Deltas have been important in the history of mankind. Historians believe that it was in a fertile delta in Mesopotamia that people learned to use the power of water for irrigation in agriculture. This makes sense because these are very fertile, green and lush areas. The region of origin of Mesopotamia is known as the Fertile Crescent because it was so fertile. There is evidence of human agricultural activity dating back to 9000 BC. In this region, which includes parts of Iraq, Syria, Lebanon, Palestine, Jordan, Israel, Egypt, Kuwait, Turkey and Iran. The slowing of the river`s velocity and the accumulation of sediment allow the river to exit its only channel as it approaches its mouth. Under good conditions, a river forms a delta lobe. A mature delta lobe includes a distributive network – a series of smaller, flatter channels, called distributors, that branch off from the river`s mainstream. In rare cases, the river delta is located in a large valley and is called the inverted river delta. Sometimes a river divides into several branches in an inland area, only to flow into the sea and continue towards the sea. Such an area is called an inner delta and is often found on ancient seabed. The term was first coined by Alexander von Humboldt to refer to the middle course of the Orinoco, which he visited in 1800.
[40] Other important examples include the Inner Niger Delta,[41] the Athabasca Peace Delta,[42] the Sacramento-San Joaquin River Delta,[43] and the Sistan Delta in Iran. [44] The Danube has one in the valley on the Slovak-Hungarian border between Bratislava and Iža. [45] A tidal freshwater delta[32] is a sedimentary deposit that forms at the boundary between an upland stream and an estuary in the area known as the “lower mouth.” [33] Drowned coastal river valleys flooded by sea level rise in the late Pleistocene and later Holocene tend to have dendritic estuaries with many tributaries. Each tributary mimics this gradient of salinity from its brackish connection with the main mouth of the trunk to the cool stream that feeds the head of the tide. As a result, tributaries are considered “sub-flows”. The origin and evolution of a tidal freshwater delta include processes typical of all deltas[4] as well as processes unique to the tidal freshwater environment. [34] [35] The combination of processes that create a tidal freshwater delta results in a distinct morphology and unique environmental characteristics. Many of the tidal freshwater deltas that exist today are directly caused by the beginning or change in historical land use, especially deforestation, intensive agriculture and urbanization.
[36] These ideas are well illustrated by the many freshwater deltas that descend into the Chesapeake Bay along the east coast of the United States. Research has shown that the sediments that accumulate in this estuary come from post-European colonization deforestation, agriculture and urban development. [37] [38] [39] Extensive river management threatens deltas. River management involves monitoring and managing the flow of a river (often through dams). River management increases the amount of land available for agricultural or industrial development and controls access to drinking water, industry and irrigation. Another reason why these areas are important is the fact that they help supply the world with sand and gravel. Older deltas are often converted into sand and gravel quarries for use in construction, paving and landscaping. These areas are powerful for this purpose, as deltas naturally sort sand and gravel sediments into different areas, so much of the work is already done! Erosion is also an important control in tidal-dominated deltas, such as the Ganges Delta, which can be mostly underwater, with distinctive sandbanks and ridges.
This tends to create a “dendritic” structure. [31] Tidal deltas behave differently from deltas dominated by rivers and waves, which tend to have some major distributors. As soon as a distribution channel dominated by waves or flows silts up, it is abandoned and a new channel is formed elsewhere. In a tidal delta, new distributors form at a time when there is a lot of water nearby, such as floods or storm surges. These dispensers slowly silt up at a more or less constant rate until they turn off. [31] Another delta of non-traditional appearance is the inverted delta. The distribution network of an inverted delta is located inland, while a single stream reaches the ocean or other body of water. The sacramento-San Joaquin River Delta in northern California is an inverted delta.
The rivers and streams of the Sacramento and San Joaquin distribution systems meet in Suisun Bay before flowing into the Pacific Ocean through a single breach in the Coast Range, the Strait of Carquinez. River deltas are important in human civilization because they are important centers of agricultural production and population. [5] They can provide coastal defenses and affect the supply of drinking water. [6] They are also ecologically important, with assemblages of different species depending on their landscape location. Tidal dominated deltas usually form in areas with a large tidal range or an area between high and low tide. The huge Ganges-Brahmaputra Delta in India and Bangladesh is a tidal dominated delta formed by rising and falling tides in the Bay of Bengal. River management in Egypt, for example, has radically changed the way land is managed around the Nile Delta. The construction of the Aswan Dam in the 1960s reduced annual flooding of the delta. These floods had spread silt and nutrients along the banks of the Nile. Today, Egypt is much more dependent on fertilizers and irrigation. The Nile Delta is also shrinking due to the Aswan Dam and other river management techniques.
Without silt and other sediments that enrich it with prodelta, the waves of the Mediterranean erode the delta faster than the Nile can replace it. The booming city of Vancouver, British Columbia, Canada, is located on the Fraser River Delta, which empties into the Strait of Georgia, part of the Pacific Ocean. The Fraser Delta contributes to making Vancouver one of the busiest and most cosmopolitan ports in the world, exporting goods from within Canada and importing goods from around the world. River-dominated deltas are found in areas where tidal range and low wave energy are low. [21] When the density of river water is almost equal to the basin water, the delta is characterized by a homopycnal current, in which river water quickly mixes with basin water and abruptly discharges most of its sedimentary load. Where river water has a higher density than basin water, usually from a high sediment load, the delta is characterized by a hyperpycnal flow in which river water embraces the bottom of the pool like a density stream that deposits its sediments in the form of turbidites. When the river water is less dense than the pool water, as is typical of river deltas on a sea coast, the delta is characterized by a hypopynic current, in which the river water mixes only slowly with the denser pool water and spreads in the form of surface fans. As a result, fine sediments can be transported over a considerable distance before settling out of the suspension.
Beds in a hypocynal delta plunge to a very shallow angle, about 1 degree. [21] While some authors describe both lacustririn and the marine sites of the Gilbertian deltas,[24] others note that their formation is more characteristic of freshwater lakes, where it is easier for river water to mix more quickly with lake water (as opposed to a river falling into the sea or a salt lake). where the less dense fresh water brought in from the river stays longer). [28] Gilbert himself first described this type of delta in 1885 at Lake Bonneville. [28] Similar structures are found elsewhere, for example at the mouths of several streams that flow into Okanagan Lake in British Columbia and form prominent peninsulas at Naramata, Summerland and Peachland.