In some cases, a river flowing into a flat arid area splits into channels that evaporate as it progresses into the desert. The Okavango Delta in Botswana is one example.[47] See endorheic basin. The concept of a delta is simple, but the geometry of large deltas in the real world is rather complicated and highly varied, owing to a variety of factors. More on that Ulcer index indicator later; first, some material on the hydrodynamics of deltas. These examples are programmatically compiled from various online sources to illustrate current usage of the word ‘delta.’ Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Any interactives on this page can only be played while you are visiting our website.
- Ancient deltas are a benefit to the economy due to their well sorted sand and gravel.
- Now that you have a good mental picture of the hydrodynamics of the delta environment, think about sediment deposition in the delta environment.
- The margin of the delta forms an arc, because sediment builds not just forward but also with a lateral component.
- As you can imagine, the mixing at the margin of the jet tends to even out the properties of both the jet and the ambient fluid—which might be temperature, salinity, or concentration of suspended solids.
- At this point we need to address the various complicating factors that set in when the delta-building river is larger and the water body into which the delta is building is not as placid as was assumed in the preceding section.
Sand and gravel is often quarried from these old deltas and used in concrete for highways, buildings, sidewalks, and even landscaping. More than 1 billion tons of sand and gravel are produced in the United States alone.[64] Not all sand and gravel quarries are former deltas, but for ones that are, much of the sorting is already done by the power of water. At this point we need to address the various complicating factors that set in when the delta-building river is larger and the water body into which the delta is building is not as placid as was assumed in the preceding section.
Anthropogenic activities can also destabilise river channels through sand mining,[59] and cause saltwater intrusion.[60] There are small-scale efforts to correct these issues, improve delta environments and increase environmental sustainability through sedimentation enhancing strategies. Finally, Figure 8-39 shows a plan view of the delta in Figure 8-38 after the delta has built out into the water body for an appreciable distance. The margin of the delta forms an arc, because sediment builds not just forward but also with a lateral component. Basically because the tendency for deposition along the axis of the flow leads to a slight axial ridge, and then the flow tends to flow laterally off that ridge, down the slight sideways slopes of the delta body. At first, of course, it just makes a pile of sediment at base of the water body below the jet entrance. From then on, as the sediment comes to rest as the jet emerges into the water body, a wedge of coarse sediment builds forward into the water body, as sediment is deposited at the brink of a sediment body and slides down an angle-of-repose slope.
Meaning of delta in English
Deltas form as rivers empty their water and sediment into another body of water, such as an ocean, lake, or another river. Other rivers, particularly those on coasts with significant tidal range, do not form a delta but enter into the sea in the form of an estuary. Notable examples include the Gulf of Saint Lawrence and the Tagus estuary. Now we have to contend with the various effects that the water body itself has on the growing delta. We need to be concerned particularly with the action of waves, tides, and currents.
A more important reason, applicable to large mixed-load rivers, is that there is a strong tendency for upbuilding of the flow-carrying channel, deposition of natural levees, and occasional avulsion to relocate the channel to slightly lower areas on the aggrading delta plain. Typically there are two or more active channels, carrying water and sediment, operating on the delta plain at the same time. All the time, these shift around, avulse, and become abandoned, and new ones form.
Figure 8-38 shows three stages in the process, and Figure 8-39 shows a plan view. Now that you have a good mental picture of the hydrodynamics of the delta environment, think about sediment deposition in the delta environment. Think in terms of the jet shown in Figure 8-36 or Figure 8-37, with the reservation that the depth of water in the water body is not infinitely deep—perhaps only several times the depth of flow in the approach channel (that is, the stream or river).
British Dictionary definitions for delta (1 of
This results in the smoothing of the planform (or map-view) shape of the delta as the channels move across its surface and deposit sediment. Because the sediment is laid down in this fashion, the shape of these deltas approximates a fan. The more often the flow changes course, the shape develops as closer to an ideal fan, because more rapid changes in channel position result in more uniform deposition of sediment on the delta front.
