Total dissolved gas (TDG) supersaturation downstream of hydropower plants may cause gas bubble disease (GBD) and harmful effects in fish. Little is known about tolerance levels of TDG supersaturation ...on Atlantic salmon (
Salmo salar
Linnaeus, 1758) in natural rivers. The present study investigated the effects of TDG supersaturation on the survival of Atlantic salmon smolts at two field sites in Norway. Here, we kept smolts in cages at increasing distances from hydropower plants known to cause TDG supersaturation and at control sites. We recorded fish mortality and examined for GBD using a stereo microscope. Mortality and symptoms of GBD commenced in fish exposed to an average of 108.3% TDG (maximum 111.0%, water depth 0.55 m) for 2 days. Significant differences in time before mortality at the control sites and test sites commenced at 110.2% TDG (maximum 111.8%) for 3 days. The study indicates that Atlantic salmon may be more vulnerable to TDG supersaturation than Pacific salmonids, which are considered at risk when the TDG is above 110%. In addition, the study provides important data to link effects caused by TDG in the laboratory and in the field.
River regulation alters flow and sediment regime, habitat availability, and ultimately the ecological functioning of rivers. Various restoration and mitigation measures have been developed to improve ...ecological function, and among them is the addition of gravel to enhance the reproduction of gravel bed spawning fishes. However, information on long-term efficiency, costs, and maintenance needs of gravel additions are scarce. Here, we study the functioning of gravel additions at eight sites in three rivers in western Norway for up to 18 years. Gravel was added between 2002 and 2010 to enhance spawning of Atlantic salmon and anadromous Brown trout. We monitored changes in the size of the gravel-covered areas, sediment composition, interstitial oxygen, egg survival, and juvenile fish densities. Additionally, we report monetary costs, identify potential maintenance needs, and calculate a cost-benefit ratio. Juvenile densities of Atlantic salmon and Brown trout increased significantly after the gravel augmentations. After 10–18 years, the median egg survival was still high (> 90%) and sediment conditions were still suitable for salmonid fish reproduction. The areas were, however, shrinking across time (median area reduction 26%), mostly caused by scouring of gravel in the steep, supply-limited, and partly regulated rivers. The average construction costs of spawning gravel augmentations were 11.2 NOK (1.12 €) m
−2
year
−1
. Compared to similar measures elsewhere, the measures have had a long life span (up to 18 years) at relatively low costs. Gravel augmentation was concluded to be a successful management measure that contributed to significantly increased Atlantic salmon and Brown trout reproduction.
For channelized, flood‐regulated rivers, morphological changes are avoided as much as possible. Extraordinarily high flows in the past, however, have demonstrated that channelized rivers may also ...become morphologically active, especially when a discharge exceeds the design discharge, such as in hundred‐year floods. However, the morphodynamic potentials and critical flows in such cases have hardly been investigated, and the flood risk to human settlements is therefore poorly understood. The present study aims to analyse the critical flow conditions in Flåmselva, western Norway and the consequences of morphological adjustment on the Froude number from an extraordinary flood event in 2014. Based on a step‐backwater modelling approach, three different high‐resolution river bathymetries of Flåmselva were investigated: (i) pre‐flood, (ii) post‐flood and (iii) re‐channelized morphology. The results showed that due to the 2014 flood, large parts that were in critical flow conditions in the pre‐flood stage (>1), exhibited significantly lower Froude numbers in the post‐flood stage. It turned out that the artificially created plane‐bed morphologies in flood‐regulated channelized rivers can act as drivers for critical flow conditions and that structural as well as non‐structural measures should consider non‐fluvial, semi‐fluvial and fluvial sediment deposits in rivers and floodplains in terms of flood risk management.
