- A river delta is low-lying plain or landform that occurs at the mouth of a river near where the river flows into the ocean or other body of water. Deltas are important to both human activities and fish and other wildlife because they are normally home to very fertile soil as well as a large amount of vegetation.
- Prior to understanding deltas it is first important to understand rivers. Rivers are defined as fresh bodies of water that generally flow from high elevations toward the ocean, lake or another river. In some instances however, they do not make it to the ocean – they instead flow into the ground. Most rivers begin at high elevations where snow, rain and other precipitation run downhill into creeks and small streams. As these small waterways flow farther downhill they eventually meet and form rivers.
- In many cases these rivers then flow toward larger the ocean or another body of water and oftentimes they combine with other rivers. At the lowest part of the river is the delta. It is in these areas where the river’s flow slows and spreads out to create sediment-rich dry areas and biodiverse wetlands.
When a river reaches a lake or the sea the water slows down and loses the power to carry sediment . The sediment is dropped at the mouth of the river. Some rivers drop so much sediment that waves and tides can’t carry it all away. It builds up in layers forming a delta.
Some deltas are so large that people can live on them. The Nile delta is a very important farming area in Egypt.
Formation of River deltas
- The formation of a river delta is a slow process. As rivers flow toward their outlets from higher elevations they deposit particles of mud, silt, sand and gravel at their mouths because the flow of water slows as the river joins the larger body of water. Over time these particles (called sediment or alluvium) build up at the mouth and can extend into the ocean or lake. As these areas continue to grow the water becomes more and more shallow and eventually landforms begin to rise above the surface of the water. Most deltas are only elevated to just above sea level though.
- Once the rivers have dropped enough sediment to create these landforms or areas of raised elevation the remaining flowing water with the most power sometimes cuts across the land and forms different branches. These branches are called distributaries
- After the deltas have formed they are typically made up of three parts. These parts are the upper delta plain, the lower delta plain and the subaqueous delta. The upper delta plain is the area nearest to the land. It is usually the area with the least water and highest elevation. The subaqueous delta is the portion of the delta that is closest to the sea or body of water into which the river flows. This area is usually past the shoreline and it is below water level. The lower delta plain is the middle of the delta. It is a transition zone between the dry upper delta and the wet subaqueous delta.
Types of Deltas
- Deltas have different shapes depending on how much sediment is deposited by the river compared to how much sediment is eroded and redeposited by waves and tides.
- A river dominated delta, such as the map below of the Mississippi delta, Louisiana, USA, is sometimes called a bird’s foot delta because of its shape. Long, narrow mounds of sediment called levees build up along the sides of the narrow distributary channels.
- Tide dominated deltas have long and narrow offshore bars or islands at the mouth of the river, such as in Western Papua New Guinea, shown on the map below.
- Wave dominated deltas look like a flattened triangle. The lines running across the delta are ridges that formed as the delta built up, shown in Caravelas, Brazil, on the map below.
Biodiversity and importance of River Delta
- In addition to these human uses river deltas are some of the most biodiverse areas on the planet and as such it is essential that they remain healthy to provide habitat for the many species of plants, animals, insects and fish that live in them. There are many different species of rare, threatened and endangered species living in deltas and wetlands. Each winter, the Mississippi River delta is home to five million ducks and other waterfowl (America’s Wetland Foundation).
- In addition to their biodiversity, deltas and wetlands can provide a buffer for hurricanes. The Mississippi River delta, for example, can act as a barrier and reduces the impact of potentially strong hurricanes in the Gulf of Mexico as the presence of open land can weaken a storm before it hits a large, populated area such as New Orleans.