The flood paradox: Bad or good for the environment?

As images of river markers swallowed by rivers and homes awash with water that had nowhere else to go, become a more common occurrence, it is hard to imagine there is anything potentially beneficial about flooding. Indeed, of all climate-related weather extremes recorded in the European Union between 1980 and 2022, it accounted for the greatest economic loss. 

Yet, its consequences for the natural non-built environment are not always negative.

James Dalton, Director of the Global Water Programme at the International Union for Conservation of Nature, says many ecosystems in fact rely on seasonal rising waters for the occurrence of normal ecological processes such as distributing soil nutrients.

"We use river systems for a steady supply of nutrient flow that enriches ecosystems further down the river system, but also flows into estuaries and deltas, which are the most biologically productive parts of the world," he said. 

"We need floods because they flush down sediments and they flush down nutrients and they flush down certain species as well, which are critically important." 

Flood waters recede leaving behind sediment and nutrients, which provide a rich, natural fertilizer, improving soil quality and promoting plant growth. It is one of the reasons that many towns and cities have grown up around rivers. As well as providing much needed water and a means of transporting goods, the deposited nutrients serve as fertile ground for farming.  

Flooding is also useful in replenishing groundwater supplies. While the process happens naturally in some regions, in California, regulators are planning to divert and store excess floodwaters to help refill natural underground reservoirs to store water for periods of drought. 

Wildlife at risk: the adverse effects of flood events

But beyond the natural order of things, in a world where global temperatures continue to rise, causing heavier and often unpredictable rain patterns in some regions of the world, the impacts are not so beneficial.

"We are seeing floods become more destructive and last for longer and be faster in terms of impact because of changes in rainfall," said Dalton. "And that is having an impact on biodiversity.”

One of the most obvious impacts of flooding on wildlife is that some species cannot escape the rising waters fast enough. In 2012, for example, hundreds of animals, including the endangered one-horned rhino, were killed when the Kaziranga National Park in northeastern India was hit by severe floods. 

Even if animals manage to escape, flooding can destroy their habitats and breeding grounds. 

"The more damaging floods scour riverbanks and floodplains, so can shift sediment and soil in a way that can affect migratory species because they can't get to where they need to be at a particular time for spawning and breeding. So you affect the natural reproduction cycles of species," said Dalton.

Plants can also be impacted by flood waters containing agricultural pesticides, industrial chemicals or sewage. If animals eat contaminated plants, they are exposed to toxins and impurities, which also then make their way into the food chain.

Eroding an essential food and environmental resource: soil 

While some soil benefits from flooding, faster-flowing waters leave a more destructive imprint, potentially wiping out the top five to 10 centimeters of the most nutrient-rich earth.

"Imagine when you have a flooding event," said Michael Berger, consultant for sustainable agriculture at WWF Germany, "the soil can just be flooded away and there's nothing anymore. There's still soil but the most precious part is gone." 

"It cannot be rebuilt by humans and it's the basis of producing food." 

As the most species-rich habitat in the world, soil is also essential for preserving biodiversity. According to a new report published by the Heinrich Böll Foundation, linked to Germany's Green party, healthy earth can store more carbon dioxide than forests.

Soil erosion destroys habitats and landscapes, but also prevents the earth from absorbing water when there are big flooding events, exacerbating the problem.  

"We need healthy soils in order to adapt to the climate crisis. They can store up to 3,750 tons of water per hectare and release it again as required," said Imme Scholz, president of the Heinrich Böll Foundation, in a press statement. 

What can we do to protect ecosystems from flood events? 

Dalton says there are ways to limit the impacts of flooding and that "many reasons why floods are potentially worse than they should be is because we've built things in the way of floodwaters or we put things in the floodplains.”  

A lack of affordable housing in cities all over the world has led to more and more developers being allowed to build on floodplains, which means homes and businesses are going up in potentially dangerous places. 

"Reconstituting the way that rivers function and going back to how they should be meandering and connecting them together, connecting rivers to their floodplains, connecting groundwater to surface water is important because that's all effectively the network of letting water flow," said Dalton. 

Diversifying agriculture away from industrial methods to such practices as agroforestry, which intentionally integrates trees into farming, can also prevent against the impacts of flooding. 

"When you have a flooding event, it can slow down the speed of the water. And the slower the water is running, the less erosion you have," said Berger. 

"We as humans facing climate change with all its flooding events or drought events, we should definitely go for nature-based solutions instead of putting concrete everywhere and believing we can solve everything with our technology," he said. 

Edited by: Tamsin Walker