Downstream River Research Articles

Evaluating the influence of meteorological drought on river flow in the transboundary Sunkoshi basin, Nepal

ABSTRACT The transboundary Sun-Koshi River basin, characterized by intricate topography and geo-climatic diversity, has encountered distinct periods of droughts significantly impacting downstream river discharge. This study focuses on assessing temporal and spatial patterns of meteorological drought using SPI and SPEI indices, gauging their influence on river discharge through hydro-meteorological station data and the HEC–HMS model. Severe drought episodes were notably observed in 2010 and 2015. In 2010, prominent drought occurrences extended beyond Nepal's border into China. Conversely, in 2015, Jiri, Okhaldhunga, and Salleri experienced heightened drought conditions. Spatially, over 99% of the basin area experienced drought, varying from moderate to extreme magnitudes during 2010 and 2015. The estimated annual rainfall and basin outlet discharge were 1,800 mm, 1,907 mm, and 1,899 m3/s, 1,086 m3/s in 2010, and 2015, respectively. During drought periods, the stations indicated significantly reduced discharge, indicating a marked departure from normal conditions across the basin. Ultimately, station data performance and the HEC–HMS SCS curve number model showed that discharge in the Sunkoshi River basin is profoundly impacted by drought, notably influencing rainfall intensity on monthly, seasonal, and annual scales. The smaller basins discharge more accurate results compared to the larger outlet basins.

Linking Suspended Sediment Conditions to Hyporheic Dissolved Oxygen and Fine Sediment Deposition in Salmonid Spawning Habitat Below an Irrigation Dam, Park County, Wyoming

ABSTRACTDams are essential for water resources management but impose notable effects on fluvial sediment transport and downstream river morphology by reducing or altering the timing of sediment loads. We explored the relationship between dam sediment management and downstream sediment dynamics in the context of riverine fisheries management. We quantified the effects of dam sediment management operations on downstream salmonid spawning habitat during two differing water‐level drawdown events: (1) an experimental drawdown leading to various high concentration sediment releases, and (2) a typical slower drawdown intended to minimize release of sediment. The experimental drawdown increased deposited fine sediment and decreased hyporheic dissolved oxygen levels. However, the typical drawdown did not increase fine sediment deposition or decrease hyporheic dissolved oxygen. We quantify the immediate impacts of dam operations using a number of water column and substrate metrics, and demonstrate the potential for sediment flushing operations to have short‐term seasonally persistent effects on salmonid spawning habitat. Common surrogates of suspended sediment concentration (i.e., turbidity) were poor indicators of salmonid spawning habitat, especially when sand was the dominant grain size. Instead, other measures of suspended sediment concentration such as acoustic backscatter and depth‐integrated samples, combined with discharge, appear to be better suited for monitoring and inferring the impacts of sediment releases on salmonid spawning habitat. We demonstrate the importance of understanding sediment particle sizes, monitoring relevant water column conditions in real‐time, and provide options for effectively monitoring the downstream impact of dam operations. This work can help managers balance dam sediment management operations with ecological priorities.

Multilayer entropy-weighted TOPSIS method and its decision-making in ecological operation during the subsidence period of the Three Gorges Reservoir

Coordinating the downstream ecological demand and the power generation demand of hydropower stations is an important task in the operation of reservoirs, and how to evaluate the ecological satisfaction of the scheduling process is a difficult problem that needs to be solved urgently. A multi-objective optimal reservoir scheduling model was constructed to coordinate the spawning flow demand of " Four Major Chinese Carps"; The model takes the maximum power generation and the maximum membership degree of downstream river ecological water demand as the objective functions, and uses the dynamic programming multi-objective solution algorithm based on penalty factors to solve the problem, and obtains the non-inferior solution set in each scenario. The multilayer entropy-weighted TOPSIS method was used to study the non-inferior solution of the multi-objective scheduling model of the Three Gorges Reservoir, and the satisfactory solution ranking of the river flow rise process, ecological flow-related requirements, and power generation water requirements was obtained under the four schemes including 4d ~ 7d, which improved the reliability of the evaluation results and made up for the shortcomings of the traditional TOPSIS method in terms of hierarchy and weight science. The research results of this paper can provide a reference for decision-making on the ecological management of the spawning period of the " Four Major Chinese Carps" in the Three Gorges Reservoir of the Yangtze River.

