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Can rivers always recover from floods?

Rivers are complex and crucial parts of our planet. From playful otters to towering salmon, countless creatures call rivers home, relying on their ever-flowing waters for survival. 

But what happens when powerful floods disrupt this delicate balance? Scientists from the University of Nevada, Renohas created a model in their lab to answer this question. 

The results challenge our previous ideas about how well rivers can recover from floods and other disturbances. The research provides a new perspective on the delicate balance of life in these flowing ecosystems.

The resilience of the river ecosystem 

After floods, the rising water in a river might seem insignificant, but beneath the surface, a battle for survival is waged. Plants and algae, known as autotrophic biomass, face the flood’s fury. How quickly they recover after the flood subsides determines the resilience of the entire river ecosystem.

The researchers set out to gain a better understanding of the resilience of the river ecosystem. They developed a clever method using underwater sensors that monitor oxygen levels of 143 rivers. 

A window into post-flood recovery 

This data, fed into their specially designed latent biomass time series (LB-TS) model, revealed a hidden treasure: the daily growth rate of the vital biomass. This model acted as a window into their post-flood recovery, a real-time glimpse into their fight for survival.

“Previously, you would have to go to a river and scrub rocks to measure the algae, and do that several times for an extended period of time in order to estimate changes in biomass growth and loss,” said study lead author Joanna R. Blaszczak. “This is very time consuming, so the data have been extremely limited relative to how extensive our sensor networks are.”

“The dissolved oxygen sensors show the peak during the day, and the low during the night, and from those patterns, you can estimate how much new algae and other biomass grew that day.”

“With the sensors measuring data continuously in hundreds of rivers for years now, we can get a much bigger, clearer picture. The data is there, and we can use it to model the size of flood needed to disturb the biomass in a river, as well as the rate at which a river recovers from flood disturbances, which can help us manage rivers more effectively.”

Recovering from disruptions

Traditionally, scientists believed rivers could handle natural floods pretty well. But the research shows rivers might be more sensitive to changes in water flow than we realized.

Different rivers recover from disruptions like floods at different speeds. Some bounce back quickly (2 days), while others take longer (14 days).

Normally, we judge a river’s flood resistance based on how often it can handle major floods without damage. But it turns out, even smaller changes in water flow can hurt how much life the river supports. 

This means our current ways of managing rivers might not be enough. If they’re truly more sensitive than we thought, we need to change our approach. This could involve using less water, fixing floodplains, and making plans that better consider the specific needs of each river.

River width and dams

Wider rivers bounce back faster from floods than narrow ones, especially if they don’t have dams upstream. This study shows that both a river’s size and human impacts affect how well it recovers from floods.

Wider rivers spread flood waters out more, which makes the water flow slower and gentler. This gentler flow causes less damage to the plants and animals living in the river, so they can recover more quickly. 

Dams, on the other hand, mess up the natural flow of water, sand, and nutrients in the river, making it harder for the ecosystem to bounce back after a flood.

Additional contributing factors

Rivers deal with many factors- light, water, plants, and animals all play crucial roles. Sunlight acts like fuel for the river ecosystem, powering the growth of plants and algae. Water flow acts like a delivery service, bringing nutrients, shaping the riverbed, and controlling the pace of everything happening.

When a flood disrupts this balance, changes in sunlight and water flow can affect the ecosystem’s “food production” (how much organic matter is generated) and its structure (how different organisms are arranged).

Animals like fish and insects play a role in recovering from the disturbance. They eat algae, preventing it from taking over completely, but they can also overeat and slow down the growth of new plants.

Talking about plants, floods can mix things up by moving them around, favoring certain species over others. Initially, fast-growing types might dominate, but as things settle down, a more diverse group usually arrives. This shift in plant life affects everything from nutrient movement to animal habitats.

Study implications

Healthy rivers are vital for clean water, diverse plant and animal life, and strong communities. This study helps us understand how floods affect rivers, which is crucial for protecting their health.

The study focuses on “thresholds”: points where floods become harmful to the river ecosystem. Knowing these thresholds is key to managing and protecting rivers effectively. By identifying them, we can take action beforehand, like adjusting dam releases or reinforcing riverbanks, to prevent major damage.

This knowledge is even more important with climate change increasing the number and intensity of floods. We need strong conservation strategies to protect rivers in the face of these challenges.

The study is published in the journal Ecology Letters. The river analysis is published in Proceedings of the National Academy of Sciences.

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