River Impoundment Research Articles

Effects of Climate Change on Temperature and Oxygen Stratification of Mid-latitude Reservoirs.

River impoundments (i.e., reservoirs) are the dominant lake type in mid-latitude regions, but have received comparatively little attention in recent assessments of climate change effects on inland waters. We analyzed temperature and oxygen profiles collected since the 1970s from over 100 reservoirs within the Commonwealth of Virginia. We found that unlike naturally-formed lakes in northern regions, which have experienced surface warming and bottom cooling, reservoirs show warming trends in both surface and bottom layers. Their response may be indicative of the combined effects of changes in heat exchange at the lake surface, and warming of river inflows. Surface temperatures increased more rapidly among deep, near-dam stations (0.56 ± 0.15 oC decade-1) relative to shallow stations in the upper reservoir (0.35 ± 0.24 oC decade-1), whereas bottom temperatures increased faster among shallow stations (0.50 ± 0.14 oC decade-1) relative to deep stations (0.15 ± 0.06 oC decade-1). Among deep stations, faster surface warming and slower bottom warming resulted in strengthening temperature gradients and greater oxygen depletion in bottom waters. At shallow stations, faster warming at the bottom relative to the surface resulted in a weakening of temperature gradients and increasing oxygen at the bottom. Weakening of thermal gradients among weakly stratified mid-latitude lakes may compress the range of depths over which stratification occurs. Latitudinal differences in the strength of stratification may be an important factor determining the sensitivity of lakes to climate warming.

Effects of Challawa and Tiga dams on the physicochemical water qualities of the below-dam sections of Challawa and Kano rivers, Nigeria

River impoundment resulting from dam construction leads to substantial alterations in the seasonal flow pattern as well as the physical and chemical qualities of the downstream water. This study assessed the effects of Challawa Gorge-dam and Tiga Dam on physicochemical parameters of surface water in the below-dam sections of Challawa and Kano Rivers, with emphasis on seasonal variations. Eight physicochemical water parameters were assessed at six stations over six months, distinguishing between reservoir and below-dam sections. In situ measurements included temperature, dissolved oxygen (DO), pH, electrical conductivity (EC), salinity, and total dissolved solids (TDS) while alkalinity and hardness were determined ex situ using titrimetric methods. Significant differences (p< 0.05), were observed between reservoir and below-dam sections of the two rivers in some parameters, the others exhibited no significant differences (p>0.05). Surface water temperature ranged from 26.90 to 30.10oC and 26.60 to 28.00oC for Challawa and Kano Rivers, respectively with significant differences (p< 0.05) during the dry and wet seasons in Challawa River. Decreasing water temperature at the below-dam sections was observed during the dry season and attributed to hypolimnetic water discharge from the reservoirs. Elevated atmospheric temperature in the dry hot season increases water loss through evaporation and consequently leads to increased concentration of EC, TD, and salinity in the reservoirs. A plausible reason for the higher concentrations of these parameters could be the non-point discharge from surrounding irrigation farms. Higher DO during the dry season was partly attributed to higher rates of photosynthetic production of oxygen as a result of elevated solar radiation, atmospheric temperature, and nutrient related parameters during the period. The variability exhibited by all the parameters in all the sections of the two rivers indicated that the parameters are within favourable ranges suitable for the survival and development of aquatic biota and fit as drinking water for man. Thus, the findings of this study considered water in all sections of the rivers fit for the development and sustainability of freshwater biota. The study concluded by recommending the need for further investigation on the effect of these dams on the distribution and abundance of different aquatic biota in the below-dam sections to guide in the adequate fisheries management measures. Key words: Impoundment, reservoir, hypolimnetic, downstream, dam, discharge, physicochemical, season

Temporal variation of mercury levels in fish, soil, and sediments in an Amazon reservoir: insights from 35years of river impoundment in Pará State, Brazil.

