Rivers and streams provide fresh water for drinking, irrigating crops, producing hydropower, and supporting fisheries. We meet these needs by intercepting the water cycle to store, move, and use water in complex ways, and times and places of our choosing. Consequently, many of the Earth’s rivers have flows that are “unnatural” in magnitude, frequency, duration, and timing. River networks are increasingly fragmented due to water overuse and associated flow intermittency, have dampened flow seasonality due to regulation by dams, and experience dramatic increases in peak flows due to urbanization. These changes result in direct and indirect impacts on biodiversity and ecosystem processes. Today, hydro-ecological science is pushing its boundaries to understand how the flow structure of a river or a stream—its “flow regime”—can affect biota, ecosystem processes, and the interplay between these two.
Albert Ruhi, an assistant professor in the Department of Environmental Science, Policy, and Management, published a review on restoration of river flow regimes in Science today in collaboration with Margaret Palmer, the director of the National Socio-Environmental Synthesis Center at the University of Maryland. The College of Natural Resources (CNR) sat down with Ruhi to learn more about the topics covered in their review.
CNR: Why are river ecosystems increasingly degraded?
Albert Ruhi: Many sources of river degradation exist globally, but the most important are related to excess nutrients and associated poor water quality, water overuse, dams, and urbanization. Our paper focuses on the latter two because their impacts are growing fast and have an important aspect in common: they both cause dramatic alterations to flow regimes. We often refer to flow regimes as a “master variable,” because they influence habitat structure, biodiversity, and ecosystem-level processes. Because dams and urbanization alter flow regimes, they have profound effects on these different aspects of a river ecosystem.
About 3 million dams exist globally and thousands more are planned or in construction, especially in countries with developing economies. As a consequence, less than a quarter of the Earth’s long rivers run into the ocean without interruption. Dams provide numerous benefits to humans, but they fragment river habitats and degrade the river banks (known as riparian habitats), they trap sediments and nutrients, and they create artificial flow regimes downstream. Additionally, today over half of the world’s population lives in cities, and by 2050 this figure will reach almost 70%. In urban environments, flow regimes tend to be highly variable, with sudden floods, and unsurprisingly, water quality is often impaired.
CNR: What are the impacts of this ecosystem degradation?
AR: The impacts of this degradation are numerous for both river biodiversity and ecological processes. Native species are replaced by non-natives, top predators often go locally extinct, and algal production frequently increases. Whole riverine food webs are affected by this degradation, not just individual species. All these changes can compromise the ability of river ecosystems to function adequately. For example, rivers may become less efficient at decomposing leaf litter, or may be unable to keep fluctuations in water chemistry within the bounds that sensitive organisms can tolerate.
CNR: What types of river restoration efforts are there?
AR: River restoration practice has traditionally focused on “improving” the stream channel: changing its form and location, adding structures to promote biodiversity by making riverine and riparian habitats more complex. However, in the paper we argue that restoring flow regimes, or at least ecologically-important facets of a flow regime, provides far greater benefits. In urban environments, flow regimes can be partially restored via green infrastructure that promotes infiltration and decreases peak flows. In regulated rivers, it is often achieved via controlled releases from the reservoir; for example, by recreating seasonal flows that match with the timing of fish reproduction or dispersal.
CNR: Your paper notes that river ecologists are using sophisticated methods to study river biodiversity and ecosystem metabolism. What are some of these methods?
AR: An increasing availability of environmental and biological time series data is spurring the use of quantitative methods that were common in other disciplines, but required too much data to be used in river science until recently. We now frequently combine environmental data collected by in-stream and remote sensors, with biodiversity records obtained via long-term monitoring programs—or even by citizen science.
CNR: What did you find when looking at the research related to this topic?
AR: In the paper we show an example where the application of time-series analyses in the frequency domain (that is, focusing on how predictable or unpredictable environmental variation is) allowed detecting changing frequencies in the environment—particularly, seasonality becoming weaker due to flow regulation. Then, we can relate such changes to responses of the stream invertebrate and fish communities. On the ecosystem processes side, recent research partitioned the contribution that different branches of a river network made to network-level nutrient removal and transformation. We propose this could allow for identifying habitats that are crucial for a particular process, and thus should be prioritized for protection or restoration.
CNR: What are some of the challenges and opportunities for those working to restore river ecosystems?
AR: We realized that there is abundant research connecting flow regime alteration to organisms and ecological communities (e.g., fish invasions) as well as to ecosystem processes (e.g., to algal production or nutrient uptake). However, very few studies have linked the three parts of the story, or what we call the “flow-biota-ecosystem processes nexus.” Tackling this research challenge will improve our ability to predict ecosystem-level responses to river degradation—or outcomes that should be expected if we choose to restore the flow regime.
A great opportunity to advance river restoration science is in the use of environmental releases by dams, or the design of green infrastructure, as scientific experiments rather than just management tools. This way, restoration outcomes can be assessed and used to adjust restoration designs in an adaptive way. We should not be surprised that river restoration has had limited success so far, however. This is, after all, still a relatively new science. Successful restoration is the ultimate test of our understanding of a system—speculating about why an ecosystem may not ‘work’ is much easier than putting it together successfully, in practice!