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Two-round system
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For other types of Run-off voting systems, see Run-off voting (disambiguation).
An example of runoff voting. Runoff voting involves two rounds of voting. Only two candidates survive to the second round.
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The two-round system (also known as the second ballot, runoff voting or ballotage) is a voting method used to elect a single winner, where the voter casts a single vote for their chosen candidate. However, if no candidate receives the required number of votes, then those candidates having less than a certain proportion of the votes, or all but the two candidates receiving the most votes, are eliminated, and a second round of voting is held.
The two-round system is used around the world for the election of legislative bodies and directly elected presidents. For example, it is used in French presidential, legislative, and departmental elections. In Italy, it is used to elect mayors, but also to decide which party or coalition receives a majority bonus in city councils.[1] A two-round system is used also to elect the presidents of Afghanistan, Argentina, Austria, Benin, Brazil, Bulgaria, Burkina Faso, Cape Verde, Chile, Colombia, Costa Rica, Croatia, Czech Republic, Cyprus, Dominican Republic, East Timor, Ecuador, Egypt, El Salvador, Finland, Ghana, Guatemala, Haiti, India, Indonesia, Kyrgyzstan, Liberia, Macedonia, Peru, Poland, Portugal, Romania, Russia, Senegal, Serbia, Slovakia, Slovenia, Turkey, Ukraine, Uruguay and Zimbabwe. Historically it was used to elect the Reichstag in the German Empire between 1871 and 1918, and in New Zealand in the 1908 and 1911 elections.
Contents
[hide]
1 Terminology
2 Voting and counting
3 Examples
3.1 Example I
3.2 Example II
3.3 French 2002 presidential election
4 Similar methods
4.1 Exhaustive ballot
4.2 Instant-runoff voting
4.3 Contingent vote
4.4 Nonpartisan blanket primary
5 Compliance with voting method criteria
6 Tactical voting and strategic nomination
7 Impact on factions and candidates
8 Majoritarianism
9 Practical implications
9.1 Costs
10 See also
11 Notes
12 External links
Terminology[edit]
The two-round system is known as run-off voting in the United States, where the second round is known as a run-off election. Run-off voting is also sometimes used as a generic term to describe any method involving a number of rounds of voting, with eliminations after each round. By this broader definition the two-round system is not the only form of run-off voting, and others include the exhaustive ballot and instant run-off voting (also known as the alternative vote). However the subject of this article is the two-round system.
In Canada, for example, when there are more than two candidates for political party leadership use an exhaustive ballot system (often called a run-off voting method) where one candidate must win a simple majority (over half). Candidates with the fewest votes or candidates who want to move their support to other candidates may also move to remove themselves from the next vote.
Voting and counting[edit]
In both rounds of an election conducted using runoff voting, the voter simply marks an "X" beside his/her favorite candidate. If no candidate has an absolute majority of votes (i.e. more than half) in the first round, then the two candidates with the most votes proceed to a second round, from which all others are excluded. In the second round, because there are only two candidates, and absent a tie vote, one candidate will achieve an absolute majority. In the second round, each voter is entirely free to change the candidate he or she votes for, even if his preferred candidate has not yet been eliminated but he has merely changed his mind.
Some variants of the two-round system use a different rule for choosing candidates for the second round, and allow more than two candidates to proceed to the second round. Under such methods, it is sufficient for a candidate to receive a plurality of votes (more votes than anyone else) to be elected in the second round. In Czech and Kenyan presidential elections, the candidates in the first and second places are permitted to stand in the second round, and all other candidates are eliminated in the first round, thus providing a contingency for a first- or second-place tie; a plurality is then sufficient to be elected. However, in Ghana, which also employs this contingency method, the majoirty requirement still holds in the second round, and a third round would be held if it isn't obtained, etc.. Under some variants of runoff voting, there is no formal rule for eliminating candidates, but candidates who receive few votes in the first round are expected to withdraw voluntarily.
The President of Weimar Germany was popularly elected in 1925 and 1932 by a two-round system; in the second round, a candidate could run, even if they had not done so in the first round, and they did not require an absolute majority to win. In both elections, the communist candidate, Ernst Thälmann, did not withdraw and ran in the second round. In 1925, that probably ensured the election of Paul von Hindenburg (with only 48.3% of the vote), rather than Wilhelm Marx, the centrist candidate. Hindenburg had not run in the first round of the 1925 election.