The increase of mercury (Hg) concentrations in abiotic and biotic compartments of aquatic ecosystems following the river impoundment for building a hydroelectric reservoir is one of many environmental and social impacts that the construction of hydroelectric plants can trigger. Yet, long-term studies in Amazon reservoirs are still scarce. The present study aimed to understand the effects of dam impoundment in THg concentrations in an Amazon reservoir up to 35years of its creation. On March 2019 (35th year after filling), samples of fish, soil, and sediments were collected in the Tucuruí reservoir. Total mercury (THg) concentrations were determinate in those samples and compared with data extract from previous studies referring to the 6th, 16th and 18th years after the reservoir filling. Fish from different guilds at the 6th year after filling the Tucuruí reservoir had high THg concentrations, and those decreased in the 16th and 18th years, then the concentrations increased again in the 35th year after filling. For soils and sediments, a decline in THg concentrations was observed. These results differ from previous studies that predicted that Hg concentrations in fish would return to natural concentrations within 30years in temperate zones and no decline of THg concentrations would be observed for Amazonian reservoirs. The Tocantins river drainage basin has been subjected to multiple anthropic disturbances and land use changes over the past decades, such as the implementation of new hydroelectric plants, deforestation, and fires, which can explain our observations. This study contributes valuable long-term insights into the dynamics of Hg concentrations in an Amazonian reservoir, highlighting the complex interactions between environmental changes and Hg accumulation over decades.

Using two- and three-dimensional landmark data to examine the shape variation of Elliptio complanata from two heavily impounded rivers in northern New York (USA)

ABSTRACT Despite vast improvements in the quantification of shape variation with geometric morphometric methods, evaluating morphological variation in unionids has remained difficult due to the lack of measurable landmarks on shells and their overall morphological plasticity. In this study, we supplement traditional two-dimensional landmark data with three-dimensional landmark data. Using photogrammetry, we generate three-dimensional models that are landmarked. This approach adds a new, third dimension to capture shape changes, providing a more detailed understanding of the shell topography. With this method, we explore the differences in shell shape along river stretches of two heavily impounded rivers in northern New York (USA). We ask whether isolation by distance, presence of dams and/or difference in land use around sampling localities impact the shell morphologies of Elliptio complanata populations. Our data suggest that, although separated for almost a century, these unionid populations do not show a pattern of increased morphological difference with distance. But we find that broad, rounded morphs that are posteriorly extended occur more frequently in reservoir environments while unaltered river habitats have a higher number of slim and more elongated variants.

Fish biodiversity in modified ecosystems: a case study of Nigeria's Challawa and Tiga dams in the Challawa and Kano rivers

River impoundment is usually accompanied with consequences of alteration in the hydrological system and change in the biodiversity composition of the ecosystem. This study investigated the impact of the dams built on Challawa (that is, Challawa George-Dam) and Kano (that is, Tiga Dam) on the fish biodiversity of the reservoir and below-dam sections of the two rivers. Fish sampling was done using long lines, Malian traps, cast and gill nets from stations on the reservoir and below-dam sections of the rivers. Ten fish species were identified from the reservoir and the below-dam sections of Challawa River belonging to nine families and four orders. Fish communities of the reservoir and below-dam sections of Kano River consisted of 28 species belonging to 12 families and 7 orders. Results of the Shannon diversity index (H^') indicated that the two reservoirs have relatively higher fish species diversity measures (that is, Challawa=1.948 & Kano=2.294) than the below-dam parts (that is, Challawa=1.833 & Kano=2.247) of their respective rivers. Non-metric Multidimensional Scaling (NMDS) analysis produced two-dimensional maps with stress values of 0.0554 and 0.0537, an indication that the model provided good representation of the original data in reduced-dimensional space. The ordination map indicated wide dissimilarities between fish communities of the impounded and below-dam sections. Furthermore, a one-way analysis of similarity indicated significant difference (P=0.0001) and high dissimilarities (R=0.643) between fish communities in the two sections of Challawa River and also in Kano River (P=0.001, R=0.929). Analysis of Similarity Percentage indicated 74.45% of the dissmilarities between fish communities in the two sections were cummulatively contributed by four species in Challawa River and 72.18% by five species in Kano River. The distinct differences in fish abundance and richness between the two sections is ascribed to increased ecosytem productivity often associated with the inundated portion of dammed river. It is recommended that more water flow across the dams should be encouraged as a mitigating measure to boost basin sizes of the rivers at the below-dam sections. Key words: Diversity, dam, impound, reservoir, community, freshwater and fish species

Stream and river flows mediate adult lamprey spawning migrations: Considerations for management

