Rivers have been important to humans throughout history. They provide food, freshwater, and fertile land for growing crops. Water is needed for life, but it can also be destructive. In many cases, the flooding of rivers can have terrible effects.

Flooding is one of the most common types of natural disaster, and the results are often deadly. A 1931 flood in China was one of the worst flooding events in recorded history. As many as several million people died.

Floods affect humans, but they also affect the environment. Their effects are not always bad, however. In fact, some ecosystems depend on flooding during certain seasons.

Floods Can Cause Harm

Flooding can harm wildlife. The water can drown animals and destroy habitats. For example, a flood in India in 2012 killed many one-horned rhinos (Rhinoceros unicornis).

Floodwaters can pick up dirt from riverbanks. Particles of mud and dirt float in the water. These particles are known as sediment. If the water becomes filled with sediment, it appears dirty. Too much sediment blocks rivers and streams from flowing. This can kill water-dwelling animals and destroy habitats.

Floodwater can carry pollution too. These pollutants can include pesticides, garbage, or sewage. Sometimes, polluted floodwater can run into the ocean. This can harm marine life.

Floods can also lead to disease outbreaks. Some deadly diseases live in water. These include hepatitis A and cholera. Floodwaters can carry these diseases and make people sick.

Floods are not always bad. Sometimes, seasonal floods are necessary to bring new life to ecosystems.

Some Floods Are Helpful

Floods carry important nutrients to the surrounding land. When the floodwaters dry up, they leave sediment behind. This sediment is a natural fertilizer. It improves the quality of the dirt and encourages plants to grow. Egypt's Nile River floods every year. The ancient Egyptians built their first cities along the Nile. The river's floods made the nearby soil perfect for growing food.

Floods can refill pools of freshwater underground. Floodwater sinks into the ground. Then, it drips through layers of dirt and rock. After a while, the water reaches underground bodies of water called aquifers. These aquifers supply clean freshwater to springs, lakes, and rivers. In dry seasons, this groundwater may be the only supply of freshwater to a region.

Helping Fish, Boosting Water Supplies

Some animals use seasonal floods as a sign. The floods may signal that it is time to mate or migrate. When an animal migrates, it moves from one habitat to another. For example, in 2016, a flood triggered thousands of birds to migrate from their wetland habitat to the Macquarie Marshes in Australia.

Small seasonal floods can be good for fish. During floods, sediment is left behind on riverbeds. Sediment can provide a place for baby fish to grow. The floodwater also carries nutrients that small animals eat.

Floods can help refill freshwater supplies. They can keep lakes from drying up. Floods also help marshes and swamps. Many animals depend on marshes and swamps. In dry seasons, they might dry up. But floods help refill the wetlands and keep the ecosystem going.

Floods are a force of nature. They can both help and harm the ecosystems they affect. Floods can be destructive to humans and the environment. But they are also necessary to keep some ecosystems alive.