When a river floods, the water rises over its banks and flows out onto the surrounding land. Sediment (composed of clay, sand, and silt) filled floodwater is deposited on the land adjacent to the river, known as a floodplain. Coarser, heavier sediment settles first and builds up the banks of the river, whereas finer, lighter sediment is carried farther away from the river and is not deposited until the flow slows down. This deposited sediment left behind is called an overbank deposit.

Some rivers flood seasonally because of snow melt, excessive rainfall, or monsoons. Famous examples of seasonally flooded rivers are the Mississippi River in North America, the Amazon River in South America, and the Nile River in Africa. When a river floods regularly, the overbank deposits can build up in layers on the floodplain year after year. These sediment layers can grow to be several meters thick.

This layering process can create natural levees, consisting of tall sediment ridges that form along the river bank and prevent flooding. Along the Song Hong River in Vietnam, for example, overbank deposits have created large natural levees. When river levels rise over a natural levee then recede, the water of the other side pools allowing fine sediment to settle. This is another type of overbank deposit, called a backswamp.

Sometimes floods erode a levee, causing the levee to fail. The resulting sediment-heavy water breaks through and is deposited onto the floodplain in a large fan shape, called a crevasse splay. Crevasse splays are often the beginning of a new branch of a river, known as an avulsion channel.

Overbank deposits contain a variety of nutrients and organic materials that support plant growth. Because of this, floodplains are typically fertile and ideal for agriculture. For this reason, many ancient civilizations began on the banks of seasonally flooded rivers. In some places, the sediment also contains precious metals and gemstones. In fact, the world’s main supply of tin ore is found in overbank deposits.

When humans modify river ecosystems, the changes can interfere with the natural process of sediment deposition. For example, measures to reduce flooding, like building dams, can negatively impact floodplain habitats, reducing the availability of nutrient-rich sediment that instead pools in the channel. At the same time, human activities can erode the floodplain, increasing sediment load within the river, which can reduce water quality and damage aquatic habitats.

Unfortunately, today’s rivers and corresponding floodwater is often contaminated with toxic substances, including pesticides from farmland, industrial chemicals, heavy metals, and untreated sewage. These substances can be harmful to the plants, animals, and humans that live and grow in or near the river. These pollutants can also damage marine life when contaminated water flows into the ocean. Rivers can naturally help remove these toxic substances from the water and store them as overbank deposits on the floodplains.