AbstractOrganisms have adapted to dynamic river flows as part of the natural flow regime. However, climate change and humans' use of fresh water are associated with decreases in the quality and quantity of surface waters. In addition, river impoundments and water withdrawals for human use regulate and dampen the dynamism of flow in many rivers. These changes to the natural flow regime can impact the spawning migrations of lampreys (Petromyzontiformes). Here, our goal is to review the ecology of lamprey spawning migrations associated with river flows, with considerations for controlling invasive sea lamprey (Petromyzon marinus) and conserving native lampreys (including native, anadromous sea lamprey). We identify five common themes: (1) natural flow regimes provide a competitive edge to native fishes over non‐native fishes; (2) high flows (up to a threshold) attract adult lamprey and large streams attract large numbers of lamprey; (3) larval lamprey pheromones attract adult lampreys; (4) behavioral responses by adult lamprey to flow (including localized hydraulics) can inform dam passage and trapping methods; and (5) changes to rivers induced by climate change have been implicated in changing the phenology of run timing and spawning location. Controlling invasive sea lamprey and conserving native lampreys can be done with pheromones and flow management. Pheromones may be of more use under focused management efforts to control invasive sea lamprey, whereas flow management can provide a foundation for conservation of native lampreys.

Innovative control strategy for flood mitigation through a combination of barrage management on impounded rivers and polders

The partially catastrophic effects of large floods, such as the events of 2002 and 2013 in Germany and Austria for example, clearly show the vulnerability of settlement structures along large rivers. In the case of impounded rivers with run-of-river hydropower plants, the main question is whether a reduction of floods can be achieved with flood-adapted, intelligent control of barrages - to date, the operational objective has been the safe discharge of floodwater; targeted reduction is currently an unoccupied field due to its complexity. Specifically, the interaction with the operation of a polder for flood retention is of special interest, as polders are a much-discussed and effective element of flood protection, especially at higher discharges, that directly influences the hydrograph shape. Extensive analyses and sensitivity investigations of a multitude of parameters and measured values, using a complex model system, are needed in order to investigate the reduction potential of the named technical flood protection measures and the development of control strategies relevant to practice. It is essential to base the developed controlling strategies on operationally available data, as this is the only way to make the determined reductions relevant for practical operation.

Unique diet and Philonema sp. infections in reservoir-rearing juvenile Chinook Salmon

Abstract Objective Dams and reservoirs can alter juvenile growth and survival of migratory salmonids through several physical and biological mechanisms. Juvenile Chinook Salmon Oncorhynchus tshawytscha that are produced upstream of large hydropower dams may have associated passage mortality, but the reservoirs created by these dams can support rapid growth. Characterizing the biotic drivers of growth and mortality in reservoirs may aid in understanding the cumulative effects of river impoundments on migratory salmonid populations. The purpose of this study was to understand how reservoirs facilitate rapid growth in juvenile Chinook Salmon. Methods We analyzed stomach contents to determine diet composition throughout the summer and fall. We also recorded prevalence of the parasitic nematode Philonema sp. in the coeloms of fish. Result We found that juvenile Chinook Salmon frequently consumed young-of-year centrarchids, which likely contributed to rapid growth. Piscivory was highest from July through October and decreased with surface temperature from November through December. Correspondingly, zooplankton and arthropod consumption increased in November and December. Prevalence of visible Philonema sp. infections in the coelom was high (34.6%), negatively associated with time, and nonlinearly associated with fork length. Conclusion These findings reveal unique diet patterns and suggest potential parasite-associated mortality in reservoir-rearing Chinook Salmon, but more detailed studies across a longer time scale are needed to robustly assess the population-level effects of this parasite.

Clumped isotopes reveal relationship between mussel growth and river discharge

Freshwater mussels preserve valuable information about hydrology, climate, and population dynamics, but developing seasonal chronologies can be problematic. Using clumped isotope thermometry, we produced high-resolution reconstructions of modern and historic (~ 1900) temperatures and δ18Owater from mussel shells collected from an impounded river, the Brazos in Texas, before and after damming. We also performed high-resolution growth band analyses to investigate relationships between mussel growth rate, rainfall, and seasonal temperature. Reconstructed δ18Owater and temperature vary little between the modern (3R5) and historic shell (H3R). However, a positive relationship between reconstructed δ18Owater and growth rate in H3R indicates that aside from diminished growth in winter, precipitation and flow rate are the strongest controls on mussel growth in both modern and pre-dam times. Overall, our results demonstrate (1) the impact, both positive and negative, of environmental factors such as flow alteration and temperature on mussel growth and (2) the potential for clumped isotopes in freshwater mussels as a paleohydrology and paleoclimate proxies in terrestrial environments.

