Do floods play a vital role in ecosystems?

Floods are often seen as a force of destruction. From photographs of crops under water and houses being swamped by swollen rivers, to stories of road, business and public amenity closures, the news during flooding understandably emphasises human loss.

But as river ecologists, we find it hard not to see the positive side of flooding. Why? Because although floods cause destruction, they are also creators, of which we are all beneficiaries.

Floods as destroyers

Rivers have played pivotal roles in most civilisations throughout human history due to the universal need for drinking water and other resources like food. Rivers feature in the mythology, religion, philosophy and culture of so many societies and also play political roles, acting as borders between tribes, states and nations.

Virtually all of the world’s major cities were founded on soils made fertile by flooding. In fact, floods – and the fertility that they bring - have been one of the most important reasons why human societies exist where they do today.

But despite their benefits to humans, rivers also bring death and destruction. In terms of lives lost, the top two worst natural disasters on record are floods.

The worst was in 1931, when at least 4 million people died and almost 30 million people were affected by floods in China.

In the United States, the Great Mississippi Flood of 1927 affected about 630,000 people and covered an area of almost 70,000 square kilometres. That flood’s destructive power was exacerbated by the failure of levees, as has commonly happened elsewhere.

By contrast, death tolls from Australian floods have been comparatively light. Purportedly the most lethal flood in Australia’s history was the 1852 Gundagai flood, which claimed almost 90 lives. Many drowned because the town was previously built on the lowland flood plain of the Murrumbidgee River.

Deaths and destruction occur to the extent they do because of our desire to live in the very areas that are most prone to flooding. But with living on flood plains comes risk, and sooner or later, a big flood will come.

Floods as creators

Generally, rivers flood every one to two years. It is just what they do. The reason is because of the interaction of geology, geomorphology and climate.

When rivers flood, water moves out onto the flood plain. But so does sediment and a lot of organic matter, nitrogen and phosphorus - the energy and materials that fuel river ecosystems and productive farm land. There is in fact mutual exchange of these rich materials between rivers and flood plains, which is why river flats are valued so much by farmers, and often why these areas became permanent settlements.

Some fish and other animals move backwards and forwards between the main channel and flood plain too, but all benefit from the rich materials transported by flooding.

Nature over nurture

In our ambition, we think that we can live on and exploit flood plains through controlling flooding. But this has been shown time and time again to be deluded.

Since the industrial revolution, vastly ambitious and expensive engineering projects around the world have sought to separate rivers from their flood plains, to reclaim land on which to build houses or to farm, and to prevent flooding. In most cases, levees have been built to effectively raise the level of river banks.

While these reduce the incidence of minor floods in some areas, they mostly fail to stop the major ones, and generally make flooding much worse in areas downstream.

Flood damage in the European Union from 2000-12, for example, cost an average US$6.8 billion a year, despite the extensive networks of levees designed to prevent flooding. Similar networks of dams and levees are ineffective at preventing large-scale flooding in Australia. Climate change is set to make the costs even higher.

Going with the flow

If we’ve learned anything from floods, it is that trying to prevent flooding, especially the big ones, is enormously expensive, rarely works and causes ecological and socio-economic damage. There are, however, ways in which people can live and enjoy the benefits of rivers without causing damage.

For example, the Yolo Bypass in Sacramento, California is a clever way of harnessing the floodplain’s capacity to buffer the effects of flooding, rather than trying to prevent flooding in the first place. The bypass, built in the 1930s, transports a large percentage of high flows away from the city, and into a reconnected flood plain. The flood plain is, during non-flood periods, used for agriculture and other activities.

Researchers argue that there are many human uses consistent with periodic flooding, such as the growing of pasture and timber, but building infrastructure on flood plains is not one of them.

Solutions such as these are far less costly than trying to prevent flooding and mopping up after inevitable failure. But of course, this requires a transformation in thinking when planning the design of towns and in developing flexible agricultural practices.

Floods are reminders that nature can be both creator and destroyer. Herodotus referred to Egypt as “the gift of the Nile”. It would be wise of us to view our own flood plains in the same way: that they are the gift of our rivers.

We should learn to accept that there will be times when the landscape on which we live, farm or play is reclaimed by the river that created it. On the flipside, we can rejoice when the river spends its time confined to its banks, and make hay while the sun shines.

Paul Humphries, Senior lecturer in Ecology, Charles Sturt University, Nicole McCasker, Post-doctoral researcher, Charles Sturt University, and R. Keller Kopf, Postdoctoral research fellow, Charles Sturt University.

This article first appeared on The Conversation.