An in-situ study in the Xijiang River basin revealed adverse effects of total dissolved gas supersaturation on fish.

High dam discharge can lead to total dissolved gas (TDG) supersaturation in downstream rivers, causing fish to suffer from bubble trauma and even mortality. Focusing on the Datengxia hydropower station in the Xijiang River basin, we conducted in-situ experiments to explore the tolerance patterns of economic fish species, including Ctenopharyngodon idella, Hypophthalmichthys molitrix, and Cirrhinus molitorella, under the influence of TDG supersaturation at different compensation depths. Moreover, the development and recovery patterns of bubble trauma and the swimming ability of fish exposed to TDG supersaturated water were investigated. In-situ experiments showed that TDG supersaturation ranged from 112.2 % to 125.2 %, averaging 118.3 % at the experiment site. The results revealed that compensation depth is favorable in fish avoidance of TDG supersaturation. The survival rate of the experimental fish at the surface was lower than for those at the 0-3 m water depth. The survival rates of Ctenopharyngodon idella, Hypophthalmichthys molitrix, and Cirrhinus molitorella at the surface were only 30 %, 47.5 %, and 70 %, respectively, while at the 0-3 m water depth, the survival rates were 97.5 %, 87.5 %, and 87.5 %, respectively. Additionally, the survival rate of fish was related to their preferred water depth. The bubble trauma scores of the experimental fish in TDG supersaturated water significantly increased with exposure time and significantly decreased after recovery in freshwater. The relative and absolute critical swimming speed (Ucrit) of Ctenopharyngodon idella ranged from 10.91 to 12.98 BL/s and 83.3-102.9 cm/s respectively, and there were no significant changes in the Ucrit with increasing TDG supersaturation exposure.

The Identification of River Flow Capacity using the HEC-RAS 1D Model: Case Study of Sringin River Downstreamdy of the Sringin River Downstream

The Sringin is a downstream river located in East Semarang. The Sringin River becomes the channel waste of the East Semarang district area. The Sringin River bank is surrounded by the Terboyo industrial area and settlement. The flood occurred in both areas, especially at Ngilir village, Genuk subdistrict. It is caused by low-capacity sedimentation channels and buildings occupying the channel body. The study aims to estimate flood discharge during repeat period 2, 5, 10, 25, 50, and 100 years on Sringin river downstream, analyze the capacity cross-section river, and identify the river water level profile when debit flowed with various period repeat flood. The hydrology calculation showed that flood discharge plans in the repeat period 2, 5, 10, 25, 50, and 100 years are 12.42; 16.67; 19.81; 30.7; 31.6; 33.22 m 3 /s. At the same time, the flow river capacity is 2.95 m³/s in the upstream section and 15.52 m³/s in the downstream section. Thus, floods occurred in some places when large discharges flowed. In period 2, the water level is about 1.0 to 1.5 meters. In more repeat periods, the level is higher. The conclusion is that the Sringin River cannot flow with perfect discharge, which exceeds its flow capacity.

Change in sediment transport regime of the Keelung River in Taiwan induced by the operation of Yuanshantze flood diversion tunnel.

The impact of flood diversion channels on river sediment transport has been rarely reported. This study uses the Yuanshantze flood diversion tunnel (YFDT), which was commissioned in July 2005 in Taiwan, as an example. This study calculates the sediment transport in the Keelung River from 1997 to 2018 by using seasonal rating curves, in the form of aQb. Changes in rating curve coefficients are also analyzed to understand the impact of YFDT on sediment transport regime. The results show that after the construction of YFDT, the annual sediment transport dropped from 0.59 ± 0.47 [Mt y-1] to 0.17 ± 0.09 [Mt y-1], leading to dampened inter- and intra-annual variation. Before flood diversion, the Keelung River requires ~1% cumulative time to export 50% cumulated sediment loads, but it takes ~4.5% cumulative time after flood diversion. Exponent b decreased from 1.23±0.18 to 1.15±0.13, and log a decreased from 0.71±0.15 to 0.51±0.11, suggesting that the Keelung River is akin to a different river in terms of sediment transport regime. While the design of the diversion tunnel mainly considered its impact on flow, its impact on sediment transport is far greater than its impact on flow and should not be overlooked. Whether this new normality will affect the downstream river continuum requires continuous attention.