The pace of global river meandering influenced by fluvial sediment supply

Meandering rivers move gradually across the floodplains, and this river movement presents socioeconomic risks along river corridors and regulates terrestrial biogeochemical cycles. Experimental and field studies suggest that fluvial sediment supply can exert a primary control on lateral migration rates of rivers. However, we lack an understanding of the relative importance of environmental boundary conditions, such as floodplain vegetation and sediment supply, in setting the pace of river meandering across different environmental settings. Here, we combine the analysis of satellite imagery and global-in-scale sediment and water discharge models to evaluate the controls on lateral migration rates of 139 meandering rivers that span a wide range in size, climate, and bank vegetation. We show that migration rates normalized by the channel width monotonically increase with the volumetric sediment flux normalized by the characteristic size of the river. This relation is consistent across rivers in vegetated and unvegetated catchments, indicating that enhanced lateral migration rates in unvegetated basins is likely not only facilitated by lower bank mechanical strength, but also by higher normalized sediment supply in ephemeral rivers. Using three case examples, we also demonstrate that width-normalized meander migration rates respond to spatial gradients in sediment supply caused by river impoundments, highlighting the prominent role of sediment supply in setting the pace of meander migration. Our results suggest that sediment-supply variations caused by climate, land-cover and land-use changes can lead to predictable changes in meandering river evolution and ultimately drive architectural changes in sedimentary stratigraphy.

Peaks and transient dynamics of ecological and biogeochemical variables following impoundment in boreal reservoirs

Across the globe, reservoirs represent nearly 10 % of the world's freshwater. River impoundment strongly alters the hydrological regime of aquatic ecosystems which subsequently affect the ecological (e.g., primary production, fish biomass) and biogeochemical variables (e.g., nutrient, mercury, and carbon cycles which includes Green House Gas emissions; GHG). We examined the transient dynamics and co-variation of biogeochemical and ecological variables from unique long-term time series (40 years of data) from Hydro-Québec boreal reservoirs, with data before and after impoundment. To do so, we applied curve fitting analysis on the data from eight plausible scenarios and model selection. Following impoundment, most variables increased, peaked, and then decreased over time (clear hump-shaped patterns; six over eight variables). Model predictions peaked between three- and 11-years post-impoundment and returned to pre-impoundment levels after about nine- to 40-years. Variables also followed a clear sequence where GHG emissions (CO2, CH4) peaked first, immediately after impoundment, followed by an increase in phosphorus and Chl-a. Total mercury in fish peaked a few years later for non-piscivorous fish and was followed closely by piscivorous fish. This work provides the first comprehensive and holistic description of the transitory nature and co-variation of ecological and biogeochemical variables following reservoir impoundment.

Factors associated with Paddlefish (Polyodon spathula) restoration success in Oklahoma

AbstractThe Paddlefish (Polyodon spathula) has been extirpated from portions of its native range due to anthropogenic habitat degradation and fragmentation, most notably the impoundment of rivers. To mitigate some of these losses in Oklahoma, Paddlefish have been stocked into reservoirs throughout the state, with variable success in establishing self‐sustaining populations. Two factors thought to contribute to success of Paddlefish stocking are spawning substrate and prey availability, which were quantified in six reservoirs and nine reservoir tributaries. Side‐scan sonar and supervised classification of aerial imagery were used to classify 4517‐ha of river substrate upstream of the river‐reservoir interface in reservoir tributaries. Zooplankton community structure, water clarity, and nutrient availability were also assessed in the same reservoirs and tributaries. One tributary had suitable spawning substrate (>40%), and the rest had minimal (<1.5%), which suggested that availability of suitable spawning substrate was not directly correlated with Paddlefish stocking success. Reservoirs with self‐sustaining Paddlefish populations had higher abundance of large zooplankton (copepods and cladocerans) than reservoirs without a reproducing population. Notably, tributaries associated with Lake Texoma, the one known example of failed restoration, were much more turbid than other rivers. We conclude that abiotic factors such as water clarity may contribute more to variable recruitment than spawning substrate or zooplankton abundance by mediating foraging success of Paddlefish post‐larvae.

