Highlights: What are the main findings? Multitemporal surface reflectance shows trends in oxbow lake area and water types. Oxbow lake connectivity peaks during the flooding period under natural conditions. What are the implications of the main findings? Connectivity-based groups of oxbow lakes represent the availability of fish habitat. Oxbow lake diversity and connectivity are essential for river ecological integrity. In meandering river floodplain systems, remote sensing is a valuable tool for assessing connectivity processes relevant to fish ecological functions. This study used the Google Earth Engine platform and multispectral Landsat 7 imagery. A random forest classifier was used to evaluate water types and area changes in oxbow lakes of the Beni River in Bolivia. Water type dynamics were mainly associated with lake age and distance from the main channel. Seasonal variations highlighted the role of wind-driven sediment resuspension and overflow during high discharge conditions. Long-term lake area changes reflected typical oxbow lake evolution as well as alterations caused by the main channel. Multiannual changes showed a notable area decrease during years of low discharge. Relationships between discharge and lake area dynamics allowed the classification of three lake groups with different levels of connectivity and overbank flow influence. The ecological relevance of these groups was evaluated based on fish habitat preferences and migration patterns. Results emphasize the importance of preserving natural hydrologic variability, with flooding associated with increased habitat availability. Overall, this study demonstrates the usefulness of satellite remote sensing for detecting ecohydrological processes and offers insights to preserve ecological functions in data-scarce regions.

The natural flow regimes of Andean-Amazon tributaries play a vital role in sustaining their rich biodiversity and productive local fisheries, but ongoing and proposed alteration of river flow regimes by large dams threatens to negatively impact river ecosystems. Despite its importance, our understanding of how hydrologic variability influences ecological functions in the Andean Amazon is limited, particularly in regions with scarce data. In these regions, growing research highlights the value of fishers' local ecological knowledge in addressing these gaps. This study focused on increasing our knowledge of ecohydrological relationships in the Beni River of Bolivia through the analysis of fishers' knowledge through 28 individual semi-structured interviews. Results indicate how key species rely on hydrologic variability, connectivity, and flooding dynamics to carry out their life stages of reproductive migration and access different habitats in the floodplains. Fishers mentioned using hydrologic indicators at multiple scales to guide their fishing activity. For instance, flooding extent and duration help anticipate fish abundance in the next years; connectivity between the main channel and oxbow lakes indicates fish migration; and within-site observations of water level on the bank, water depth, flow direction, flow velocity, and backwater effects are used to select a fishing location. In addition, the fishers described characteristics of habitat such as substrate, vegetation, and turbidity, as well as fish feeding habits and sequential migration patterns that represent valuable observations about fish ecology. The comparison with scientific information not only confirmed existing insights but also extended our understanding of ecohydrological relationships and helped explain possible causes of relevant long-term trends. In conclusion, our findings highlight the vital role of the flooding dynamics in the fishing practice and draw attention to the potential negative impacts of hydrologic alteration on the livelihoods of fishers.

Denitrification in large tropical river systems is likely important for nitrogen retention estimates, but is limited by the need for measurements and the ability to scale these estimates to relate seasonal changes to river geomorphology and discharge. Geomorphic units (GUs), that describe the structure of a river system based on their inundation frequency and vegetation cover, may be useful to characterise features that influence denitrification rates. In this study, we tested the hypothesis that measurements of potential denitrification rate (PDR) using denitrification enzyme assays from different GUs could be used to1) relate PDR to soil, vegetation and different land use and land-cover (LULC) types as controlling factors and 2) that these characteristics could be assessed using remote sensing data to model PDR over a large spatial scale (along a 50 km reach) for the Padma River (Bangladesh). Specifically, 245 PDR measurements were made from the four LULC types with in eight GUs during the dry/winter season 2020. Linear regression using a mixed-modelling approach showed that PDR was highly related to vegetation cover and soil moisture across all GUs. Sentinel-2 data were then used to develop relationships between the Normalised Difference Vegetation Index. (NDVI) and vegetation cover and, specifically, between Sentinel-2 band 11 and soil moisture, which also reasonably described PDR rates. We then used this satellite data to estimate reach-scale PDR in post-monsoon, dry/winter and pre-monsoon seasons. The satellite-based model showed that PDR increased in GUs from post-monsoon 2019 to pre-monsoon 2020. The vegetation islands and the bars were the most important GUs for denitrification in all seasons. The satellite-assisted approach developed in this study can be applied to the GUs in large lowland rivers where inundation occurs frequently.