Geochemical and magnetic properties in fluvial and lacustrine systems as environmental quality proxies in the Atacama Desert

Rivers play a crucial role in landscape evolution and human development, especially in arid zones, where hydrological resources are scarce and in high demand. The Atacama Desert is one of the world’s oldest and driest non-polar deserts, and aquatic systems therein have been historically subjected to anthropogenic pressure mainly associated with natural resource exploitation, such as water consumption for industrial mining activities. The mining industry has experienced a systematic development since the early 20th century, making Chile one of the main worldwide copper producers. This study analyzed sediments from two Atacama Desert rivers, the Loa and Salado Rivers (Antofagasta Region, Northern Chile). Sedimentary short-cores were obtained from sampled locations at varying distances from the confluence of the rivers. The characterization of chemical components, grain size, mineralogy, and magnetic properties of the rivers’ sediments was assessed in surface and subsurface samples to determine their respective signatures in the Inka-Coya Lake near the rivers’ confluence. The magnetic mineralogy present in the sediments of both rivers is composed of detrital magnetite and maghemite interspersed with those of authigenic origin. However, the downstream Loa River concentrated more authigenic minerals than the Salado and increased the abundance of silt-sized particles. The grain size of the Loa’s channel bed suggests low stream competency and high formation of depositional habitats. The magnetic signal and mineralogical composition of sediments from the lake are dominated by detrital pyrite, magnetite, and authigenic greigite. In contrast, the river’s sediments were dominated by magnetite and maghemite of detrital origin intercalated with those of authigenic origin. The granulometry, mineralogy, and rock magnetic properties of Inka-Coya Lake sediments indicate recent detrital input alternating with authigenic mineral-rich layers, mainly reflecting shifts in hydrological regimes. The highest concentrations of copper were observed in the upper, more recent sediment layers. Future scenarios of risky climatic conditions associated with increasing global metal demands could modify the availability of potentially toxic elements and transport capability in fluvial sediments, increasing the threats to water resource conservation in the world’s most arid desert.

Study of Enrichment and Conversion Mechanisms of Heavy Metal Elements in Mines in Cold Regions Under Freezing and Thawing

In cold-region mines, the dual effects of heat islands and cold islands, caused by cycling of hot and cold temperature extremes, facilitate the outward dispersion of heavy metal dust. Freeze–thaw cycles occurring in ice provide a conducive environment for the enrichment and conversion of heavy metals, allowing them to enter downstream rivers with meltwater. This process significantly impacts human activities and ecosystems in areas traversed by these rivers. This study is conducted in the typical alpine mining area in Xinjiang, China, and focuses on the impact of freeze–thaw cycles on the enrichment and conversion of heavy metals in alpine mining regions. It employs a comprehensive approach combining field measurements, environmental simulations, theoretical analysis, and laboratory experiments. The findings indicate that the environmental behavior of heavy metals is influenced not only by the freeze–thaw cycles themselves but also by factors such as temperature, pH, and redox conditions. Heavy metal elements may enter water bodies during the melting process and pose risks to downstream ecosystems and human health.