This method can be coupled with a vote par circonscriptions in which an area may provide multiple seats. For example, all French departements and territories, but the five less populated ones, have several deputies, like the 18 of Paris. French Polynesia, in which most of the population is on two islands, has three deputies, one from each of three constituencies. The constituencies, however, exist only for that election and are not administrative subdivisions.
Examples[edit]
Example I[edit]
Imagine an election to choose which food to eat for dessert. There are 25 people having dessert and four candidates: Ice cream, Apple pie, Fruit and Celery. Runoff voting is used to find the winner.
Round 1: In the first round of voting each diner votes for the one candidate they most prefer. The results are as follows:
Ice cream: 10 votes
Apple pie: 6 votes
Fruit: 8 votes
Celery: 1 vote
Round 2: No candidate has an absolute majority of votes (in this election that would be 13) so the two candidates with the most votes, Ice cream and Fruit, proceed to a second round, while Apple pie and Celery are eliminated. Because their favorite candidates have been eliminated Apple pie and Celery supporters must now vote for one of the two remaining candidates. The sole Celery supporter is health conscious, so now gives his vote to Fruit. However Apple pie supporters are split: three prefer Ice cream and three vote for Fruit. Of those who supported Ice cream and Fruit in the first round no-one decides to change their vote. The results of the second round are therefore:
Ice cream: 13
Fruit: 12
Result: Ice cream now has an absolute majority so is declared the winner.
Example II[edit]
Imagine that the population of Tennessee, a state in the United States, is voting on the location of its capital. The population of Tennessee is concentrated around its four major cities, which are spread throughout the state. For this example, suppose that the entire electorate lives in these four cities, and that they would all like the capital to be established as close to their own city as possible.
The candidates for the capital are:
Memphis, the state's largest city, with 42% of the voters, but located far from the other cities
Nashville, with 26% of the voters
Knoxville, with 17% of the voters
Chattanooga, with 15% of the voters
Round 1: In the first round of voting the results will be as follows:
Memphis: 42%
Nashville: 26%
Knoxville: 17%
Chattanooga: 15%
Round 2: No candidate has an absolute majority in the first round (this would be greater than 50%), so Memphis and Nashville proceed to the next round, while Knoxville and Chattanooga are excluded. Both eliminated cities are closer to Nashville than they are to Memphis. Therefore, all of those who vote for either of the eliminated cities chose to vote for Nashville in the second round. None of the Memphis or Nashville supporters change their votes. The results are therefore:
Nashville: 58%
Memphis: 42%
Result: After round two Nashville has an absolute majority and is the winner. Note that while Nashville received only 26% of the votes and was significantly behind Memphis in the first round, it won the election by winning the second round.
This example demonstrates how the first two candidates from the first round might not necessarily be those expected to be the most popular by virtue of their party affiliations.
French 2002 presidential election[edit]
Main article: French presidential election, 2002
Unlike the two examples above, this is from real life. In the 2002 French presidential election, the two contenders described by the media as possible winners were Jacques Chirac and Lionel Jospin, who represented the largest two political parties in France at the time. However, an exceptional number of 16 candidates were on the ballot, including even Jean-Pierre Chevènement (5,33 %) and Christiane Taubira (2,32 %) from the Plural Left coalition of Jospin, who refused by excess of confidence to dissuade them. Because the left vote was divided among a number of candidates as a consequence, a third contender, Jean-Marie Le Pen, unexpectedly obtained slightly more than Jospin in the first round of elections:
Jacques Chirac (Centre-right, Gaullist): 19.88%
Jean-Marie Le Pen (Far-right, National Front): 16.86%
Lionel Jospin (Centre-left, Socialist): 16.18%
Several other candidates received smaller percentages of the first round vote.
Because no candidate had obtained an absolute majority of the votes in the first round, top two candidates contested the second round. Most supporters of the parties which didn't get through to the second round (and Chirac's supporters) preferred voting for Chirac rather than Le Pen, and Chirac had a very large victory.
