Floodplain: Definition, Development & Features

Instructor: Terry Dunn

Terry has a master's degree in environmental communications and has taught in a variety of settings.

Floodplains can be a small part of the landscape or a gigantic feature. Here, you will learn what floodplains are, how they are formed, why they are important and where you can find some major floodplains in the world.

What is a Floodplain?

Let's say you're on the hunt for your dream house. You picture it being alongside a babbling brook, in a wide, green valley, with a little farmland along the edges. Perhaps your idyllic, green oasis is surrounded by mountains, desert, or forest, but your perfect house, would be close to the water so you can hear it flowing by. If you find that dream house, you might want to buy extra insurance, because it's probably in the middle of a floodplain.

At the most basic level, a floodplain is an area where there is a river or stream that regularly overflows, and the flooding is often seasonal. It is usually a flat area with steeper sides on the perimeter. A floodplain can be small, large, and sometimes massive. That doesn't mean that every time there's a flood, water fills the floodplain. But eventually, there will be a flood that extends to the edges of the floodplain.

A floodplain flooding
A typical floodplain

The Dynamics of a Floodplain

A floodplain, it turns out, often creates good agricultural land, not just because water is present, but also because those floodwaters carry sediments that enrich the soil. The waters also sometimes carry pebbles and rocks, but that doesn't outweigh the rich sediments, so to speak. The debris that is left by running water is called alluvium.

Floodplains tend not to be formed by fast-moving waters, which usually cut deeper rather than allowing floodwaters to flow more broadly. In a floodplain, the water flows more slowly, allowing the sediment to settle out. The heavier debris settles out first, often forming piles along the edges, which are called levees. You'd be surprised how much sediment the streams and rivers of the world move each year: 16 billion tons!

The edges of the Nile River showing the rich agricultural land of the floodplain
Limits of the Nile floodplain

Floodplains usually have certain characteristics:

Channel: the main flow of the stream

Stream bed: the bottom of the stream

Stream banks: the sides of the stream

Examples of Floodplains

Small floodplains can be little enough that they barely show up on a map, but there are floodplains in the world that define whole regions and sometimes almost entire countries. The Mekong River Delta in Vietnam is a case of a floodplain that nearly covers the entire country - it's a whopping 12,000 square miles in size. In the United States, the lower Mississippi River has a large floodplain, and so does much of the Amazon River basin in South America. Another notably large floodplain in South America is that of Paraná River - it spans 11 miles.

Satellite view of the Parana River floodplain in Argentina
Parana River floodplain Argentina

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