Discovery of Marine Macrozoobenthos Fossils in the River of Wonocolo Geosite, Indonesia

Wonocolo Geosite is a geoheritage known for the existence of the Kawengan Anticlinal Trap, which is the only one in the world, causing oil to be found in shallow locations. This condition has allowed for traditional mining since the Dutch colonial era and continues today. Geological history states that the anticlinal structure was formed when the Indo-Australian plate collided with the Eurasian plate, resulting in folds and uplift of the seafloor above the surface. However, there has not been much research supporting the truth of the theory. Therefore, evidence is needed to support the uniqueness of the Wonocolo Geosite. The fossil collection method was carried out simultaneously with the collection of substrates at the upstream, middle, and downstream riverbeds using 20 cm diameter PVC pipes at a depth of 10 cm.This study found marine macrozoobenthos fossils during substrate sampling in the upstream, middle, and downstream river beds using a PVC pipe with a diameter of 20 cm at a depth of 10 cm. The samples were then identified in the laboratory, revealing fossils from the deep sea. The fossils found include Azooxanthellate (Cnidaria), which lives at depths of up to 2000 m; Dentaliida (Mollusca) which lives at depths of 500-7000 m and Foraminifera which also live in the deep sea. This phenomenon is unusual, considering that the fossils were found in a river far from the sea at an altitude of 250 MASL. Therefore, the discovery of marine macrozoobenthos fossils at the bottom of the Wonocolo River strengthens the theory of the formation of the Wonocolo Formation from a raised seabed which is also an important asset supporting the Wonocolo Geoheritage.

Anthropogenic Perturbations Complicated the Downstream Greenhouse Gas Dynamics of a Large Subtropical Reservoir

AbstractRiver damming can significantly alter the hydrology and nutrient levels of river water, resulting in substantial greenhouse gas (GHG) emissions to the atmosphere. However, the dynamics of greenhouse gases in the discharged water downstream of dams remain poorly understood, despite being recognized as a crucial source of GHG emissions in river‐reservoir systems. In this study, we conducted comprehensive measurements of GHG concentrations and water chemistry in a large subtropical reservoir and its upstream and downstream rivers to investigate the spatiotemporal patterns of GHG concentrations and fluxes and to identify their governing mechanisms, with a primary focus on downstream GHG dynamics. Our analysis revealed that the distribution of pCO2 among the reservoir and its upstream and downstream rivers was predominantly controlled by aquatic metabolism and atmospheric CO2 exchange. Conversely, the distribution of CH4 and N2O levels was largely influenced by anaerobic metabolism. Seasonal fluctuations in GHG dynamics were linked to hydroclimatic conditions, including water temperature, hydrologic connectivity between land and rivers, and reservoir thermal stratification. Anthropogenic activities (e.g., agricultural land use) were found to affect the downstream trend of GHG concentrations. Higher GHG fluxes in the downstream rivers compared to reservoir were attributed to the anaerobic production of CH4 in the reservoir and increased gas transfer velocity in the downstream rivers. These findings underscore the critical influence of anthropogenic activities on downstream GHG dynamics and emphasize the necessity of integrating anthropogenic impacts and seasonal variability in downstream GHG emissions to enhance our understanding of the carbon budget in river‐reservoir systems.

MATHEMATICAL MODEL FOR PREDICTING CHANGES IN THE ECOLOGICAL STATE OF SURFACE WATER BODIES, TAKING INTO ACCOUNT THE IMPACT OF UPSTREAM TRIBUTARIES

The article investigates the impact of surface water bodies in the sub-basins of the Dnipro River, particularly the influence of an upstream tributary on the ecological status of the main downstream river. Such research is essential to identify factors affecting the water composition of rivers and to assess the ecological status of surface water bodies, including left tributaries of the Dnipro, such as the Desna, Sula, Psel, Vorskla, and Samara. The study aims to determine the impact of the left tributaries of the Dnipro on the water quality of surface water bodies within the sub-basins, in order to identify the primary sources of their ecological deterioration. The object of the study is the surface water bodies (the left tributaries of the Dnipro – the Desna, Sula, Psel, Vorskla, and Samara rivers). The subject of the study is the impact of the upstream left tributaries of the Dnipro on the downstream ones on changes in their ecological status within the sub-basins. The scientific novelty lies in the development of a mathematical model for predicting changes in the ecological status of surface water bodies within a sub-basin, taking into account the influence of upstream tributaries. The obtained research results for the Desna, Sula, Psel, Vorskla, and Samara tributaries, based on the constructed isolines, confirm the influence of upstream tributaries on downstream ones at observation posts. It has been established that the influence of upstream tributaries on those located downstream follows an exponential law. The developed mathematical model allows for predicting the impact of upstream tributaries on downstream ones. The results of the Kolmogorov-Smirnov criterion calculations indicate that the proposed model for predicting the ecological status of surface water bodies in the Dnipro sub-basins is reliable and effective and can be implemented in risk assessments for natural and man-made emergency situations.

