Our hydrobiologists reveal, thanks to the Horizon 2020 project DRYvER, the impacts of current and future river drying up to the year 2100

The extent, species diversity, and ecosystem services provided by river ecosystems to human societies will be significantly affected in the coming years due to climate change and the associated increased occurrence of river drying. In addition to drying, these systems will face new and often synergistic stressors from global changes, such as pollution, invasive species, and landscape changes, which together pose a major threat to local aquatic communities. 

Introduction: Four publications from the prestigious Horizon 2020 DRYvER project 
In spring 2025, four distinct articles on drying rivers from the international Horizon 2020 project DRYvER were published, which we introduce in this article. Outputs of the project include a mobile DryRivers application, enabling public help in mapping drying rivers, and the practical Catalogue of Nature-Based Solutions for Drying Rivers. 

The first article, “Projections of streamflow intermittence under climate change in European drying river networks”, presents models estimating future hydrological trends in six model catchments based on three climate scenarios (pessimistic, realistic, optimistic). These drying scenarios cover six European climate types from north to south and east to west, including the Velička River basin in the White Carpathians. 

The second study (Chalmandrier et al. 2025) titled “Natural Disturbances and Connectivity Drive Seasonal Taxonomic and Trait Patterns of Aquatic Macroinvertebrate Communities Across Europe” analyzes seasonal responses of aquatic invertebrates to ongoing drying effects under different climate regimes, considering drought frequency and duration.  

In a more detailed study focused exclusively on three Central European catchments and a model group of caddisflies, the article by Harsagy et al. (2025) titled “Connectivity-Driven Assembly of Trichoptera Metacommunities Across River Networks With Different Drying Pattern” follows up, concentrating on survival strategies and recolonization of drying sites. 

The series is then concluded by the publication of Escobar-Camacho et al. (2025) titled “ River Drying Causes Local Losses and Regional Gains in Aquatic Invertebrate Metacommunity Diversity: A Cross-Continental Comparison” which compares different patterns of invertebrate community formation depending on the region’s climatic development and historical occurrence of drying. 

The following sections present detailed results of these studies. 

Hydrological models of European river drying within the DRYvER project: scenarios and ecological impacts in the Velička River basin 
The international Horizon 2020 DRYvER project developed a comprehensive hydrological model of drying dynamics for six European basins ranging from Mediterranean southern regions through Central Europe to northern Finland. The modeling is based on three future climate scenarios—from an optimistic sustainability scenario with strong greenhouse gas emission reductions, through a neutral realistic variant, to a pessimistic fossil-fuel scenario with minimal emission cuts. 

In the Czech context, the Velička River basin is especially relevant. It covers 177 km² with approximately 145 km of river network and was analyzed by our hydrobiologists at the Department of Botany and Zoology, Faculty of Science, Masaryk University. The basin experiences drying due to a complex combination of factors: direct human water withdrawals, landscape changes, river regulations, and continental climate with hot summers, predominant southwest slope exposure, and flysch clay geology with low water retention. 

Models project that by 2100, under the realistic scenario, flow reductions of about 25% will occur, and under the pessimistic scenario, more than 60% reduction is expected. 

This could cause a drastic decline of up to 60% in continuously flowing river segments, especially in the agricultural lower basin, where currently about 35–40% of streams do not dry but might drop to 10–15%. Over 35 km (8.4% of the basin’s total river length) could shift from permanent to intermittent flow regime. 

Drying onset is expected to advance by 2–3 months, and dry periods will extend by 2–3 months toward late year. Uniquely, Velička drying will increase not only in warm summer months but also in colder parts of the year, potentially lasting through winter holidays until early January. This seasonal change raises the risk of freezing dried riverbeds, which devastates aquatic fauna not adapted to subzero conditions. Even larger, wetter river sections are projected to experience drying length increases from weeks to months. 

Retrospective hydrological modeling of the extreme drying event in 2012 predicted smaller dry extents than actually observed (when two-thirds of the Velička basin was dry). This suggests that future modeling projections may underestimate actual drying extent. 

Effects of drying frequency and duration on aquatic invertebrate community composition 
Chalmandrier et al. (2025) studied six European drying basins within the DRYvER project, providing a comprehensive view of how drying forms and occurrence, spatial–temporal site connectivity, and biogeographic-climatic contexts collectively shape taxonomic and functional traits of aquatic invertebrate communities through the year. 

Key findings include: 

• Sites with repeated but temporary drying host richer species communities with higher seasonal turnover compared to sites with long, continuous drying. Alternation between dry and flooded phases reduces abundance, diversity, and functional roles but allows survival of resistant species with longer life cycles. Persistent drying favors species with higher fertility and dispersal, rapidly recolonizing after dry periods. 

• Species and functions peak in summer, likely related to higher temperatures fostering biodiversity, with drying rivers dominated by smaller, short-cycle species able to exploit the brief suitable periods. Permanent rivers host longer life cycle species favored by stable conditions. 

• Connectivity between drying and permanent sites is crucial, allowing recolonization from refugia and maintaining even less adapted species by balancing disadvantages with good dispersal or resistant stages (e.g., drought-resistant eggs). 

