters in a week. The tail water from the upper reservoir also exbibits substantial change.

3.3 During energy production

As power is generated, water runs over the turbine and is released through the tail waters. Most often, hydropower source water is drawn from the hypolimnion of the production reservoir and that water layer is either hypoxic or anoxic. This water may be methane-rich. The hydrostatic pressure change during use and turbulence forces large portions of the CH₄ gas to be released [42, 43]. These emissions can be significant during the summer months when the reservoir is stratified.

Observations suggest methane concentrations at the turbine intake reached their maximum value during greatest stratification and dry weather [39]. This is likely a combination of build-up in the hypolimnion and additional methane that dissolves when bubbling occurs [41]. While ebullition is the most significant source of methane, water traveling over the turbines adds to methane emission by releasing the dissolved fraction [44]. Movement of water causing ebullition and turbulence at the turbines cause an extensive release of methane.

4. Impacts from climate change

As climate change intensifies, it is expected that precipitation patterns will become more variable with more extreme rainfall events and a warming of the planet surface [45]. These two probabilities will have a profound impact on water resources through time. Increasing frequencies of extreme weather will likely bring in more material to lakes and reservoirs increasing eutrophication. Warming temperatures will cause that material to metabolize more intensively. This has the potential to create the conditions where we will see GHG emission from reservoirs intensify.

4.1 Warming temperatures

Warming temperatures and declining water clarity will directly contribute to an increased loss of oxygen in reservoirs [46]. Increasing oxygen loss from rising temperatures will occur as a warmer epilimnion strengthens density differences between layers. Strengthening and lengthening stratification allows the breakdown of more organic material under anoxic conditions and more GHGs particularly methane to be produced [47].

While eutrophication will contribute to this problem, the increasing temperatures will drive greater productivity, potential algal blooms, loss of oxygen and GHG release. Hence, it may be expected that warming temperatures may have significant impact on exacerbating the effects eutrophication and GHG release from reservoirs. Increased GHG emissions can be expected from rising temperatures alone [48]. Thus, the combination of greater production of GHGs and release will certainly have impacts on the global carbon budget.

Further, climate change may force systems from diffusion dominated to ebullition dominated system [37]. Increased methane production generating more methane bubbles will boost CH₄ ebullition. This will be very pronounced in shallow reservoirs and shallow portions of large reservoirs [47]. Further, temperature increases at depth may generate more diffusive methane releases from sediments and enhance methane build-up in the hypolimnion [49]. This has multiple consequences increasing methane release during lake turnover and creating much greater methane release during energy production as water flows over the turbines. Increased warming will be a catalyst to intensify already potent processes for GHG production and release.

4.2 Stormwater and flooding

Inputs from stormwater have profound impacts on reservoirs [50]. It is expected as climate change intensifies so will the intensity and frequency of extreme weather and hence flooding events [51]. Flooding can both increase the oxygen concentration on rainy days and accelerate the consumption of oxygen after the storm [52]. Organic material from storms entrained into the reservoir during rain events will consume oxygen during stable stratification [53]. As flooding increases, so will oxygen loss that will exacerbate production of GHGs.

Coupled with increased oxygen loss, flooding will exacerbate eutrophication with increases in nutrients and organic matter flowing into reservoirs from storms [54, 55]. High winds and precipitation associated with more frequent and intense storms will influence water-column mixing and the behavior, amount, and composition of runoff swept into lakes [19]. Additionally, reservoirs will experience greater flushing disrupting stratification and releasing GHGs.

These small, hydrological changes have the potential to restructure entire phytoplankton communities in both the short- and long-term [56]. Large and even extreme weather events will further disrupt current reservoir dynamics and even generate new patterns [57]. Because reservoirs are so interdependent on watersheds, we may see simple changes in precipitation patterns change a somewhat healthy reservoir into an unhealthy one as climate change persists. Evidence suggest climate driven changes to reservoirs will create ideal conditions for increased production and ultimately emission of GHGs beyond what is occurring now [56].

Looking at patterns in reservoir data to support or refute impacts due to temperature and precipitation, a principal component analysis was conducted on a current data set from a pump-storage reservoir in central Virginia USA (Table 1). From this analysis, patterns suggest that reservoir variables at the river and headwater stations are most strongly correlated to precipitation with the remaining reservoir stations correlated to temperature. Most importantly, reservoir oxygen percentages correlated to external temperatures and only in the portions of the reservoir with lake attributes and extensive summer stratification and oxygen loss in the hypolimnion. This suggest oxygen loss in these reservoirs may be strongly dependent on temperature in the region and may intensify as we see the effects of climate change.