Wildfire and Its Impact on the Ecosystem in Guizhou Province

ABSTRACTIn February 2024, a total of 221 recorded excess wildfires occurred throughout Guizhou Province. The atypical weather conditions attributed to climate change, seasonal drought during the non‐monsoon period, and human activities associated with the Spring Festival are identified as the primary factors contributing to this period of extensive wildfires. To facilitate a comprehensive assessment of the local ecosystem, analyses were conducted on precipitation, temperature, air quality index records, and water quality monitoring of downstream lakes and rivers. The early onset of the rainy season in April exacerbated soil erosion in Guizhou Province, where 81% of the terrain is mountainous. The repercussions of the excess wildfires on the downstream surface water ecosystem may persist for several months. The findings revealed significant differences in biomass accumulation and response times between rivers and lakes. A more thorough understanding of the impacts of wildfires on water and soil is essential for the formulation of effective recovery policies aimed at safeguarding downstream water resources.

‘Par mesure de prudence’. Flood prevention and river regulation in Demer-Dijle confluence area (NE Belgium), 1840s–1880s

Abstract This article analyses the views of experts and political discourse on river regulation, its objectives, and flood prevention in the rural Demer-Dijle catchment areas, the 1840s–1880s – a period marked by climatological extremes and agricultural transition. Via a close read of state archives and parliamentary discussions, we unravel different opinions and interests and find that not all flooding was unwanted. While the Ministry of Public Works, at first, aimed to better regulate water discharge and to improve watercourses for navigational purposes, it later prioritised agricultural interests. However, managing rivers coherently and efficiently was challenging because river powers were dispersed over ministerial departments, provinces, and municipalities that made coordinating upstream and downstream river works difficult. Floods often resulted in the drawing up of new plans, but their implementation often failed to materialise because the government had other worries and insufficient resources available while parliamentarians prioritised the interests of their constituencies.

Long-term effects of Abay River flow regulation at Lake Tana on the geomorphic and ecological responses of the downstream river channels, Upper Blue Nile basin, Ethiopia

Alluvial rivers adjust their geometry in response to environmental and anthropogenic disturbances. Flow regulation results in a new geomorphic condition that affects the aquatic ecosystem and its nature. This paper examines the long-term effects of flow regulation on the geomorphic and ecological responses of the downstream river channels of the Abay River, which is the only natural outflow of Lake Tana, Ethiopia. Since 1996, the river's discharge regime has been affected by constructing a head-rise weir (Chara Chara) at its natural outlet for hydropower production and dam construction on the Lake Tana tributary rivers. Hydrologic data collected at the outlet of the river, SPOT, and Google Earth images were used for the study. River banks and bed topography were extracted via ArcGIS for selected study periods. The study revealed that existing water resource development on the tributary rivers of Lake Tana modified (decreased) the outflow discharge on the Abay River, and resulted a changed morphological and moderate level ecological impact on the river system. Future ongoing and planned water resource developments will exacerbate the pressure on the lake. Without careful management, these changes are likely to have severe morphological, ecological and social consequences.

Interannual variations of terrestrial water storage in the East African Rift region