Dynamic N transport and N2O emission during rainfall events in the coastal river

The coastal zone exhibited a high population density with highly impacted by anthropogenic activities, such as river impoundment to prevent saline intrusion, which resulted in weak hydrological conditions. Rainfall events can result in dramatic changes in hydrological and nutrient transportation conditions, especially in rivers with weak hydrological conditions. However, how the nitrogen transport and N2O emissions or biogeochemistry responds to the different types of rainfall events in the weak hydrodynamics rivers is poorly understood. In this study, the hydrological, nitrogenous characteristic, as well as N2O dynamics, were studied by high-frequency water sampling during two distinct rainfall events, high-intensity with short duration (E1) and low-intensity with long duration (E2). The results displayed that the hydrologic condition in E1 with a wider range of d-excess values (from −9.50 to 32.1 ‰), were more dynamic than those observed in E2. The N2O concentrations (0.01–3.33 μmol/L) were higher during E1 compared to E2 (0.03–1.11 μmol/L), which indicated that high-intensity rainfall has a greater potential for N2O emission. On the contrary, the concentrations of nitrogen (e.g., TN and NO3−-N) were lower during E1 compared to E2. Additionally, hysteresis was observed in both water and nitrogen components, resulting in a prolonged recovery time for pre-rainfall levels during the long-duration event. Moreover, the results showed that the higher average N2O flux (78.3 μmol/m2/h) in the rainfall event period was much larger than that in the non-rainfall period (1.63 μmol/m2/h). The frequency dam regulation resulted in the water level fluctuation, which could enhance wet-dry alternation and simulated N2O emissions. This study highlighted the characteristic of N dynamic and hydrological responses to diverse rainfall events occurrences in the coastal river. Rainfall could increase the N2O emission, especially during high-intensity rainfall events, which cannot be ignored in the context of annual N2O release.

Characterization of fish assemblages in eleven multi-use reservoirs from North Carolina, USA

Managing impounded river systems is a recurring challenge for aquatic resource professionals because reservoirs serve multiple functions with different ecological and socioeconomic outcomes. However, research on fishes in reservoirs has disproportionally focused on recreationally and economically important species, with less attention directed toward fish assemblages despite the potential for management at the assemblage level. As such, evaluation of relationships between reservoir fish assemblages and biotic and abiotic factors and testing whether assemblage structure is affected by changing environmental conditions may deepen ecological understanding and provide insights for reservoir fisheries management. Our overall objective was to assess these relationships in 11 reservoirs from North Carolina, USA. We sampled fish assemblages in the reservoirs, which spanned five river basins representing a range of habitat conditions, using experimental gillnets and pulsed DC nighttime electrofishing. Multivariate statistical analyses indicated that taxonomic differences in fish assemblage composition among river basins followed a gradient of productivity. The top contributing species to reservoir dissimilarity were bluegill (Lepomis macrochirus), gizzard shad (Dorosoma cepedianum), black crappie (Pomoxis nigromaculatus), and white perch (Morone americana). These four species were positively associated with factors that reflect increasing eutrophic conditions in the 11 reservoirs and could, therefore, serve as indicators of reservoir productivity, anthropogenic influence, and fish assemblage structure, in addition to their key role in reservoir fisheries management. Whereas ­fisheries research has historically focused on assessing fish ­populations, our results illustrate the ecological and management insights derived from simultaneously collecting assemblage- and population-level data. Research on reservoir fish assemblages in relation to biotic and abiotic conditions may help advance fish ecology and management alike.

Amazonian ichthyoplankton assessment via DNA metabarcoding: A baseline for detecting spawning sites of migratory fishes