	River	Tail Water	Headwater	Transition	Dam
% Variability	32.70%	26.40%	27.90%	33.60%	29.30%
Precip. (cm)	16.077	8.699	21.139	2.917	3.367
Temp C	0.641	29.305	0.397	25.958	32.535
DO%	0.572	15.545	0.494	24.369	15.720
Turbidity	29.263	1.884	29.464	20.439	15.560
TP	29.069	3.842	4.714	0.473	6.336
Secchi	22.656	28.121	39.033	12.071	23.876
Chl a	1.722	12.603	4.759	13.773	2.607

Table 1.

Results from principal component analysis (PCA) showing similarities among variables using % contribution of each variable in first factor. Principal component analysis is a multivariate technique analyzing data tables to look for patterns of similarity among the observations [58]. The first factor accounts for the greatest similarity among all of the variables tested. Each station represents a station on the pump storage reservoir (Leesville Lake in Central Virginia, USA) with river input at the headwaters (river), release from the upper dam (tail water), portion of reservoir where both inputs converge (headwater), further down the reservoir in the transition area (transition) and then near the dam (dam). Observations with the largest correlations are highlighted. Data analyzed from samples taken on a monthly basis at the end of the month from April-October 2015–2023 for a total of 63 total samples. Temperature is the monthly mean air temperature and precipitation is the total for the month. All other variables are measured in the reservoir.

Conversely, precipitation did correlate with reservoir parameters in the riverine and headwater portions of the reservoir. Importantly, nutrient inputs as total phosphorus (TP) correlated with changes in precipitation. This suggests the importance of river input and eutrophication entering these reservoirs. Water clarity (turbidity and Secchi) was correlated to both temperature and precipitation. This suggests that both temperature and precipitation have influence in these reservoirs and these are the parameters of concern as climate change progresses. Data here have implications with eventual methane flux as it is positively correlated with temperature and lake nutrient status [59].

5. Trending (sustainability of hydropower)

With concerns over the production of GHGs and potential of worsening with climate change, the sustainability of hydropower must be carefully analyzed and improved if this will be the energy source of the future. Jager and Smith [32] define sustainability as the operation of reservoirs to meet societal needs for water and power while protecting long-term health of the river ecosystem. At its most basic sense, hydropower changes the natural flow regime of the river optimizing water storage for release during peak demands for electricity – storage or movement at night and release during the day. And then in a most critical sense, these systems are massive GHG emitting systems that must be controlled. The production of energy through hydropower must be viewed in the context of the entire operation and managed accordingly.

5.1 Managing hydropower holistically

It is critical that when considering the sustainability of hydropower projects all aspects and factors be part of the considerations [60]. Compared to other renewable forms of energy, hydropower provides an almost continuous supply of energy, relatively low cost and with lower emissions compared to conventional forms of energy production [61]. Reservoirs created for these projects provide water sources for recreation, drinking water supply and even fish and wildlife enhancements with multiple societal benefits [62]. And even with concerns in communities during construction, eventually these reservoirs can improve the quality of the landscape and become accepted [63]. Thus, the attractiveness of the power source and environmental enhancements all exist for a favorable evaluation.

Conversely, the building of hydroelectric dams has widespread impacts on populations, ecology and climate. Dams cause fragmentation of many river systems that are now free-flowing impacting the ecology and hydrology of the region [3]. These construction projects are high in capital cost, involve considerable amounts of machinery and disturbance of the areas with loss of arable land, residential neighborhoods and scenic river valleys [32]. The emission of CO₂ and CH₄ are considerable both from the reservoirs and during operations. The three greatest barriers to future progress are (1) the valuation of ecological benefits, (2) understanding the ecological effects of flow releases sufficiently well to quantify them and (3) lack of incentive for power producers to operate sustainably [32].

While our understanding of environmental concerns with hydropower continues to increase, all of this information has not resulted in significant management actions [56]. In fact, a comprehensive and integrative holistic tool for hydropower reservoir management is not available [64]. Thus, two distinctive management needs are apparent as we move forward in a transition to more renewable forms of energy. Proper management of existing hydropower projects and in particular large dam and pump-storage operations to become more sustainable. And potential decentralization of the power projects as breaking these operations into smaller projects lowers the environmental costs.