Jacques Chirac (Centre-right, Gaullist): 82.21%
Jean-Marie Le Pen (Far-right, National Front): 17.79%
Similar methods[edit]
Exhaustive ballot[edit]
Main article: Exhaustive ballot
The exhaustive ballot (EB) is similar to the two-round system, but involves more rounds of voting rather than just two. If no candidate receives an absolute majority in the first round then the candidate(s) with the fewest number of votes is eliminated and excluded from further ballots. The process of exclusion and reballot continues until one candidate has an absolute majority. Because voters may have to cast votes several times, EB is not used in large-scale public elections. Instead it is used in smaller contests such as the election of the presiding officer of an assembly; one long-standing example of its use is in the United Kingdom, where local associations (LCAs) of the Conservative Party use EB to elect their prospective parliamentary candidates (PPCs). Exhaustive ballot is also used by FIFA and the International Olympic Committee to select hosts. EB often elects a different winner from runoff voting. Because the two-round system excludes more than one candidate after the first round, it is possible for a candidate to be eliminated who would have gone on to win the election under EB.
Instant-runoff voting[edit]
Main article: Instant-runoff voting
Instant-runoff voting (IRV) (also known as Preferential voting or Alternative Vote (AV)) like the exhaustive ballot involves multiple reiterative counts in which the candidate with fewest votes is eliminated each time. Whilst the exhaustive ballot and the two-round system both involve voters casting a separate vote in each round, under instant-runoff voters vote only once. This is possible because, rather than voting for only a single candidate, the voter ranks all of the candidates in order of preference. These preferences are then used to "transfer" the votes of those whose first preference has been eliminated during the course of the count. Because the two-round system and the exhaustive ballot involve separate rounds of voting, voters can use the results of one round to decide how they will vote in the next, whereas this is not possible under IRV. Because it is necessary only to vote once, IRV, like the two-round system, is used for large-scale elections in many places. IRV often elects a different winner to the two-round system and tends to produce the same results as the exhaustive ballot.
Variants of Instant Runoff voting can be designed to reflect the same rules as a two-round voting system. If no single candidate has an absolute majority of votes then only the two highest polling candidates progress to the second count, while all other candidates are excluded and their votes redistributed according to the recorded preferences for continuing candidates. One variant that works this way is called the Contingent vote, detailed below.
In Australia it is called Preferential voting and is used to elect members of, among other institutions, the House of Representatives. In Ireland it is known as the Alternative Vote or AV and is used for presidential elections.
Contingent vote[edit]
Main article: Contingent vote
The Contingent vote is a variant of instant-runoff voting that has been used in the past in Queensland, Australia. Under the contingent vote voters cast only one vote, by ranking all of the candidates in order of preference. However it involves only two rounds of counting and uses the same rule for eliminating candidates as the two-round system. After the first round all but the two candidates with most votes are eliminated. Therefore, one candidate always achieves an absolute majority in the second round. Because of these similarities the contingent vote tends to elect the same winner as the two-round system, and often produces different results to instant-runoff voting. A variant of the contingent vote, called the supplementary vote, is used to elect some mayors in the United Kingdom. Another variant elects the President of Sri Lanka. A criticism of this method is that "it requires two polls, and gives opportunity for intrigue of various kinds."[2]
Nonpartisan blanket primary[edit]
Main article: Nonpartisan blanket primary
In the United States, the nonpartisan blanket primary, introduced in Louisiana for partisan state elections in 1975 and federal elections in 1978 (with a short return to a closed primary system in 2010), is virtually identical to the two-round system. Instead of the standard American system of primary elections to choose each party's candidate, followed by a general election contest between the winners of the primaries, the Louisiana primary allows voters to select any candidate, regardless of party affiliation. The two highest vote-winners in the primary – in effect, the first round of a two-round system – are then the only candidates whose names appear on the ballot at the general election, effectively requiring one candidate to win an absolute majority to take office.
The state of Washington adopted a system virtually identical to Louisiana's in 2008, which came into effect after legal difficulties in 2010. California approved a similar system in 2010, coming into effect for the 36th congressional district election in February 2011.
The main difference between a nonpartisan blanket primary and a standard two-round system is that a second round of voting is required, even if one candidate wins an absolute majority of votes in the primary.
Compliance with voting method criteria[edit]
Most of the mathematical criteria by which voting methods are compared were formulated for voters with ordinal preferences. Some methods, like Approval voting, request information than can't be unambiguously inferred from a single set of ordinal preferences. The two-round system is such a method, because the voters are not forced to vote according to a single ordinal preference in both rounds.
Since the two-round system requires more information from each voter than a single ordinal ballot provides, one can't fit the criteria that are formulated expressly for voters with ordinal preferences without making a generalization as to how the voters will behave. The same problem exists in Approval voting, where one has to make assumptions as to how the voters will place their approval cutoffs.