Examples of delta
The jet described in the preceding paragraph is a good model of a river entering a fresh-water lake or reservoir. But, as we all know, the ocean is salty, and, because it’s salty its density is significantly greater, by an important few percent, than that of fresh water. That impedes vertical mixing of the jet with its surroundings, while not affecting its horizontal mixing greatly. The reason is that the density stratification that develops between the overlying fresh-water jet and the underlying salt-water medium is gravitationally stable, and it takes work to disrupt or break down that stratification by mixing. The jet may be able to do some vertical mixing, but the extent is much reduced. The jet ends up being largely in the form of a kind of horizontally oriented fan, spreading laterally but not downward (Figure 8-37).
The delta described in the preceding section, and shown in Figures 8-38 and 8-39, is representative of the way deltas develop when a relatively small stream or river enters a water body in which currents and waves are minor. Such deltas are called Gilbert deltas, in honor of an early geomorphologist, G.K. Gilbert, who made the first systematic studies of such small deltas in the western U.S. in the late 1800s. In addition to interference with delta aggradation, pumping of groundwater,[57] oil, and gas,[58] and constructing infrastructure all accelerate subsidence, increasing relative sea level rise.
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The Selenga River delta in the Russian republic of Buryatia is the largest delta emptying into a body of fresh water, in its case Lake Baikal. As you can imagine, the mixing at the margin of the jet tends to even out the properties of both the jet and the ambient fluid—which might be temperature, salinity, or concentration of suspended solids. Note in Figure 8-35 that there is a residual core in the jet, which has not yet been affected by the marginal mixing. Delta BluesDelta blues is a style of music developed by African American artists living and performing in the Mississippi Delta region of the southern United States. Robert Johnson, widely recognized as one of the greatest guitarists of all time, played the Delta blues.
In that way, over time all of the delta plain is built up uniformly but with very complex internal structure in detail. The individual flow-carrying channels are called distributary channels. The next time you look at a map of Louisiana, note how the most recent active area of the Mississippi delta is building out into the Gulf of Mexico in the form of a hand-like body with several active distributaries (locally called “passes”). It is so named because its triangle shape resembles the uppercase Greek letter delta, Δ. A delta is a body of sediment deposited at a point along a body of water where a sediment-transporting channelized flow of water enters the water body. Deltas range in size from those little decimeter-scale bodies that you can see forming when a rivulet of rainwater enters a puddle during and right after a rain, to giant bodies at the mouths of major rivers like the Mississippi.
The Mississippi and Ural River deltas, with their bird’s-feet, are examples of rivers that do not avulse often enough to form a symmetrical fan shape. Alluvial fan deltas, as seen by their name, avulse frequently and more closely approximate an ideal fan shape. In fluid dynamics, a jet is a fluid motion created where a high-speed flow in a pipe or other conduit enters a large body of relatively still fluid.
Each of these factors is able to mold the morphology of the delta, in complex ways. Urban areas and human habitation tends to locate in lowlands near water access for transportation and sanitation.[65] This makes deltas a common location for civilizations to flourish due to access to flat land for farming, freshwater for sanitation and irrigation, and sea access for trade. Deltas often host extensive industrial and commercial activities as well as agricultural land which are often in conflict. Some of the world’s largest regional economies are located on deltas such as the Pearl River Delta, Yangtze River Delta, European Low Countries and the Greater Tokyo Area. Ancient deltas are a benefit to the economy due to their well sorted sand and gravel.
The reason deltas form—and I suppose that this is obvious—is that as the channel flow enters the water body it spreads out and thereby decelerates, dropping much of its sediment load at or not far from its mouth. As the gradient of the river channel decreases, the amount of shear stress on the bed decreases, which results in the deposition of sediment within the channel and a rise in the channel bed relative to the floodplain. If the river breaches its natural levees (such as https://www.forexbox.info/bitcoin-calculator/ during a flood), it spills out into a new course with a shorter route to the ocean, thereby obtaining a steeper, more stable gradient.[13] Typically, when the river switches channels in this manner, some of its flow remains in the abandoned channel. Repeated channel-switching events build up a mature delta with a distributary network. In both of these cases, depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition.