A river is more than just excess rainwater: the Earth actively destroys its own land surface, and the river is a system for transporting the debris of erosion, or sediment. Sediment load constitutes a major portion of most rivers; otherwise, you could always see to the bottom.
A river's ability to carry sediment (whether suspended in the water or rolling along the bottom) depends on a variety of factors, but the most important one is the speed of the water. When the river reaches a place where its speed drops significantly, such as its mouth, it also loses some of its ability to carry sediment. This sediment drops out of suspension (flocculates) or ceases to roll.
A pile of sediment often builds up at the river's mouth, or at the bottom of the mountain range the river flows out of. The combined effects of continued piling and sea level rise often cause the pile to have a triangular shape when viewed from above, with the apex at the point where the river enters it. Often, the river breaks up into smaller sub-channels (distributaries).
The ancient Greek historian Herodotus noticed the triangular shape of the pile at the mouth of the Nile and named it delta after the Greek letter it resembled. The term has since come to mean all such piles at the mouths of rivers, triangular or not.
There are four main types of deltas, in two broad categories:
- Constructive deltas
dump more sediment into a body of water than it can carry away. This causes distributaries to become blocked by sediment, leading to their constant rearrangement.
- Bird's-Foot delta
The delta of a river (such as the Mississippi River's principal channel) which empties into a body of water carrying mostly fine sediment. This sediment is deposited in such a way as to produce natural levees along the banks of the principal distributaries. The levees and channels tend to extend themselves into the sea, producing long protrusions of land with a distributary running down the middle. As the levees elongate, gravitational potential along the levees further up the river increases until some event causes a levee to break somewhere. In nature, this usually causes a new distributary to form, and sometimes causes the old downstream portion to be abandoned. The overall effect is a branching system of fingers of land resembling a bird's foot.
- Lobate delta
The delta of a river (such as the Nile) whose sediment contains a lot of coarse material. The river changes course frequently within the delta, causing it to build up more or less evenly, like a slice of pie. Mini-deltas often form at the mouths of individual distributaries.
- Destructive deltas
fight a losing battle against the sea. The distributaries don't get blocked as much and are stabler than those of constructive deltas.
- wave-dominated delta
The delta of a river that empties into a body of water with a longhsore current stronger than the river itself. For a very small river, the "delta" is little more than a place where the beach is a little wider, with the river running through the wide part. However, sediment buildup has to be enough to counteract any rise in sea level; otherwise the mouth of the river floods and becomes an estuary.
- tidal delta, usually called a mangrove delta
The delta of a river (such as the Niger or Ganges) whose erosion is primarily influenced by tidal forceds. Tidal flow is concentrated in the distributaries, which are scoured, deepened, and kept straighter and more parallel than the distributaries of other deltas. Often found in tropical areas, the regions between the distributaries of a tidal delta are often filled in with mangrove forests, which stabilize the distributaries further. In fact, the stabilizing effect of the mangrove maintains the dominance of tidal forces over waves.
A river's floodplain is formed form the same sediment as its delta, and can be considered an extension of the delta. In the classic idealized model of river evolution, a "youthful" stream carves a deep V-shaped valley as it grades itself. Follwing the logical implications of this scenario, the floodplain has its genesis in the delta as sediment backs up at its apex, and slowly fills up the youthful stream's valley moving continuously upstream. The situation is rarely this idealized but is a useful way to think of floodplains.
- desert deltas
An alluvial fan is indistinguishable from a lobate delta, except that it discharges from a mountain valley into a desert rather than a body of water. The deltas around the Mediterranean Sea were alluvial fans when that sea dried up, and the difference is is nominal.
- fossil deltas
During the last Ice Age, increased rainfall created pluvial lakes such as Lake Bonneville; in addition, water backing up behind ice dams formed glacial meltwater lakes. When these lakes dried up or emptied, deltas were often left behind. From time to time, piles of sediment are often found nowhere near a body of water. But if fossil beaches are nearby, the odds are that the pile was the delta of a river that emptied into a body of water there. The Moorhead River of Northwestern Minnesota leaves such a fossil delta where it used to enter Lake Agassiz.
Deltas can be left behind by tectonic forces or the simple march of years, and turned into sedimentary rock, usually sandstone or conglomerate.