If the voters determine their preferences before the election and always vote directly consistent to them, they will emulate the Contingent vote and get the same results as if they were to use that method. Therefore, in that model of voting behavior, the two-round system passes all criteria that the contingent vote passes, and fails all criteria the contingent vote fails.
Since the voters in the two-round system don't have to choose their second round votes while voting in the first round, they are able to adjust their votes as players in a game. More complex models consider voter behavior when the voters reach a game-theoretical equilibrium from which they have no incentive, as defined by their internal preferences, to further change their behavior. However, because these equilibria are complex, only partial results are known. With respect to the voters' internal preferences, the two-round system passes the majority criterion in this model, as a majority can always coordinate to elect their preferred candidate. Also, in the case of three candidates or less and a robust political equilibrium,[3] the two-round system will pick the Condorcet winner whenever there is one, which is not the case in the Contingent vote model.
The equilibrium mentioned above is a perfect-information equilibrium and so only strictly holds in idealized conditions where every voter knows every other voter's preference. Thus it provides an upper bound on what can be achieved with rational (self-interested) coordination or knowledge of others' preferences. Since the voters almost surely won't have perfect information, it may not apply to real elections. In that matter, it is similar to the perfect competition model sometimes used in economics. To the extent that real elections approach this upper bound, large elections would do so less so than small ones, because it's less likely that a large electorate has information about all the other voters than that a small electorate has.
Tactical voting and strategic nomination[edit]
Main article: Tactical manipulation of runoff voting
Runoff voting is intended to reduce the potential for eliminating "wasted" votes by tactical voting. Under the "first past the post" (plurality) method voters are encouraged to vote tactically by voting for only one of the two leading candidates, because a vote for any other candidate will not affect the result. Under runoff voting this tactic, known as "compromising", is sometimes unnecessary because, even if a voter's favourite candidate is eliminated in the first round, they will still have an opportunity to influence the result of the election by voting for a more popular candidate in the second round. However the tactic of compromising can still be used in runoff voting because it is sometimes necessary to compromise as a way of influencing which two candidates will survive to the second round. In order to do this it is necessary to vote for one of the three leading candidates in the first round, just as in an election held under the plurality method it is necessary to vote for one of the two leading candidates.
Runoff voting is also vulnerable to another tactic called "push over".[citation needed] This is a tactic by which voters vote tactically for an unpopular "push over" candidate in the first round as a way of helping their true favourite candidate win in the second round. The purpose of voting for the "push over", in theory, is to ensure that it is this weak candidate, rather than a stronger rival, who survives to challenge a one's preferred candidate in the second round. But in practice, such a tactic may prove counter-productive. If so many voters give their first preferences to the "weak" candidate that it ends up winning the first round, it is highly likely they will gain enough campaign momentum to have a strong chance of winning the runoff, too, and with it, the election. At the very least, their opponent would have to start taking the so-called "weak" candidate seriously, particularly if the runoff follows quickly after the first round.
Runoff voting can be influenced by strategic nomination; this is where candidates and political factions influence the result of an election by either nominating extra candidates or withdrawing a candidate who would otherwise have stood. Runoff voting is vulnerable to strategic nomination for the same reasons that it is open to the voting tactic of "compromising". This is because a candidate who knows they are unlikely to win can ensure that another candidate they support makes it to the second round by withdrawing from the race before the first round occurs, or by never choosing to stand in the first place. By withdrawing candidates a political faction can avoid the "spoiler effect", whereby a candidate "splits the vote" of its supporters. A famous example of this spoiler effect occurred in the 2002 French presidential election, when so many left-wing candidates stood in the first round that all of them were eliminated and two right-wing candidates advanced to the second round. Conversely, an important faction may have an interest in helping fund the campaign of smaller factions with a very different political agenda, so that these smaller parties end up weakening their own agenda.
Impact on factions and candidates[edit]
Runoff voting encourages candidates to appeal to a broad cross-section of voters. This is because, in order to win an absolute majority in the second round, it is necessary for a candidate to win the support of voters whose favourite candidate has been eliminated. Under runoff voting, between rounds of voting, eliminated candidates, and the factions who previously supported them, often issue recommendations to their supporters as to whom to vote for in the second round of the contest. This means that eliminated candidates are still able to influence the result of the election. This influence leads to political bargaining between the two remaining candidates and the parties and candidates who have been eliminated, sometimes resulting in the two successful candidates making policy concessions to the less successful ones. Because it encourages conciliation and negotiation in these ways, runoff voting is advocated, in various forms, by some supporters of deliberative democracy.
