Floodplains are also known as the kidneys of the landscape. This is due to the fact that they are submerged in the event of a flood, and the suddenly much larger area lets the flow velocity drop so that substances carried along can settle. The height and type of vegetation also influence the roughness and flow rate and thus the settling not only of sediments, but also of nutrients and pollutants. This cleans the rivers, which are often exposed to a much too high nutrient load that can contribute to high algae loads after they flow into the oceans. In the floodplains, for example, the nutrients phosphorus and nitrogen are deposited and, depending on the shape, temporarily incorporated into plants. Or they are completely removed from the system by certain bacteria. This happens with nitrate that gets reduced to atmospheric nitrogen. Part of the phosphorus is bound to sediments and is deposited until the next flood takes the sediments away again.

In this Video (in German only), produced under leadership of the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, the ecosystem function described above (by myself) as well as further functions are explained in a illustrative way through the example of the Lahn river.

In addition to this characteristic, which is very useful for us humans, floodplains are very important for biodiversity. This is because on a small scale different heights and thus flooding frequencies alternate. That means that a wide variety of biotopes exist side by side. Even very dry locations may exist in a floodplain.