Fluvial processes are considered decisive for the formation of river channel patterns. The concepts of fluvial morphology contain a functional relationship between catchment area, local bed slope and ...grain size that leads to characteristic channel patterns. In the postglacial landscapes of Western Norway, however, the channel patterns are highly diverse, do not follow the traditional concepts of fluvial models and are distributed seemingly randomly over the river environments. The objective of this study was to investigate whether Western Norwegian rivers have a characteristic morphology determined by non-fluvial features and whether existing fluvial morphology concepts must be supplemented to represent these characteristics. River channel patterns in Western Norway were classified for 53 rivers on the basis of (i) aerial photos, (ii) LiDAR bathymetry data, (iii) sediment sampling, (iv) validation of pre-classified patterns in the field and (v) maps of geologic deposits. Sediment composition was sampled using a novel, modified Wolman–Count method, focusing on the distribution of the largest grains (Dmax). The results show that Western Norwegian rivers are dominated by glacial and colluvial deposits and partly bed rock. These non-fluvial features determine the longitudinal profile shape, bed slope and channel patterns. The collected data allow the definition of two novel types of channel forms: (i) the ‘diamictic plane bed’, with a large variation in sediment size, and (ii) the ‘mixed riffle-pool type’, with pool riffle morphology but boulder elements related to post-glacial or colluvial processes. It is presumed that the characteristic morphology evolved during the Holocene due to riverbed incision and low fluvial sediment yields, which are typical for the region and caused by the lack of dynamic tectonics and by rock resistance to weathering. The results are in contrast to the fluvial channel formation processes observed in many rivers of the world. The study thus contributes to an improved understanding of river morphology and channel pattern formation in a post-glacial landscape. It was concluded that the non-fluvial nature of the rivers studied has significant implications for flood protection measures, hydropower utilization, river ecology and ecological restoration since it determines sediment characteristics and channel stability.
•First systematical assessment of river morphology in Western Norway•Novel theory on the genesis of post-glacial, supply limited rivers•Novel sediment sampling approach for the classification of river channel patterns•New perspectives on non-fluvial morphological impacts in river engineering.
“The systematic analysis and understanding of channel‐forming processes of rivers must be expanded by including semi‐ and non‐fluvial geomorphological processes. Such processes were particularly ...driven by glaciation during the Pleistocene and led to diamictic non‐fluvial deposits in the post‐glacial valleys. In the Holocene, rivers either covered these deposits with fluvial sediments or incised into them and exposed the non‐fluvial deposits. These processes have strong and so far overlooked implications for the understanding of the genesis, morphology and sediment composition of many rivers – and thus for river utilization, ecology, restoration and management.”
Abstract
Many Norwegian rivers and lakes are regulated for hydropower, which affects freshwater ecosystems and anadromous fish species, such as sea trout (
Salmo trutta
). Lakes are an important ...feature of many anadromous river systems. However, there is limited knowledge on the importance of lakes as habitat for sea trout and how hydropower affects the behaviour of sea trout in lakes. To investigate this, we conducted an acoustic telemetry study. A total of 31 adult sea trout (532 ± 93 mm total length) were captured by angling in river Aurlandselva, Norway, and tagged between July 20 and August 12, 2021. The tags were instrumented with accelerometer, temperature, and depth sensors, which provided information on the sea trout’s presence and behaviour in lake Vassbygdevatnet. Our results indicate that there was a large prevalence of sea trout in the lake during the spawning migration, and that the sea trout were less active in the lake compared to the riverine habitats. An increase in activity of sea trout in the lake during autumn might indicate that sea trout spawn in the lake. However, the discharge from the high-head storage plant into the lake did not affect the depth use or activity of sea trout in the lake. Furthermore, the large prevalence of spawners in the lake during autumn will likely cause an underestimation of the size of the sea trout population in rivers with lakes during annual stock assessment. In conclusion, our results could not find evidence of a large impact of the discharge on the behaviour of sea trout in the lake.
Zusammenfassung
Künstliche Gasübersättigung in Fließgewässern und die dadurch verursachte Gasblasenkrankheit bei aquatischen Organismen ist in Europa wenig beachtet. Neue Forschungsergebnisse in ...Norwegen, China und Nordamerika zeigen jedoch, dass Gasübersättigung verursacht durch Dämme oder Wasserkraftwerke weitaus verbreiteter ist als bisher angenommen. In Norwegen wurde künstliche Gasübersättigung in 12 von 16 untersuchten Wasserkraftwerken nachgewiesen. Dabei kam es in 10 Fällen zu Fischsterben. Ursache ist der Eintrag von Luft in die Zuleitungen, welche sich unter Druck im Wasser löst. In vielen Fällen kann die Gasübersättigung vermieden werden, beispielsweise mit ausreichender Dimensionierung von Kraftwerkseinläufen und Rechenanlagen, Alarmsystemen und Deflektoren. Umwelteffekte und die Toleranz heimischer Arten sind in Europa jedoch kaum erforscht. Das Gleiche gilt für die Verbreitung künstlicher Gasübersättigung in Europa sowie für kosteneffektive Methoden zu ihrer Vermeidung an bestehenden Anlagen. Im vorliegenden Fachartikel werden verschiedene Aspekte der Gasübersättigung thematisiert. Neben einer Beschreibung der physikalischen Prozesse werden sowohl die biologischen Auswirkung als auch Messmethoden und mögliche Maßnahmen präsentiert. SUPERSAT, ein neues internationales Forschungsprojekt mit österreichischer Beteiligung, soll nun wissenschaftliche Erkenntnisse zu Umwelteffekten, Verbreitung, Umfang und Vermeidung der Gasübersättigung in Flüssen liefern.