The Neotropical fish fauna comprises approximately 6080 species, of which almost 3000 inhabit the Amazon River basin. However, the construction of hydroelectric dams has threatened this biodiversity by habitat fragmentation. To uncover the putative effect of river impoundment on migratory fish species, over 5400 eggs and larvae were collected from the Teles Pires River, including five tributaries and the hydroelectric reservoir, which were identified using the COI metabarcoding approach. Identification of early life stages allowed the detection of >40 fish species, including at least 27 migratory species. Fish eggs were assigned to nine families, 13 genera, and 23 species, whereas larvae were attributed to 12 families, 18 genera, and 34 species. Eggs and larvae of migratory fishes were primarily present in tributaries upstream of the dam, with fewer in the reservoir. A putative impact of the dam on the ichthyoplankton community was detected, with a significant difference found between the reservoir and tributaries for larval species richness (r = 0.822, p = 0.0211). In contrast, no significant difference was recovered among tributaries for both eggs (r = 0.213, p = 0.2657) and larvae (r = 0.378, p = 0.0696). However, a change in ichthyoplankton assemblage was observed between tributaries, as revealed by beta diversity analyses. These results highlight the potential of ichthyoplankton metabarcoding for studying and biomonitoring of migratory fishes, as well as providing relevant data concerning the identification of spawning sites to set guidelines for fisheries management and effective conservation of fish populations.

Dynamics of Suspended Particulate Matter in an Impounded River Under Dry and Wet Weather Conditions

AbstractThe dynamics of suspended particulate matter (SPM) and SPM‐associated biological and physicochemical processes in a river vary under dry and wet weather conditions and hydraulic structures, which moves a free‐flowing river to a semi‐lacustrine environment. This study investigated the response of SPM dynamics and riverbed morphodynamics to climate and anthropogenic stressors based on in‐situ observations and biogeochemical analyses of river water and riverbed sediment. The biochemical analyses and flocculation tests demonstrate abundant biopolymers and high flocculation potential of the river water during the pre‐flood period with algal bloom. This might promote SPM deposition and high‐concentration sediment layer (HCSL) formation on the riverbed. The high fractions of clay minerals and organic carbon in the riverbed sediment indicate the deposition of organic‐rich biomineral flocs. Vertical turbidity profiles with “long tails” of high turbidity near the riverbed also confirmed SPM deposition and HCSL formation. However, the highly turbulent flow conditions during the post‐flood period disturbed the tails of high turbidity and homogenized the vertical profiles of turbidity, temperature, and dissolved oxygen. Additionally, terrestrial humic substances were discharged from the watershed during this period, increasing the deflocculation and stabilization of SPM but decreasing deposition on the riverbed, thereby reducing the fractions of clay minerals and organic carbon in the sediment. This study demonstrated the interaction mechanism of SPM dynamics and riverbed morphodynamics with hydrological and biogeochemical processes in an impounded river under dry and wet weather conditions. The findings provide insights into water resource management to deal with climate change and anthropogenic stressors.

Can spatial and temporal differences in fish assemblage structure inform conservation of an endangered sucker in a large western reservoir?

AbstractObjectiveConversion of lotic to lentic habitat after river impoundment can lead to habitat loss and threats from introduced species to native aquatic organisms. Some native species can persist in reservoirs despite these alterations. Some of the largest reservoirs in North America were constructed in the Colorado River basin and are used by endangered Razorback Sucker Xyrauchen texanus. This species successfully spawns in several reservoirs and recruits to adulthood in Lake Mead, whereas recruitment in other reservoirs and altered river habitats is limited or nonexistent. Hypothesized mechanisms for the general lack of recruitment include flow and habitat alteration as well as competition and predation from nonnative fishes.MethodsWe analyzed 16 years of netting data from Lake Mead to assess spatial and temporal patterns in fish assemblage structure and the size structure of Razorback Sucker across four sites.ResultThere was a substantial shift in assemblage structure across all sites after the establishment of Gizzard Shad Dorosoma cepedianum in 2010–2011 and an apparent decline in Threadfin Shad D. petenense. Captures of juvenile Razorback Sucker (<450 mm total length in Lake Mead) were higher at sites where overall fish abundance was also higher, suggesting that all fishes might be responding to similar spatial and temporal environmental variation. We identified differences in Razorback Sucker size structure that varied over time and among sites, largely driven by the presence of juvenile Razorback Sucker at some sites in some years.ConclusionRazorback Sucker was among the top‐10 most abundant species at all sites sampled, but assemblages were dominated by abundant nonnative fishes. Razorback Sucker continue to recruit in Lake Mead in the face of abundant and diverse nonnative fishes, but the adult population size remains relatively small (<1000). Further investigation of limiting factors will be necessary to understand which actions might be successful at improving recruitment conditions.

