Anthropogenic versus natural flow intermittence in river networks: causes, responses, and implications

More than half of the world’s streams and rivers are naturally intermittent and the first global mapping of these watercourses was published in Nature on June 16 2021. In recent years, the causes, consequences and implications of dry phases in streams and rivers have been subjected to intensive scientific scrutiny in the fields of hydrology, biogeochemistry and ecology. The knowledge thus accrued provides a basis for various adaptation pathways in the management of river networks comprising naturally intermittent streams and rivers. By contrast, our knowledge related to the ecological consequences of river drying caused by human activity remains limited, although the impacts of global change, and particularly climate change, are becoming increasingly visible.

The team composed of members from the European DRYvER project and the Dry Rivers RCN in the United States, first investigated the range of anthropogenic drivers of river drying.  Water abstraction (both surface water and groundwater), the construction of reservoirs to store and regulate flow for irrigation, hydroelectric schemes and recreational activities, changes to land use and climate change each have hydrological signatures that differ from those seen in natural flow intermittence, imposing their own patterns on the frequency, duration, intensity and timings of the water deficits they cause.

Responses to the two different types of drying in terms of biodiversity, ecosystem functions and ecosystem services were then explored. The transition from a perennial to an intermittent flow regime represents a dramatic tipping point for aquatic biodiversity, since many species lack the adaptations needed to cope with drying. This has profound, cascading implications for ecosystem functions such as the decomposition of terrestrial litter, a vital process for aquatic ecosystems. In the realm of ecosystem services, the zones most susceptible to anthropogenic flow intermittence are often those where the ecosystem services provided by river networks (drinking water, aquifer recharge, irrigation for food production, etc.) are the most critical for the survival of human populations and their well-being, leading to significant management challenges.

The paper concludes with an assessment of the knowledge gaps and management needs related to anthropogenic flow intermittence, proposing priorities for future research on river networks in the Anthropocene. The message is clear. If the differences in hydrologic characteristics and ecological impacts between natural and anthropogenic flow intermittence are ignored, the management of global river networks could be compromised, exacerbating the risks for the ecosystems and societies that rely on them.