This study investigates the effect of large woody debris (LWD) on the abundance of juvenile Atlantic salmon (Salmo salar, L.) and anadromous brown trout (Salmo trutta, L.) in semi‐alluvial side ...channels of the river Aurlandselva (Norway) using point electrofishing and microhabitat mapping. Not the presence of LWD, but stream bed shelter availability and the distance to spawning grounds affected the fish abundance (fish/point), independent of other habitat components. LWD showed only an effect on fish abundance when in interaction with other habitat components. This discrepancy can be explained by the availability of cavities in the shelter‐rich coarse substrate which provide sufficient cover and territory for juvenile fish at the given carrying capacity of river Aurlandselva. Whilst LWD may be most effective to provide shelter in lowland streams (bed slope <0.005), maintaining or restoring shelter‐rich coarse substrates should be considered a key priority in steeper salmonid rivers and associated semi‐fluvial streams.
During the monitoring of total dissolved gas (TDG) saturation in the Vetlefjordelva River in western Norway in 2014–2015, characteristic waves of supersaturated water were discovered. These waves ...were significantly correlated with hydropower operation, which was run by hydropeaking (R2=0.82, p<0.001). The TDG saturation varied between 99% and 108%, with a median of 105%.
The term “saturopeaking” is introduced for these waves, defined as the artificial, rapid, periodic and frequent fluctuation of gas saturation caused by hydropeaking. Hydropeaking is recognized as hydropower operation that rapidly fluctuates according to the electricity market demand. Though the observed TDG saturation levels were moderate and not likely to cause acute effects on biota, we expect that the observed saturopeaking may have significant ecological impacts in general, especially in cases with TDG saturation levels >110^% which is considered as potentially lethal for fish in rivers.
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•Characteristic waves of gas supersaturation were discovered which were linked to hydropeaking•The term “saturopeaking” is introduced for these waves.•Saturopeaking may have significant ecological impacts.
Abstract The saturation of total dissolved gases (TDG) in water remains around 100%. Certain circumstances can lead to TDG values exceeding 100%, resulting in TDG supersaturation (TDGS). TDGS above ...about 110% can be toxic to animals that rely on water for gas exchange. However, saturation beyond 200% can occur in freshwater downstream of dams and hydroelectric power plants. Despite its impact, TDGS is often overlooked as a hazard to aquatic life, particularly for benthic macroinvertebrates. This study aimed to examine the effects of TDGS on nine species of benthic macroinvertebrates. We used replicated tank studies to manipulate TDGS levels from 100% to 120% and investigated the overall survival and species‐specific effects on survival and buoyancy. We also present a summary on the effects of TDGS on invertebrate species previously tested. The results indicate that seven of nine species exhibited increased buoyancy when exposed to TDGS, causing them to float on the water surface. Additionally, a Cox Proportional Hazards model revealed a significant effect of TDGS on the survival of the macroinvertebrates. The sensitivity towards TDGS varied greatly among species of benthic macroinvertebrates, and significant species‐specific effects were only observed for Isoperla grammatica , Baetis rhodani and Asellus aquaticus . Among these, the two latter species showed clear dose‐related effects caused by TDGS, enabling the assessment of LT50 (time required to kill half of the tested population). B. rhodani was most sensitive with a LT50 of 3.7 days at 119% TDGS. Both species had visible air bubbles under the exoskeleton. Our findings highlight that direct and indirect effects on benthic macroinvertebrates can occur even at low to moderate levels of gas supersaturation, likely causing reduced density, decreased species diversity and altered species composition. The emerging evidence strongly supports the implementation of regulations on TDGS in freshwaters.
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