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.
Formation of River DeltasThe 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 River DeltasAlthough the aforementioned processes are generally way in which river deltas form and are organized, it is important to note that world's deltas are highly varied "in size, structure, composition and origin" due to factors such as climate, geology and tidal processes (Encyclopedia Britannica).
As a result of these external factors there are several different types of deltas all over the world. The type of delta is classified based on what controls a river's deposition of sediment. This can usually be the river itself, waves or tides. The main types of deltas are wave-dominated deltas, tide-dominated deltas, Gilbert deltas, inland deltas and estuaries. A wave-dominated delta is one where wave erosion controls where and how much sediment remains in the delta after a river drops it. These deltas are usually shaped like the Greek symbol, delta (∆). An example of a wave-dominated delta is the Mississippi River delta. A tide-dominated delta is one that forms based on the tide and it has a dendritic structure (branched, like a tree) due to newly-formed distributaries during times of high water (Wikipedia.org). The Ganges River delta is an example of a tide-dominated delta.
A Gilbert delta is a steeper type of delta that is formed by deposition coarse material. Gilbert deltas can form in ocean areas but it is more common to see them in mountainous areas where a mountain river deposits sediment into a lake. Inland deltas are deltas that form in inland areas or valleys where a river will divide into many branches and rejoin farther downstream. Inland deltas, also called inverted river deltas, normally form on former lake beds.
Finally, when a river is located near coasts that have large tidal variation they do not always form a traditional delta. They instead form estuaries, or a river that meets the sea. The Saint Lawrence River in Ontario, Quebec and New York is an estuary.
Humans and River DeltasRiver deltas have been important to humans for thousands of years because of their extremely fertile soils. Major ancient civilizations grew along deltas such as those of the Nile and the Tigris-Euphrates rivers and the people living in them learned how to live with the natural flooding cycles of deltas. Many people believe that the ancient Greek historian Herodotus first coined the term delta nearly 2,500 years ago as many deltas are shaped like the Greek delta (∆) symbol (Encyclopedia Britannica).
Today deltas remain important to humans because they are a source of sand and gravel. In many deltas this material is highly valuable and is used in the construction of highways, buildings and other infrastructure. In other areas delta land is important in agricultural use. For example the Sacramento-San Joaquin Delta in California is one of the most agriculturally productive areas in the state.
Biodiversity and Importance of River DeltasIn 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.