• European aquatic communities show strong climatic and altitude-driven variation. Mediterranean drying river species have high fertility and dispersal; lowland species favor gentle flows and fine sediments, are longer-lived but less dispersive and resistant; alpine species show short life cycles combined with resilience and resistance in some stages. 

Chalmandrier et al. conclude that drying frequency, duration, and site connectivity strongly shape invertebrate communities in European drying rivers. Areas with recent drying onset may cross ecological thresholds, leading to unpredictable diversity shifts and altered life strategies. The study highlights the need to integrate these insights into monitoring and targeted drying river management. 

Caddisfly communities depend on connectivity and drying history 
Hárságyi et al. (2025) compared drying-sensitive caddisfly (Trichoptera) communities across three basins with distinct drying histories: Mediterranean basin in Croatia, sub-Mediterranean basin in Hungary, and continental basin in the Czech Republic. The composition reflects spatial-temporal connectivity of river segments and long-term drying patterns. 

Understanding community responses to differing drying intensities, frequencies, and regularities is critical for predicting climate change impacts combined with human activities. This knowledge enables designing effective biodiversity protection measures to slow species loss and maintain ecosystem functions amid climate change. 

• Local environmental factors (substrate type, flow velocity and depth, water temperature, vegetation presence) most strongly defined caddisfly species composition and abundance in all three basins. 

• Regional factors, particularly hydrological connectivity among sites over days to 10 years before sampling, also shaped communities. 

• In Croatia, with regular seasonal drying cycles, long-term site connectivity (2, 5, 10 years) was most important, reflecting adapted and stable resistant communities.  

• Hungary’s drying effects appeared only recently (three years before study), producing fragmented, maladapted communities relying heavily on short-term site connectivity (up to 200 days) for survival and dispersal. 

• The Czech Velička basin showed the least connectivity effect, likely due to irregular drying resulting in a mosaic of wet, occasionally dry, and regularly dry patches, leading to more random community patterns. 

This underscores that drying and connectivity must be interpreted within specific geographic–climatic contexts. Long-term regular drying, as in Croatia, allowed species to develop survival strategies and stable communities despite climate fluctuations. In contrast, new and irregular drying in Hungary is a greater ecological threat due to species’ inability to adapt, requiring rapid changes in dispersal and colonization. Irregular drying in Czech rivers may soon cross a threshold causing significant biodiversity impact, making migration corridor restoration and directed conservation crucial. 

The study emphasizes the necessity of considering local and regional connectivity factors for adequate impact assessment and biodiversity protection planning. 

Drying impacts on biodiversity – a Europe–South America comparison 
Another DRYvER-based study compared 43 aquatic invertebrate communities from European and South American (Brazil, Bolivia, Ecuador) basins, providing the first description of differing drying impacts on species richness in regions with contrasting climates and evolutionary histories. 

Key contrasts: 

• In Europe, drying typically reduces local species diversity (alpha-diversity) since most native invertebrates are not evolutionarily adapted to repeated or prolonged droughts. Mediterranean fauna, adapted to regular drying, shows smaller or no diversity loss. 

• In South America, where drought timing is predictable and part of annual cycles, drying has minimal negative effect on local alpha diversity. Diversity in temporary and permanent streams remains comparable due to resistant life stages (e.g., diapause) and strong recolonization ability. 

• At the catchment scale, European drying basins surprisingly show higher regional diversity (beta-diversity) than permanent ones, due to spatial-temporal habitat heterogeneity promoting distinct communities. In South America, diversity between drying and permanent basins is similar and overall higher than in Europe, reflecting a longer evolutionary history adapting fauna to environmental fluctuations. 

A fundamental and unexpectedly strong factor for community survival was species dispersal ability. Species that recolonized efficiently from refuges recovered populations faster, highlighting the critical importance of free river connectivity for biodiversity restoration. This supports EU Nature Restoration Law initiatives calling for the removal of river migration barriers, including in the Czech Republic. 

These findings reveal that drying influences biodiversity differently depending on climatic context and stress the need for migration corridor restoration and species protection focusing on limited dispersers. 

Conclusions and research perspectives 

The DRYvER project substantially advances understanding of European river drying dynamics and aquatic biota responses to climate change accelerating drought impacts. Model projections clearly indicate longer drought periods extending into unusual times (late autumn, winter), creating new survival challenges for aquatic life. Severe freezing of dried channels represents a significant yet underexplored threat. 

Biological studies show that aquatic species’ survival and recolonization strongly depend on hydrological connectivity of the river network, making connectivity and barrier removal key for biodiversity and ecosystem stability in highly variable drying river conditions. Maintaining and restoring free flow is not just conservation idealism but an imperative to sustain viable populations under accelerating climate stress. 

Our studies confirmed distinct drying impacts across European climates and versus South America where evolutionary adaptation enables higher drought resilience. This stresses the need for precisely targeted, region-specific conservation strategies respecting the unique climate and ecological characteristics. 

We believe the DRYvER results provide a valuable foundation for effective river ecosystem protection and restoration strategies in the Czech Republic and internationally. They also commit us to continuing research on hydrology, biodiversity, and human activities interaction to enable adequate response to rapid environmental change and minimize climate crisis damage.