Large tropical floodplain rivers act as important pathways of nitrogen transport from land to the sea. In the present study, a mass balance approach was used to evaluate nitrogen retention over a two-year period from a 50 km reach of the Padma River in Bangladesh. The relationship between concentration and discharge was estimated from 58 nitrogen concentration and discharge measurements. Daily nitrogen flux was then calculated from the hydrological inflow and outflows of the reach, and total nitrogen (TN) retention was estimated based on the flux difference of TN inflow and outflows. To validate mass-balance measurements, retention processes of nitrogen loss due to water retention (NLWR), sedimentation, potential denitrification rate (PDR), and nitrogen fixation rate (NFR) were estimated from the water column of the river. Monthly mass-balance measurements revealed substantial seasonal variation in nitrogen retention, indicating river discharge as the main controlling factor. Estimated maximum retention values (tonnes per month) of NLWR, sedimentation, PDR, and NFR were all associated with the monsoons, with 86 % occurring during that period. However, the percentage of PDR and NFR to TN retention was higher in non-monsoon months (post-monsoon, dry/winter and pre-monsoon), suggesting retention mechanisms varied seasonally. TN retention via NLWR accounted for the largest portion of total TN retention, that consistently exceeded 50 %, followed by sedimentation. PDR in submerged geomorphic units was the second-most important retention mechanism in the dry/winter and pre-monsoon seasons. The present research provides a benchmark for nitrogen-budget modelling in tropical rivers, supporting planning for sustainable river management.

Knowledge of the trophic structure and the major energy sources supporting metazoan production are important considerations for biodiversity conservation and ecosystem management. African streams and rivers face multiple stressors from agricultural intensification, deforestation, and municipal and industrial effluents coupled with uncontrolled water abstractions. Yet, the effects of these influences on ecosystem structure and functioning are poorly understood. In this chapter, we review the trophic dynamics of African riverine ecosystems with a focus on trophic structure, the major sources of energy supporting food webs, and the influence of human activities. While much of the data used for this review are from African studies, we also reference other studies in the tropics for comparison and to fill existing knowledge gaps. Based on available information, autochthony, short food chains, and an increased tendency toward omnivory characterize food webs in African streams and rivers. However, trophic interactions and dynamics in these systems are witnessing changes caused by human activities. Changes in trophic diversity and dynamics include shifts from allochthony to autochthony following the deforestation of forested headwater streams, top-down control of local fish and invertebrate populations caused by introduced predatory fish such as trout, and shrinkage of trophic niche sizes caused by land use change. Despite these developments, studies on food web structure and trophic dynamics are very limited in low-order streams, and we have identified future research needs that need to be addressed to fill knowledge gaps that would hinder biodiversity conservation and effective management of riverine ecosystems in African rivers, including their fisheries.

To cope with the groundwater depletion problem and achieve sustainable groundwater development, groundwater conservation measures and managed aquifer recharge (MAR) have been implemented worldwide. However, knowledge gaps exit how does the aquifer system respond to these interventions differently and if these interventions are adequate to lead to long-term sustainable groundwater development under future climate change. In Beijing Plain, two measures have been implemented: reduction of groundwater abstraction by substituting groundwater abstraction with transferred surface water and implementation of managed aquifer recharge (MAR) in two major rivers. This study aims to assess how do the shallow and deep aquifers respond to these measures and if these measures can lead to long-term sustainable groundwater development in Beijing Plain under future climate change. A 3-D transient groundwater flow model was calibrated and used to simulate groundwater level and budget changes from 2021 to 2050. The monthly groundwater recharge was estimated using the projected monthly precipitation from three downscaled regional climate models under two scenarios (RCP4.5 and RCP8.5). The results show that declines in groundwater head and storage can be reversed with the combined two measures, thereby contributing to achieve sustainable groundwater development. The reduction of abstractions is a deciding measure to reverse the trend of groundwater depletion, especially in the deep confined aquifers, while large scale MAR schemes can restore the cones of depressions in shallow aquifers and maintain the groundwater abstraction. Climate variation has large impacts on groundwater resources, especially, consecutive dry years can cause rapid groundwater storage depletion. The projected monthly precipitation from 2021 to 2050 is not significantly different from the past. Therefore, the projected future precipitation has minor impacts on groundwater resources in the next 30 years. The findings from the study will support the Beijing municipality to maintain the tight control on groundwater abstraction and to implement large-scale MAR schemes in two rivers. This successful example will encourage managers of other heavily exploited aquifers to take similar measures to achieve sustainable groundwater development.