5.2 Solutions

Small Hydroelectric Power (SHP) may be a solution. There is generally no agreed upon definition of SHP but in general these are small plants that use run of the river flow with minimal impoundment (Figure 1). These projects are gaining popularity for a suite of potential benefits. Most of the ecological problems associated with large reservoir operations such as oxygen depletion, increased temperature, decreased flow and GHG emission can be avoided [60, 65]. These projects can be sited in less than traditional hydropower areas and throughout the world to meet expanding demand [66]. They rely on direct river flow but if spaced thought a region (decentralization) some of this problem can be diminished.

Such projects put into sequential order and adequately spaced to maximize ecology ecosystem services (fish, macros and water quality) may be the most preferential hydroelectric projects [67, 68]. Small low head dams have the potential to improve water quality [69] and benefit local governance through adaptation to local economies, local jobs and enhancement of local infrastructure [66]. Localities can assume responsibility for local energy production.

These projects are not without concern. Factors such as mode of operation, the degree of river flow alteration, impacts on habitat connectivity, and the cumulative ecological effects of multiple SHP installations on a single river warrant close consideration and management [70]. Yet even with these concerns, this type of power production is the preferred going into the future as it minimizes large reservoir operation and management. These projects are often too large for local management and operate throughout a region making impacts quite considerable.

Existing large systems need to be reviewed for operational strategy and moved into a more sustainable approach. Several operational strategies such as Run of River (ROR) allowing water to flow continuously or spill flows (non-generating flows not released through turbines) need consideration. Adopting a more natural ROR flow regime meets environmental needs maintaining aquatic ecosystems and human needs embedded in domestic and agricultural usage [71]. It can be adjusted from a consumer peak demand model to meet energy production goals. Cost/benefit analyses are desperately needed to weigh environmental atmospheric costs, health costs and improvement in aquatic resources and fisheries habitats in exchange for energy production.

Sluicing or spill flows ameliorate water quality downstream by re-oxygenating the water avoiding excessive sedimentation behind dams and allowing sediment to flow into downstream areas [72]. This improves watershed natural function. And because of the strong relationship between reservoirs and watersheds, new reservoirs need sited away from land use and anthropomorphic conditions that cause declines in reservoir water quality [28]. Current and new large reservoir projects need careful planning to not only meet energy needs but to minimize environmental impacts.

5.3 Future trends

The damming of rivers has seen less and less popularity and large hydropower projects are not accepted as a clean and renewable energy source [65]. With concerns over meeting growing electricity demand and a desire to grow hydropower to meet this demand, more sustainable approaches need developed and accepted.

Apart from its doubtless advantages, even nominally successful hydropower projects are often associated with negative environmental consequences in the form of biodiversity loss, disruptions to fish migration, potentially large-scale land inundation, the disruption of human resettlement, and many others [73]. Sustainability studies are need to consider hydropower holistically rather than by its individual parts. If it is to be the energy source of the future, the negative aspects must be addressed and understood.

6. Conclusion

Hydropower when used well can provide a very viable renewable energy source to meet the growing demand as we move into the future. Alternatively, if the industry continues to ignore management of reservoirs associated with these projects adhering to schedules of power generation only, viability as a sustainable source of energy will diminish. It is clear that reservoirs are integrators of watersheds and as such must be maintained and managed with this in mind. The assimilation of organic material and processing into GHGs makes management imperative. And as climate change that is expected to exacerbate the GHG emission problem intensifies, management can no longer be ignored as part of these hydropower projects.

As we push for construction of new hydroelectric projects to meet demand, large projects with a myriad of environmental concerns may become less feasible. Small hydropower may be a good alternative. These projects may be sited in areas not amenable for large projects and spread throughout a region to diminish the environmental impact. These projects may fit better under local control helping local economies and providing better management as a small project. These small projects are not without concern but may be a very good alternative.

An integration of small hydropower, large hydropower and sustainable reservoir management would seem to be the best solution. Integration of environmental concerns needs to be at the forefront of these projects instead of an afterthought or retrofit once the project is in operation. The need for more power is increasing and growth using fossil fuels does not provide the path to a sustainable future. We can use hydropower to produce energy and demonstrate environmental stewardship. In this way, hydropower will lead the way toward a sustainable energy future.

Acknowledgments

The author would like to acknowledge the University of Lynchburg for providing the time and resources necessary to complete this project. I would also like to thank the Leesville Lake Association for providing funding for this project and Dr. Tony Capuco and the entire water quality committee for their support and help throughout. To the countless students have assisted me in this work, spent time on the boat and in the laboratory to quantify the water quality of the reservoirs I am very grateful. My family is always at the forefront of support and no project can be completed without them.