What Is an Alluvial Plain? - Definition and Characteristics

We can provide a basic definition of an alluvial plain by saying it is a large area formed by the accumulation of sediment deposited by a river. These sediments are known as alluvium and the fact that it is a flat landform means it is a plain. The sediments are carried by the river's current, but are then deposited in areas along the banks. They build up the most in the lower parts of the watercourse because this is where the current slows down the most.

Alluvial plains usually have a topographically uniform surface, meaning they are flat landforms. Since they are created by rivers, they only exist in the areas around them. They can be totally or partially covered by water when these rivers flood.

While some people think they are the same thing, there is a difference between an alluvial plain and a floodplain. Floodplains are the low-lying areas next to rivers which are prone to flooding. They are also created by sediment, but this sediment is left there by flooding. Alluvial plains are larger areas created by sediment deposition over a long time. For this reason, there may be floodplains within an alluvial plain, but not necessarily.

Learn more about the landforms created around rivers with our article on what is a river delta and how does it form?

While we have already provided an alluvial plain definition, we need to look at its characteristics to have a better idea of how they act as an ecosystem. The following are some defining characteristics of an alluvial plain:

• They are dynamic and unstable environments: their instability is due to changes in sedimentation patterns, fluvial processes, climatic events and human activity. These factors contribute to the variability of its topography, the renewal of nutrients in the soil and the formation of a variety of habitats that favor rich biodiversity. These also make parts of the plains susceptible to flooding.
  
  
• They are very fertile areas: one of the most outstanding characteristics of the alluvial plains is the high fertility of their soils. The deposited sediments are full of nutrients carried from different parts of the river basin, making these areas favorable sites for agriculture and the settlement of human populations.
  
  
• They provide various benefits to ecosystems: alluvial plains provide numerous benefits to ecosystems. They act as natural filters to remove pollutants from the environment, they retain water to prevent flooding, they capture and store carbon contributing to the mitigation of climate change, and they form diverse habitats for a wide variety of species.
  
  
• They function as settlements for human populations: since ancient times, humans have settled in alluvial plains. This is due to close access to water sources, the fertility of their soils, river transportation, energy development and, eventually, as sites for urbanization. These changes in land use have caused a potential risk for these societies that are vulnerable to possible floods.
  
  
• They are degraded environments: alluvial plains are currently being degraded by various anthropogenic pressures such as the advance of the agricultural frontier, urban development, pollution, the introduction of exotic species and the construction of dams, dikes and other infrastructure which alter the watercourse of rivers. These threats underscore the need to implement different effective conservation and management strategies that balance human needs with floodplain preservation.

The bridge in the photo below shows one type of infrastructure which is important in alluvial plains. Bridges are particularly important for when the river is high after heavy rainfall. Learn more about heavy rainfall with our article on what is a torrential downpour?

We have already explained that alluvial plains have great biodiversity, aided in part by an abundance of water resources. These rich alluvial ecosystems can support a great range of plants and animals. These ecosystems are crucial to the survival of many species, especially in providing food and shelter.

Flora of alluvial plains

The vegetation of alluvial plains varies significantly depending on geography, climate and the frequency of flooding. Whether they are herbaceous or woody, these plants are adapted to surviving in moist or waterlogged soils. They are able to tolerate prolonged periods of flooding. In calmer or more stagnant areas, many species of floating or submerged aquatic plants proliferate.

Vegetation in alluvial plains help to prevent flooding by absorbing and slowing the flow of water. They also purify the water and prevent soil erosion, often retaining the substrate in their roots.

Fauna of alluvial plains

The fauna of the alluvial plains is equally diverse, mainly including:

• Fish: rivers and surrounding alluvial plains are habitats for numerous species of fish, many of which depend on floods to reproduce and spawn.
  
  
• Amphibians: are attracted to temporary and permanent pools for laying eggs and developing their larvae (tadpoles). Seasonal flooding creates perfect conditions for its life cycle.
  
  
• Reptiles: they can be found in alluvial plains which provide both spaces for hunting and safe areas for thermoregulation and reproduction. See a river snake pictured in the photo below.
  
  
• Birds: waterfowl, wading birds and other bird species use alluvial plains as food and nesting sites. They also function as important points for migratory birds.
  
  
• Mammals: alluvial plain mammals range from large herbivores to various species of rodents and carnivores. They play crucial roles in these ecosystems as top predators regulating populations of other species, seed dispersers and pollinators, among other functions.

Learn more about the different types of pollinators in our related article.

Some of the most famous alluvial plains are those which are connected to the world's biggest rivers. They include some famous river deltas, a type of alluvial plain which occurs at the mouth of a river. Below are examples of alluvial plains across the world:

• Alluvial plain of the Middle Paraná, Argentina
• Amazon alluvial plain, South America
• Mississippi Delta, United States
• Yangtze River Delta, China
• Ganges River alluvial plain, India and Bangladesh
• Murray-Darling River alluvial plain, Australia
• Indus River alluvial plain, Pakistan
• Danube River alluvial plain, Central Europe
• Po River alluvial plain, Italy
• Ebro River alluvial plain, Spain

Now that you know what an alluvial plain is and its characteristics are, you may want to learn more about riparian zones with our article on the difference between a delta and an estuary.

If you want to read similar articles to What Is an Alluvial Plain? - Definition and Characteristics, we recommend you visit our Ecosystems category.