Global Water Program

Why Rivers Flood – and How to Reduce Risk

Flooding along the Mississippi River

Join Keith Ouchley for a flight over the flooded Atchafalaya River basin.


Dispatch From the Missouri Flood

Michael Reuter reports from Sikeston, MO, site of record flooding.


With the Mississippi River sending record amounts of floodwater into communities, farms and natural areas from Missouri to Louisiana, spoke with Senior Freshwater Scientist Jeff Opperman to get the fundamentals on flood science.

 Why do rivers flood?


The simple answer is that very high amounts of rain, snowmelt, or both lead to floods. All the land that drains downhill into rivers—or into the streams and tributaries that eventually join the river—is called a watershed. When that watershed receives a lot of rain, the river will rise higher than its banks and spread out into the floodplain, which is the name for low lying areas along a river.

But what most people don’t realize is that floods are not some departure from the way a river is “supposed” to behave – witness the common description in the press of a river “bursting from its banks” during a flood, like a convict breaking out of jail. The river and floodplain are actually one single highway for moving water downstream (and not just water, but also sediment, which is why the Missouri River is nicknamed the “Big Muddy.”) It’s just that the floodplain part of this highway is dry much or most of the time.

Think of the river-floodplain system as all the highway lanes at a bridge toll crossing. For much of the day and night only a few booths are open and only a few lanes needed. But for the occasional “rush” hours, the flood of cars can only be contained by using all the booths and filling all the lanes. The cars are not “bursting out of their street” – they simply use all of the lanes only during the moments of intense traffic flow.

For most rivers, the channel between the river banks cannot contain even small floods – on average, natural rivers will spread out onto their floodplains anywhere from once a year to once every 3 years.

In fact, these floods are what create the floodplain. During each flood, the river deposits a layer of sediment outside of its banks. Over the centuries, these layers pile up like sheets and blankets thrown on top of each other, resulting in a thick bed of rich soil. This thick soil is what attracts farmers to floodplains and why the floodplains of the world’s great rivers, like the Mississippi and Yangtze, are so important for global food production.

Why do floods last so long even after rain stops?


Are you asking: “How can you have blue skies in New Orleans at the same time that you’ve got record flooding?” The answer is that the flood began hundreds of miles away and there is a lag between the rainstorms and when the crest (the highest point of the flood) arrives in a given location.

Even after a flood crests the water can linger where the river moved out into the floodplain. Rivers move water relatively quickly because they have a constant downhill slope and they have few obstructions to the movement of water. Floodplains don’t have a constant downhill slope and have flat areas where water can pool and remain for a long time. Floodplains also have lots of obstructions, such as trees, that slow water down. And levees can also prolong the flooding on a floodplain as they can prevent the water from draining back to the river.

Do floods hurt fish?


Actually, the opposite is true: floods are good for fish. Anyone who likes to fish will tell you that that many are found along the edges of the river, where there is slow-moving, shallow water with lots of cover from logs, aquatic plants, and overhanging branches.

This type of habit supports more food sources for fish, so think of it as similar to the grazing land needed by cows. Most rivers provide this “grazing land” only in narrow strips along the edges of the river, where the water nestles against the land.

Imagine being a cattle rancher whose grazing land was restricted to the thin strip of grass alongside a highway. When a river floods, the water spreads out across the land, creating a lot more of this shallow, slow-moving, vegetated habitat. For a rancher, it would be like removing the fences along the highway margin and allowing the cows to spread out on vast pastures of good grazing land – much more productive for the cows, just as floodplains are much more productive for fish.

In fact, rivers with extensive floodplains support the most important freshwater fish harvests in the world, such as the Mekong River, which provides the primary source of protein for 60-70 million people.

What can be done to prevent floods?


We can’t prevent floods. Rain will always come. The question is how to increase protection for people, nature, farms and businesses. But dealing with floods defies simple answers. For example, trying to keep a river completely confined between levees – and keeping all floodplains dry – not only deprives nature and people of the benefits of natural floods, it can also increase the risk of disaster. And relying only on levees to always work can create a false sense of security because almost all levees can, and eventually will, fail or be overtopped by floodwaters, resulting in catastrophic flooding.

Ultimately, there’s no silver bullet, especially in the case of massive floods like this. We have to take a holistic approach on a big scale. As we’ve seen, what happens upstream triggers a domino effect – the flood keeps moving downstream. The tributaries are also flooding, either on their own or because the Mississippi is so high the tributaries get backed up. For example, people have described the Yazoo River, a tributary to the Mississippi in northern Mississippi, as flowing backwards because the main river was so high it began to flow up its own tributary.

So we have to think strategically and work across boundaries to find the right mix of solutions. Floodplains themselves are part of the solution. You can see this right now on the Mississippi with the use of floodways and spillways that reconnect the river to a portion of its historic floodplain. As floodwaters spread out into these areas, it takes the pressure off of levees that protect towns, farms and cities.

Strategic designation of such floodways, with policies that compensate people who live within them, can be critical to reducing the damage of massive floods. We also need to make these floodways more “flood-proof.” For example, there may be ways to develop crops that are more tolerant of floodwater.

What is The Nature Conservancy’s role?


We know a lot about rivers and we’ve been working to keep them healthy for 60 years. We have more on-the-ground river and lake projects in the U.S. than any other conservation organization.

A big piece of that work is the protection, restoration and reconnection of floodplains, which is some of the richest habitat in the world. This obviously has the additional benefit of letting floodplains do what they do best: take in water when the river is high.

We have dozens of projects the whole length of the Mississippi, including a 7,000-acre floodplain restoration project and an ambitious effort to remove a section of levee and return 25-square-miles to natural, connected floodplain. These are happening at either end of the Mississippi River basin. There are dozens of other diverse projects in between.

We also have strong working relationships with all kinds of people, from the Army Corps to farmers. That’s really important because dealing effectively with flooding, which is expected to get more intense, requires that people from all sectors of society work together to find a way forward.

Jeff Opperman

Jeff Opperman, senior freshwater scientist, has been working to protect rivers and lakes for nearly 15 years. He has provided strategic and scientific guidance to freshwater conservation projects across the United States as well as in China, Africa and Latin America. In his role at The Nature Conservancy much of Jeff’s focus is on improving the environmental sustainability of hydropower both by advancing sound policies and by supporting on-the-ground projects.

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