Evaluation of re-depositing hydropower trapped sediments at large rivers to improve flood protection and aquatic habitats – Case study Danube/Austria

Sedimentation in impoundments of run-of-river hydropower plants is an ongoing and progressing management issue for hydropower operators because of its consequences for e.g., the maintenance of flood protection and waterway parameters. Current practices in sediment management are often unsustainable, associated with high costs, and can pose risks for downstream biota (e.g. during flushing). The present study elaborated a conceptual model of a novel sediment management strategy for impounded river sections consisting of a current-state and deficit analysis, and the application of a novel sediment management practice, which was practically implemented at the study site at the Austrian Danube River. This novel practice consists of (i) local dredging of gravel at locations in the impoundment, which are problematic in terms of flood protection and waterway maintenance, and (ii) the re-deposition of dredged sediments by artificial placement of gravel structures. The present study included morphological analyses of the impounded section at the study site by applying the channel profile budget technique. The knowledge of the long-term morphological development served as a basis for the evaluation of the sediment management measures and for the elaboration of the conceptual model. The combination of the morphodynamic characteristics in the impoundment with the implementation of the novel sediment management practice helped to derive generalized statements for the potential implementation of the presented conceptual model in large rivers with similar impoundment characteristics. We further defined several aspects, which are related to sediment dynamics in impounded river sections, flood protection, waterway demands, and ecological criteria that require consideration for an efficient realization of the proposed conceptual model.

Basin-specific pollution and impoundment effects on greenhouse gas distributions in three rivers and estuaries

Large uncertainties exist regarding the combined effects of pollution and impoundment on riverine greenhouse gas (GHG) emissions. It has also been debated whether river eutrophication can transform downstream estuaries into carbon sinks. To assess human impacts on the riverine and estuarine distributions of CO2, CH4, and N2O, two source-to-estuary surveys along three impounded rivers in Korea were combined with multiple samplings at five or six estuarine sites. The basin-wide surveys revealed predominant pollution effects generating localized hotspots of riverine GHGs along metropolitan areas. The localized pollution effect was pronounced in the lower Han River and estuary adjacent to Seoul, while the highest GHG levels in the upper Yeongsan traversing Gwangju were not carried over into the faraway estuary. CH4 levels were elevated across the eutrophic middle Nakdong reaches regulated by eight cascade weirs in contrast to undersaturated CO2 indicating enhanced phytoplankton production. The levels of all three GHGs tended to be higher in the Han estuary across seasons. Higher summer-time δ13C-CH4 values at some Nakdong and Yeongsan estuarine sites implied that temperature-enhanced CH4 production may have been dampened by increased CH4 oxidation. Our results suggest that the location and magnitude of pollution sources and impoundments control basin-specific longitudinal GHG distributions and estuarine carryover effects, warning against simple generalizations of eutrophic rivers and estuaries as carbon sinks.

Ecosystem Metabolism Is the Dominant Source of Carbon Dioxide in Three Young Boreal Cascade‐Reservoirs (La Romaine Complex, Québec)

AbstractThe impoundment of rivers for multipurpose reservoirs has significant consequences to the carbon cycle, one of the most relevant being the increase in greenhouse gases emissions. Reservoirs have been shown to be net sources of such gases to the atmosphere, emitting between 0.8 and 1.08 Pg carbon dioxide (CO2) equivalents per year. Even though emission estimates have become common, less is known about the processes driving this CO2 excess, a prerequisite for understanding and ultimately predicting and managing CO2 emissions from reservoirs. In the present study, we aimed at exploring ecosystem metabolism (gross primary production, ecosystem respiration, and net ecosystem production [NEP]) and its environmental drivers in three young hydroelectric reservoirs in a cascade configuration but with distinctive morphometries (shape, depth, and size). By combining our metabolic measurements with a hydrological mass balance approach, we quantified the relative contributions of internal (ecosystem metabolism) versus external sources (tributaries and groundwater) to the reservoir surface diffusive CO2 emissions. There was a predominance of net heterotrophy in all sites, and metabolism played a key role in fueling CO2 fluxes in all three reservoirs, NEP alone being able to account for the measured fluxes in approximately 50% of all sites. Internal production was thus the main process explaining total reservoir CO2 diffusive emissions (∼100%), with groundwater and tributaries contributing similarly but more modestly (∼9% each). Our results contribute to our understanding of the processes underlying boreal reservoir carbon footprints, and in particular, for apportioning the emissions can be attributed to the reservoir itself.