Abstract. The US–German GRACE (Gravity Recovery and Climate Experiment, 2002–2017) and GRACE-FO (GRACE Follow-On, since 2018) satellite missions observe terrestrial water storage (TWS) variations. Over 20 years of data allow for investigating interannual variations beyond linear trends and seasonal signals. However, the origin of observed TWS changes cannot be determined solely with GRACE and GRACE-FO observations. This study focuses on the northern part of the East African Rift around the lakes of Turkana, Victoria, and Tanganyika. It aims to characterise and analyse the interannual TWS variations compared to meteorological and geodetic observations of the water storage compartments (surface water, soil moisture, and groundwater). We apply the STL (Seasonal-Trend decomposition using LOESS) method to decompose the signal into a seasonal signal, an interannual signal, and residuals. By clustering the interannual TWS dynamics for the African continent, we define the exact outline of the study region. We observe a TWS decrease until 2006, followed by a steady rise until 2016, and then the most significant TWS gain in Africa in 2019 and 2020. Besides meteorological variability, surface water storage variations in the lakes explain large parts of the TWS decrease before 2006. The storage dynamics of Lake Victoria alone contribute up to 50 % of these TWS changes. On the other hand, the significant TWS increase around 2020 can be attributed to nearly equal rises in groundwater and surface water storage, which coincide with a substantial precipitation surplus. Soil moisture explains most of the seasonal variability but does not influence the interannual variations. As Lake Victoria dominates the surface water storage variations in the region, we further investigate the lake and the downstream Nile River. The Nalubaale Dam regulates Lake Victoria's outflow. Water level observations from satellite altimetry reveal the impact of dam operations on downstream discharge and on TWS decreases in the drought years before 2006. On the other hand, we do not find evidence for an impact of the Nalubaale Dam regulations on the strong TWS increase after 2019.

The ecological operation of ChiTan hydropower station based on hydrological alteration using the PCA method

ABSTRACT Water infrastructure affects the quantity, quality, and environment of water. During reservoir construction, only the social-economic benefits were considered and the health of the downstream river was neglected. Due to the severe downstream river system, the operators and managers faced the challenge of the downstream ecosystem. Many scholars are concerned with the reservoir's optimum functioning, which considers social and economic advantages. Numerous significant opinions and research findings are being presented on ecological scheduling at the moment, both in China and overseas, which surely deepen the connotations of ecological scheduling and serve as the research basis and critical reference of this work. The influence of hydropower development on hydrological conditions, the categorization and calculation of environmental runoff, and the building and solution of ecological scheduling models are all discussed and researched in this work. The study examined how the ChiTan hydropower project influenced the Jinxi River's flow. The total hydrologic alteration calculated using RVA is 50.53%. The component analysis is also utilized to eliminate the statistical redundancy in the hydrologic indicators. The indicators’ monthly flow for July and August's mean minimum flow for 7 days are relevant indicators for the Jinxi River basin's hydrologic modification regime.

Ruin-of-the-rivers? A global review of run-of-the-river dams.

The classification of a hydropower scheme as run-of-the-river (or run-of-river; ROR) evokes an image of a low-impact installation; however, examination of eight case studies worldwide shows that substantial negative societal and ecological impacts are tied to them, albeit in somewhat different ways. We conclude that ROR dams not only potentially displace communities, disrupt livelihoods, and degrade environments in surrounding areas, but they also divert water from areas of need, impact aquatic ecology through habitat destruction and disruption of fish migrations, emit non-trivial amounts of greenhouse gases over the lifespan of the project, and disrupt streamflow in downstream river sections. While these negative impacts vary on a case-by-case basis, medium and large ROR dams consistently have multiple and cumulative impacts, even when not having appreciable reservoirs. We contend that many impactful dams do not qualify as low-impact ROR projects, despite being defined as such. Such mislabeling is facilitated in part by the ambiguous definition of the term, which risks the ROR concept being used by proponents of impactful structures to downplay their negative effects and thus mislead the public or gain status, including within the Clean Development Mechanism in relation to mitigating climate change.

Emergy-Based Evaluation of Xiaolangdi Reservoir’s Impact on the Ecosystem Health and Services of the Lower Yellow River

