When rivers do not flow completely into the ocean and instead form estuaries, these wetlands are considered estuarine deltas. While some authors describe both lake and marine sites in the Gilbert 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 by the river, stays standing longer). [28] Gilbert himself first described this type of delta at Lake Bonneville in 1885. [28] Similar structures are found elsewhere, such as at the mouths of several streams that flow into Okanagan Lake in British Columbia and form major peninsulas at Naramata, Summerland and Peachland. River-dominated deltas are found in areas with low tidal amplitude and low wave energy. [21] Where river water is almost as dense as basin water, the delta is characterized by a homopycnal current, in which river water mixes rapidly with pool water and abruptly releases most of its sediment load. Where river water has a higher density than basin water, usually of a heavy sedimentary load, the delta is characterized by a hyperpycnal current in which river water embraces the basin floor as a density flow that deposits its sediments in the form of turbidites. If the river water is less dense than the pool water, as is typical for river deltas on a sea coast, the delta is characterized by a hypopycnal current in which the river water slowly mixes with the denser pool water and spreads like surface fans. This allows fine sediments to be transported a considerable distance before they settle out of the suspension. Beds in a hypocynical delta dive at a very low angle, about 1 degree. [21] Tides also limit where deltas can form. The Amazon, the largest river in the world, is deltaless. The tides of the Atlantic Ocean are too strong for the mud to form a delta in the Amazon.
The Ganges Delta in India and Bangladesh is the largest delta in the world and one of the most fertile regions in the world. Deltas are also important habitats for wetlands. Plants such as lilies and hibiscus grow in deltas, as do herbs such as seasonings used in traditional medicine. Most major river deltas flow into intracratonic basins on the back edges of passive margins, as most major rivers such as the Mississippi, Nile, Amazon, Ganges, Indus, Yangtze and Yellow rivers flow along passive continental margins. [15] This phenomenon is mainly due to three factors: topography, watershed and basin height. [15] Topography along passive margins tends to be more gradual and spread over a larger area, allowing sediments to accumulate and accumulate over time to form large river deltas. The topography along the active margins tends to be steeper and less extensive, preventing sediments from accumulating and accumulating because sediments migrate in a steep subduction trench rather than a shallow continental shelf. River deltas do not cover much of the actual land area. However, they tend to house many resources that humans and animals depend on.
Some areas of the river delta are home to millions of people. River deltas provide food, ports and transportation to many countries. Estuary deltas are formed because a river does not flow directly into the ocean, but forms an estuary. An estuary is a partially enclosed wetland with brackish water habitat (partly saltwater, partly freshwater). The Yellow River, for example, forms an estuary when it empties into the Bohai Sea off the coast of northern China. The Mississippi River alone is now experiencing alarming erosion. The loss of Mississippi land is simply happening much faster than before the industrial era, and any rising oceans will have a profound impact on the Mississippi Delta. Degradation of the Mississippi Delta results from river dredging for canals, reduced sediment deposition, saltwater flooding, and wave erosion in the Gulf of Mexico. The delta has also been modified by the construction of dams and along its course to prevent community flooding.
Engineers proposed creating new lands in the delta over the next few decades by building river diversions. Extensive human activities in deltas also disrupt the geomorphological and ecological processes of deltas. [55] People living in deltas often build flood defenses that prevent sedimentation from flooding the deltas, meaning sedimentary deposits cannot compensate for subsidence and erosion. In addition to delta aggravation interference, groundwater pumping,[56] oil and gas,[57] and infrastructure construction accelerate subsidence and increase relative sea level rise. Anthropogenic activities can also destabilize river channels through sand extraction[58] and cause saltwater intrusion. [59] There is less effort to address these issues, improve the environment in the delta, and increase environmental sustainability through strategies to improve sedimentation. The world`s largest river delta is located at the mouth of the Ganges River, which spans the borders of India and Bangladesh on the Bay of Bengal. The Ganges-Brahmaputra-Meghna Delta is a tidal dominated delta that stretches for 220 miles and also includes several other rivers. These deltas form when a river flows into an estuary instead of the ocean.
An estuary is a type of wetland that is usually found near a river that meets the ocean. It consists partly of salt water and partly of fresh water (brackish water) and has a unique ecosystem. Estuary deltas are generally long and narrow. Examples of estuary deltas are the Hudson Delta and the Seine Delta. In both cases, deposition processes force a redistribution of deposition from areas with high deposition density to areas with low deposition. This results in a smoothing of the shape of the delta plane (or map view) as the channels move on its surface and deposit sediment. Since sediments are deposited in this way, the shape of these deltas resembles a fan. The more trajectory the flow changes, the closer the shape gets to an ideal fan, as faster changes in channel position result in more uniform sediment deposition on the delta front. The Mississippi and Urals deltas with their bird`s feet are examples of rivers that do not avulate often enough to form a symmetrical fan shape.
Alluvial deltas, as the name suggests, often vulze and approach an ideal fan shape. There are four main types of deltas classified according to the processes that control mud accumulation: wave-dominated, tidal dominated, Gilbert Delta, and Estuary Delta.
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