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Over Half of the World’s Rivers Stop Flowing Periodically for Part of the Year

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Jun 17 2021

According to a new study, headed by scientists from the French National Research Institute for Agriculture (INRAE) and McGill University, about 51% to 60% of the 64 million kilometers of streams and rivers on Earth have stopped flowing intermittently or dry up for part of the year. The study is the first experimental attempt to determine the worldwide distribution of non-perennial streams and rivers.

Over Half of the World’s Rivers Stop Flowing Periodically for Part of the Year — *Most rivers run dry - now and then. Image Credit: McGill University.*

Recently published in the Nature journal, the study calls for a paradigm shift in river science and management by revising foundational theories that historically assumed year-round water flow in streams and rivers.

For the first time, a map of non-perennial rivers was obtained from the study and this chart provides important baseline data for assessing upcoming changes in the intermittence of river flow. It also helps in establishing and tracking the role of these streams and rivers in global biochemical and water cycles, and also in supporting biological diversity.

Non-perennial rivers and streams are very valuable ecosystems as they are home to many distinct species that are adapted to cycles of water presence and absence. These rivers can provide critical water and food sources for people and they play an important role in controlling water quality. But more often than not they are mismanaged or altogether excluded from management actions and conservation laws as they are simply overlooked.

Mathis Messager, Study First Author and PhD Student in Geography, McGill University and French National Research Institute for Agriculture, Food, and Environment

Non-Perennial Rivers and Streams Found on All Continents

Given continued global climate and land use change, an increasingly large proportion of the global river network is expected to cease to flow seasonally over the coming decades. In fact, many formerly perennial rivers and streams, including sections of iconic rivers such as the Nile, the Indus and the Colorado River have become intermittent in the past 50 years due to climate change, land use transitions, or the temporary or permanent withdrawal of water for human use and agriculture.

Bernhard Lehner, Associate Professor, Department of Geography, McGill University

The researchers traced out the most important environmental characteristics to determine the reason for the periodic drying up of rivers. This was done by statistically equating long-term records of water flow in 5615 sites throughout the world, with data available on hydrology, geology, climate and surrounding land cover of the streams and rivers tracked at these sites.

As predicted, the researchers observed that non-perennial rivers are common in arid regions (where there is relatively more evaporation than rainfall) and that smaller streams and rivers are more likely to dry up as a result of their variable flow. However, they also occur in the Arctic where rivers are frozen for parts of the year as well as in tropical climates.

Fascinatingly, the research also indicates, based on initial estimates, that more than 50% of the global population dwells in regions in close proximity to non-perennial rivers or streams. In several languages, numerous words are used to denote these kinds of watercourses and their mark on the landscapes, emphasizing the long history of inter-dependence between seasonal freshwater systems and humans.

A Long-Standing Neglect with Significant Consequences

Over the past 10 years, several efforts have been made to point out the values and rapid ongoing degradation of non-perennial streams and rivers. Most freshwater science has focused on the conservation and functioning of perennial rivers, until now. Scientists have only recently begun to understand the major consequences of flow cessation in streams and rivers.

“Consequently, science-based methods for managing these unique ecosystems, such as tools and protocols to monitor the health of these rivers are still limited or absent. And this oversight leads to excessive water pumping, pollution, and overfishing in many cases,” added Messager.

There have also been several recent attempts to remove non-perennial rivers from environmental legislation and national water governance systems, including in the U.S. and France. By mapping non-perennial rivers and streams, our study pushes for a recognition of their prevalence and ecological significance by the scientific community. We hope that our study will trigger efforts to adequately manage these river ecosystems and halt attempts to exclude them from protective legislation.

Thibault Datry, Freshwater scientist, French National Research Institute for Agriculture, Food, and Environment

The study was financially supported by a McGill University Tomlinson Fellowship, a Doctoral Fellowship from H2O’Lyon Doctoral School, DRYvER Project, and the Natural Sciences and Engineering Research Council of Canada.