Notable river deltas
- Nile River delta
The Nile Delta is of incalculable value to history. It was the anchor of Lower Egypt and the site of its capital Memphis. Later, empires would fight each other to control the breadbasket of the Western world. The Nile delta fed Snephru, Ramses II, Euclid, Pheidias, Cleopatra, Julius Caesar, Constantine, Saladin and Suleyman the Magnificent. Today, the largest city in hte Muslim world, Cairo, sits at its apex.
- Amazon River delta
Depending on how you look at it, the Amazon either has a mangrove delta with really big distributaries, or no delta at all. Sediment has formed some large islands in the mouth of the river in northeastern Brazil, but even the mightiest river in the world can't fight the tides. Twice a day, tides cause a large waved called a bore to rush upstream. But between high tides, the miles-wide channels contain fresh water. One of the islands, Ilha do Marajo, is the size of Switzerland is not part of the Amazon delta, but is a sort of divider between the mouth of the Amazon and the mouth of the Tocatins which discharges independently into the Atlantic.
- Mississippi River delta
More than just the little bird's foot southeast of New Orleans, the Mississippi Delta is a complex of seven or eight deltas that formed as the river shifted its channel over the last 6,000 years, eventually building all of Southern Louisiana (To confuse matters, see the little sub-article at the end of this writeup). The Mississippi Delta has elongated itself to the extent that a great gravitational potential exists. In other words, the river really wants to take a shorter path to the sea. The Mississippi's largest distributary is the Atchafalaya, and it's all the US Army Corps of Engineers can do to keep it from doing just that.
- The North China Plain
Not every river delta has a mountain range in the middle. This is actually an immense combined delta of the Huang He, Huai He, and Yangtze (Chang Kiang) Rivers. A delta the size of California. The Huang He flows into the Yellow Sea north of the Shandong peninsula, but 1,000 years ago it emptied into the East China Sea south of Shandong. This former channel, the Fei-Huang He, is an important river in its own right. The natural levees of have built themselves up (with human help) so that the bottom of the river is above the surface of the plain. This has led to cataclysmic floods in northern China, taking hundreds of thousands of lives.
- Okavango River delta
Botswana's Okavango River empties into....well, nothing. The river peters out in the Kalahari Desert. But Southern Africa's wet and dry seasons have caused the river to form a pile of sediment that is exactly like a bird's-foot delta. The Okavango Delta is like other deltas in that wetlands form between the distributaries, providing habitat for plants and animals that could not survive in the desert. Stages in the evolution of such a delta can be found in Kazakstan and Uzbekistan, where the Amu Darya and Syr Darya enter the Aral Sea, and in Kyrgyzstan where the Ili River delta has filled up most of Lake Balkhash.
The original "Delta" writeup
A region of the Mississippi River floodplain in Mississippi, Louisiana, and Arkansas is named the "delta", despite the fact that the real delta starts further south. It is technically a misnomer, but as the "Delta" was formed in the same way as a delta, it's hard to quibble.
The Delta is around 200 miles (320 km) wide in a stretch from Memphis to Natchez. Not even the mighty Mississippi could have carved out such a vast valley; it had to have help.
Help, in this case, came from plate tectonics. As Pangaea tore itself apart 250 million years ago, the rift valley tearing the North American Plate from the rest of Pangaea developed a branch rift. Into this abandoned rift, the Mississippi has poured its waters, as well as its sediment. Over millions of years the rivers has filled the rift with sediment, producing the wide floodplain. The rift has been alternately been drowned by the Gulf of Mexico, making the Delta a true delta, and refilled by the Mississippi's sediment. Although the valley isn't spreading, the rift faults are still active. One of these, the New Madrid fault, produced the strongest known (1812) earthquake in the history of North America.
The Mississippi's natural levees and former channels make the delta dominated by north-south running "yazoo" rivers. A geological remnant called Crowley's Ridge sticks up throug the Delta, running for 198 miles north to south through eastern Arkansas.
The Delta has figured often in the culture and history of the United States. The fertile soil made it the largest cotton-growing area prior to the Civil War. The most crucial victory of the Civil War was fought at Vicksburg, smack in the middle of the delta. After the Civil War, the need for cheap agricultural labor made it a bastion of segregation against freed slaves. The swampy ground restricted industry and urbanization.
All of these elements of agricultural inertia contributed to the despair inculcated in the black people who were stuck there. From this anguish, however, a uniquely American form of music arose: the blues. The Delta holds the legendary Crossroads where Robert Johnson allegedly sold his soul to the Devil.