The disturbance in river ecosystems caused by reservoirs and dams has become a critical topic, attracting increasing attention. However, the extent to which reservoir and dam construction and operation impact downstream river ecosystem health and ecosystem service functions is not fully understood. This research examines the Xiaolangdi Reservoir and the Lower Yellow River (LYR) ecosystem in China as a case study. We analyzed the complex material and energy flows in the LYR ecosystem using emergy theory and developed a set of emergy-based indicators for the quantitative assessment of river ecosystem health and services under reservoir operation interference. The results indicate that the total natural capital and environmental endowments of the LYR ecosystem have remained relatively stable after the operation of the Xiaolangdi Reservoir, with an increase in renewable emergy input. The ecosystem’s vigor decreased slightly, while the biomass emergy diversity index remained stable. However, the total emergy inputs increased significantly, with external feedback inputs becoming the most important emergy source for the LYR ecosystem. The resilience of the LYR ecosystem improved, with a significant increase in emergy density and a decrease in the emergy sustainability index. These findings suggest that although the river ecosystem continues to provide supporting services to human society, the extent of these services has diminished compared to pre-perturbation levels. In this research, a methodology for analyzing the impact of key reservoir operations on the ecosystem health and services of a large river is proposed to provide support for large river sustainable development studies.

Landscape Pattern Evolution and Driving Forces in the Downstream River of a Reservoir: A Case Study of the Lower Beijiang River in China

Human activities, such as reservoir construction and riverbed sand extraction, significantly influence the hydrological and sedimentary dynamics of natural rivers, thereby directly or indirectly affecting river landscape pattern distribution. This study primarily focused on the lower Beijiang River (LBR) in China, an area characterized by intensive human activity. River landscape patterns were studied using historical topographical data and time-series Landsat remote sensing images. Natural and anthropogenic factors were considered to explore the driving forces behind the evolution of landscape patterns. The results indicated that the topography of the LBR underwent significant downcutting from 1998 to 2020. The average elevation of the study area decreased by 3.6 m, and the minimum thalweg elevation decreased by 6.7 m. Over the past 30 years, the local vegetation showed a relatively stable spatial distribution, whereas the area of sand remained relatively stable before 2012, followed by a sudden decline, and tended to stabilize in the last decade. The water area exhibited a gradually increasing trend. The transition maps indicated that the spatial changes in sand were the most significant, with only 39.6% of the sand remaining unchanged from 1998 to 2009 and 32.3% from 2009 to 2020. The corresponding landscape patterns showed that the fragmentation degree of sand increased, with the mean patch size decreasing by 69.2%. The aggregation of water intensified, as its aggregation index increased from 93.31% to 95.41%, while the aggregation of vegetation remained relatively minor, ranging from 89.52% to 90.12%. The annual average temperature, annual average maximum temperature, and annual rainfall days had the strongest correlations with the vegetation landscape pattern indices. Additionally, human activities may have been the primary driver of the landscape pattern evolution of water and sand. The findings of this study have positive implications for the maintenance of the diversity and stability of river ecosystems.

Impact of short-term total dissolved gas supersaturation on cognitive function and swimming performance in medaka (Oryzias latipes)

During the flood season, high dam discharge rates result in total dissolved gas (TDG) supersaturation. This condition causes gas bubble trauma and can lead to fish mortality, which poses a significant threat to downstream river ecosystems. Assessing the ecological risks of TDG supersaturation is a challenge in waterpower-intensive river basins worldwide. Few studies have explored the impact of TDG supersaturation on fish behaviours, such as aggression and memory, which are crucial for feeding, reproduction, and predator avoidance. In this study, behavioural tests were conducted in a T-maze to investigate the effects of acute TDG supersaturation on swimming behaviour, aggression, and memory in medaka (Oryzias latipes). The results demonstrated that medaka exposed to TDG levels of 115% and 130% for 2 h had significantly reduced swimming performance.At TDG levels of 100%, 115% and 130%, medaka activity rates in the mirror arm of the maze in the mirror test were 44.34 ± 12.88%, 40.27 ± 15.44% and 35.35 ± 16.07%, respectively. Similarly, the activity rates of medaka in the active stimulus arm of the maze in the memory test were 50.35 ± 14.75%, 40.76 ± 12.51% and 35.35 ± 18.47%, respectively. The behaviour of medaka changed with increasing TDG supersaturation. These findings contribute to the development of an ecological risk assessment model for TDG supersaturation based on memory and aggression in fish and provide data for developing management strategies to mitigate the adverse effects of TDG supersaturation.