Rivers are important sources of water and energy to the people and growing economies of Africa, but they are also vibrant and biologically diverse ecosystems in their own right. Human use invariably exerts negative pressures on riverine ecosystems, including alteration of natural flow regimes due mainly to flow regulation by dams and water abstractions. Severe alteration of natural flow regimes changes the physicochemical environment of rivers and reduces the abundance, diversity and specific characteristics of habitats used by native aquatic and riparian species, leading to unacceptable declines in ecosystem condition. In order to minimize the impacts of flow alterations and maintain acceptable ecosystem conditions, water managers are expected to maintain an environmental flow regime in rivers. African rivers exhibit a wide variety of flow regimes to which riverine species have adapted their life histories. Research over recent decades has identified important flow-ecology relationships that have been used to quantify environmental flow requirements in many river systems, and environmental flow requirements have been incorporated into the management actions of some rivers. However, the assessment and implementation of environmental flows have been highly variable across the continent, and additional effort is needed to protect environmental flows in all African rivers. This chapter explores some of the key aspects of environmental flow science and practice in Africa, both today and looking into the future.

Environmental flows (e-flows) aim to mitigate the threat of altered hydrological regimes in river systems and connected waterbodies and are an important component of integrated strategies to address multiple threats to freshwater biodiversity. Expanding and accelerating implementation of e-flows can support river conservation and help to restore the biodiversity and resilience of hydrologically altered and water-stressed rivers and connected freshwater ecosystems. While there have been significant developments in e-flow science, assessment, and societal acceptance, implementation of e-flows within water resource management has been slower than required and geographically uneven. This review explores critical factors that enable successful e-flow implementation and biodiversity outcomes in particular, drawing on 13 case studies and the literature. It presents e-flow implementation as an adaptive management cycle enabled by 10 factors: legislation and governance, financial and human resourcing, stakeholder engagement and co-production of knowledge, collaborative monitoring of ecological and social-economic outcomes, capacity training and research, exploration of trade-offs among water users, removing or retrofitting water infrastructure to facilitate e-flows and connectivity, and adaptation to climate change. Recognising that there may be barriers and limitations to the full and effective enablement of each factor, the authors have identified corresponding options and generalizable recommendations for actions to overcome prominent constraints, drawing on the case studies and wider literature. The urgency of addressing flow-related freshwater biodiversity loss demands collaborative networks to train and empower a new generation of e-flow practitioners equipped with the latest tools and insights to lead adaptive environmental water management globally. Mainstreaming e-flows within conservation planning, integrated water resource management, river restoration strategies, and adaptations to climate change is imperative. The policy drivers and associated funding commitments of the Kunming–Montreal Global Biodiversity Framework offer crucial opportunities to achieve the human benefits contributed by e-flows as nature-based solutions, such as flood risk management, floodplain fisheries restoration, and increased river resilience to climate change.

Following a 2015 Memorandum of Understanding, efforts began to develop a transboundary water allocation plan in the Mara River Basin between Kenya and Tanzania. Many lessons were learned along that way, including the importance of involving basin and national water authorities in all phases of data collection, planning and decision-making; understanding existing water management structures to promote communication and cooperation within countries; and using locally collected data whenever possible. Applying these concepts to future efforts can promote, although not ensure, ownership of the process within each country, trust between countries, and productive discussions around transboundary water resources.

