Abstract

Practically all river networks on the planet include non-perennial rivers and streams (NPRs) that periodically cease to flow or dry up. The repeated cycles of flow, non-flow, and drying that characterize NPRs are a key factor contributing to the high biodiversity and biogeochemical cycles of river networks. Consequently, the alteration of these hydrological cycles can threaten the integrity of river ecosystems, their biodiversity, and the human populations that depend on them for their livelihood and culture.
Despite their prevalence and importance, NPRs are often excluded from management practices, conservation laws, and scientific research, which are typically based on the functioning of perennial rivers. This bias arises from a negative perception of NPRs among managers and the public, and a historical lack of consideration for their specificities. As a result, NPRs suffer from chronic mismanagement and are deteriorating at an alarming rate. The aim of this thesis is to advance our understanding of the prevalence and diversity of NPRs on a global scale and to improve their integration into public policies and sustainable water management measures. Drawing on an interdisciplinary perspective that integrates hydrology, ecology, geography, and data science, this thesis addresses three main objectives across four articles (Chapters 2 to 5).

Chapters 2 and 3 provide the first robust quantitative estimate of the prevalence, distribution, and diversity of NPRs worldwide. Using a machine learning model based on global data on hydrology, climate, geology, and land use, Chapter 2 reveals that water ceases to flow for at least one day per year in 51% to 60% of the world’s rivers. This result demonstrates that non-perennial rivers and streams are the rule, not the exception, on Earth, and that they are present in all climates and biomes, across all continents. Furthermore, Chapter 3 identifies nine hydrological types of NPRs globally, which differ in frequency, duration, seasonality, and the reason for the cessation of flow.

Chapter 4 highlights the inadequate protection of NPRs in environmental legislation. Through a case study on regulatory maps defining protected watercourses under the French Water Law, this chapter reveals a disproportionate exclusion of NPRs from regulatory frameworks, the sociopolitical factors influencing regulatory mapping, and its implications for the integrity of the hydrographic network.

Chapter 5 develops a conceptual and operational framework to improve the effectiveness of ecological flow (e-flow) programs aimed at protecting freshwater ecosystems, particularly in river networks with a high prevalence of NPRs. In this chapter, I propose expanding the range of ecological processes integrated into the design, implementation, and monitoring of ecological flows in order to better protect the unique structure and dynamics of NPR ecosystems.

In conclusion, this thesis challenges the dominant conceptual models of river ecosystems by demonstrating the global prevalence and diversity of NPRs and promoting their integration into scientific frameworks, public policies, and management practices. In doing so, it contributes to a paradigm shift towards an integrated vision of river networks. This new paradigm is based on the study and management of all segments, their floodplains, and the watersheds that contribute to them, as a dynamically interconnected meta-ecosystem whose components span the aquatic-terrestrial continuum.

Publications

• Messager, M. L., Lehner, B., Cockburn, C., Lamouroux, N., Pella, H., Snelder, T., Tockner, K., Trautmann, T., Watt, C., & Datry, T. (2021). Global prevalence of non-perennial rivers and streams. Nature, 594, 391–397. https://doi.org/10.1038/s41586-021-03565-5
• Messager, M. L., Olden, J. D., Tonkin, J. D., Stubbington, R., Rogosch, J. S., Busch, M. H., Little, C. J., Walters, A. W., Atkinson, C. L., Shanafield, M., Yu, S., Boersma, K. S., Lytle, D. A., Walker, R. H., Burrows, R. M., & Datry, T. (2023). A metasystem approach to designing environmental flows. BioScience, 73(9), 643–662. https://doi.org/10.1093/biosci/biad067
• Messager, M. L., Pella, H., & Datry, T. (2024). Inconsistent regulatory mapping quietly threatens rivers and streams. Environmental Science & Technology. https://doi.org/10.1021/acs.est.4c01859