Many of the world's rivers form deltas at their mouths.
The velocity of many rivers is greatly reduced as they approach the large body of water located at their mouths. Under appropriate conditions, this reduction in velocity may result in the depositions of the sediments suspended in the river at the river's mouth. These alluvial (river) deposits constitute a river delta and lie both above and below the water's surface. The shape and characteristics of the delta are significantly influenced not only by the river, but also by currents, waves and the topography of the larger body of water.
Characteristics
Deltas throughout the world share several characteristics. First, rivers with deltas have a large drainage basin where water drains into the river. Second, the rivers carry a large amount of sediment. Third, the coastlines are geologically stable, allowing the accumulation of alluvial (river) sediments. Deltas are classified as arcuate, bird-foot, cuspate and estuarine.
Arcuate Deltas
Arcuate (fan-shaped) deltas include the triangular delta at the mouth of the Nile River, from which the term delta is derived (the Greek letter delta is triangular). These deltas form multiple, shallow distributaries (small streams that branch from the river to the open water) from the river to the open water carrying the sediments with them. The open water at the delta has weak currents and the waves push sediments back onto the shore.
Bird-Foot Deltas
Bird-foot deltas develop when the open water at the mouth of the river carries the sediments over a shallow basin that drops off abruptly. Few distributaries form along bird-foot deltas, and the sediment deposits tend to form a long elongated spit of land appearing similar to a bird's toe. The Mississippi River forms a bird-foot delta as it flows into the Gulf of Mexico in Louisiana.
Cuspate Deltas
Cuspate (tooth-shaped) deltas form as the waves in the open water push the sediments back onto the sides of the river mouth producing a broad, smooth shoreline on either side of the river mouth that comes to a point at the river mouth, resembling a tooth. One distributary is typically found with cuspate deltas. The Tiber River in Italy creates a cuspate delta as it empties into the Mediterranean Sea.
Estuarine Deltas
Estuarine deltas are greatly influenced by the tidal nature of the estuary through which they flow. The sediments are largely deposited within the estuary before reaching the coastline. As these sediments accumulate, the estuary is elongated and narrows with much of the estuary completely filled. The Seine River, which flows through Paris, forms an estuarine delta at its mouth on the English Channel.
Other Major Deltas
A number of significant river deltas occur throughout the world, excluding the Poles, with perhaps the most famous being the Nile River's delta in Egypt. Besides those mentioned, other major river systems associated with deltas include the Ganges/Brahmaputra Rivers in Bangladesh and eastern India, the Niger River in western Africa, the Yangtze River in China and the Orinoco River delta in Venezuela.
Tags: open water, river mouth, alluvial river, back onto, body water, currents waves