The Yongding River (Beijing, China) was dry most times of the year, and groundwater storage was severely depleted. To address this issue, a river rehabilitation project was initiated. A downstream environmental flow release (EFR) project from upstream reservoirs has been implemented since 2019. This study evaluated the impact of EFR by constructing transient groundwater-flow and numerical tracer transport models to simulate the hydrogeological responses to the water release events in 2019–2020. The study identified two factors that significantly influence the river leakage rate, which are operational factors (i.e., water release rate and duration) and physical factors (i.e., hydraulic properties of the riverbed, regional hydraulic gradients, and groundwater depth) that determine the maximum water availability for groundwater recharge and maximum infiltration capacity, respectively. Predictive modelling was performed to assess the long-term effects of the proposed EFR scheme from 2021 to 2050, which showed that groundwater levels along the river will increase by 10–20 m by 2050. Groundwater storage is expected to be largely recovered and groundwater/surface-water connectivity in the middle reach of the river will be restored. This restoration will not only maintain the environmental flow for the benefit of ecosystems but also enhance groundwater recharge, promoting sustainable groundwater development in the region. Overall, this study provides valuable insights into the effectiveness of the proposed EFR scheme in achieving sustainable groundwater development in the region.

Many streams and rivers outside conservation areas across the Afrotropics face multiple stressors from land use change, urbanization, and excessive water withdrawals. Thus, there is a need to develop cost-effective tools for assessing and monitoring ecological changes to inform management decisions. Studies utilizing macroinvertebrate communities as indicators of the ecological condition of streams and rivers in the Afrotropics use diverse methods, including diversity, richness, biotic and multimetric indices. However, some of these indices are region- or country-specific, which limits their general use across multiple regions or countries. In this study, we address this challenge by testing and comparing the performance of diversity and richness indices (e.g., Shannon-Wiener and Simpson), regional biotic indices (the African Scoring System Version 5 [SASS5], Tanzanian River Scoring System [TARISS] and a biotic index developed for the Ethiopian highlands [ETHbios]), and a macroinvertebrate-based index of biotic integrity (M-IBI) in assessing the ecological condition of Afrotropical rivers with the transboundary Mara River, Kenya and Tanzania, as a case study. In this study, we analyzed water and habitat quality degradation caused by multiple stressors such as land use change, organic pollution and flow alteration and the corresponding responses in macroinvertebrate communities. We utilized macroinvertebrates data collected from 143 sites covering the entire gradient of the river and its major tributaries in Kenya and Tanzania. To develop the M-IBI, we used 12 metrics that describe macroinvertebrate community richness, composition, tolerance to disturbances (indicator taxa), and the composition of functional feeding groups. Although all the biotic indices were sensitive to poor water quality and human disturbance of the river, the M-IBI performed better than biotic indices (SASS5, Tanzanian River Scoring System, and Ethiopian highlands), diversity and richness indices by having a higher discriminatory ability of site categories according to different levels and types of disturbance. Diversity and richness indices performed poorly and failed to discriminate between stressor gradients in the river. This study demonstrates a need for testing and evaluating indices or protocols before adoption and use in biomonitoring streams and rivers in other countries and regions. There is an even greater need to assess the tolerance of macroinvertebrate taxa before inclusion in biotic indices for improved performance as discriminators of multiple stressors.

The Upper Indus Basin (UIB) heavily depends on its frozen water resources, and an accelerated melt due to the projected climate change may significantly alter future water availability. The future hydro-climatic regime and water availability of the Hunza basin (a sub-basin of UIB) were analysed using the newly released Coupled Model Intercomparison Project Phase 6 (CMIP6) climate projections. A data and parameter parsimonious precipitation-runoff model, the Distance Distribution Dynamics (DDD) model, was used with energy balance-based subroutines for snowmelt, glacier melt and evapotranspiration. The DDD model was set up for baseline (1991–2010), mid-century (2041–2060) and end-century (2081–2100) climates projections from two global circulation models (GCM), namely EC-Earth3 and MPI-ESM. The projections indicate a substantial increase in temperature (1.1–8.6 °C) and precipitation (12–32%) throughout the twenty-first century. The simulations show the future flow increase between 23–126% and the future glacier melt increase between 30–265%, depending on the scenarios and GCMs used. Moreover, the simulations suggest an increasing glacier melt contribution from all elevations with a significant increase from the higher elevations. The findings provide a basis for planning and modifying reservoir operation strategies with respect to hydropower generation, irrigation withdrawals, flood control, and drought management.