Runoff voting is designed for single-seat constituencies. Therefore, like other single-seat methods, if used to elect a council or legislature it will not produce proportional representation (PR). This means that it is likely to lead to the representation of a small number of larger parties in an assembly, rather than a proliferation of small parties. In practice, runoff voting produces results very similar to those produced by the plurality method, and encourages a two-party system similar to those found in many countries that use plurality. Under a parliamentary system, it is more likely to produce single-party governments than are PR methods, which tend to produce coalition governments. While runoff voting is designed to ensure that each individual candidate elected is supported by a majority of those in their constituency, if used to elect an assembly it does not ensure this result on a national level. As in other non-PR methods, the party or coalition which wins a majority of seats will often not have the support of an absolute majority of voters across the nation.
Majoritarianism[edit]
The intention of runoff voting is that the winning candidate will have the support of an absolute majority of voters. Under the "first past the post" method the candidate with most votes (a plurality) wins, even if they do not have an absolute majority (more than half) of votes. The two-round system tries to overcome this problem by permitting only two candidates in the second round, so that one must receive an absolute majority of votes.
Critics argue[citation needed] that the absolute majority obtained by the winner of runoff voting is an artificial one. Instant-runoff voting and the exhaustive ballot are two other voting methods that create an absolute majority for one candidate by eliminating weaker candidates over multiple rounds. However, in cases where there are three or more strong candidates, runoff voting will sometimes produce an absolute majority for a different winner than the candidate elected by the other two.
Advocates of Condorcet methods argue[citation needed] that a candidate can claim to have majority support only if they are the "Condorcet winner" – that is, the candidate who would beat every other candidate in a series of one-on-one elections. In runoff voting the winning candidate is only matched, one-on-one, with one of the other candidates. When a Condorcet winner exists, he or she does not necessarily win a runoff election due to insufficient support in the first round.
Runoff advocates counter[citation needed] that voters' first preference is more important than lower preferences because that is where voters are putting the most effort of decision and that, unlike Condorcet methods, runoffs require a high showing among the full field of choices in addition to a strong showing in the final head-to-head competition. Condorcet methods can allow candidates to win who have minimal first-choice support and can win largely on the compromise appeal of being ranked second or third by more voters.
Practical implications[edit]
In large-scale public elections the two rounds of runoff voting are held on separate days, and so involve voters going to the polls twice. In smaller elections, such as those in assemblies or private organisations, it is sometimes possible to conduct both rounds in quick succession. However the fact that it involves two rounds means that, for large elections, runoff voting is more expensive than some other electoral methods. It may also lead to voter fatigue and a reduced turn-out in the second round. In runoff voting, the counting of votes in each round is simple and occurs in the same way as under the plurality methods. Ranked voting systems, such as instant-runoff voting, involve a longer, more complicated count.
Costs[edit]
One of the strongest criticisms against the two-round voting system is the cost required to conduct two ballots. The two-round voting system also has the potential to cause political instability between the two rounds of voting, adding further to the economic impact of the two-round electoral system. Under an instant run-off ballot method there is only one round. It is alleged[by whom?] that the results of the election are known in days as opposed to months depending on the size of the electorate, but this is not always the case depending on the ability to count the rounds of the more complex instant runoff voting.
Supporters of IRV have claimed that IRV can save money in reduced costs, but this has not always been the case. In the 2009, Minneapolis IRV election, the single IRV round cost US$345,726 more (adjusted for inflation) than the two rounds (2005 primary and general election) that IRV was supposed to replace, although a third of the increase was startup costs.[4]
The possibly increased costs of a two-round system have to be weighed against a debate of possibly better quality, between two clear options.
See also[edit]
Ranked voting systems
Keynesian beauty contest
List of electoral systems by country
Notes[edit]
Jump up ^ The usefulness of the link between different political forces in the second ballot is (...) a convergence of interests: Buonomo, Giampiero (2000). "Al candidato sindaco non eletto spetta sempre almeno un seggio". Diritto&Giustizia edizione online. – via Questia (subscription required)
Jump up ^ Hogbden, G. "Art. XLV.—Preferential Voting in Single-member Constituencies, with Special Reference to the Counting of Votes". Transactions and Proceedings of the Royal Society of New Zealand 1868–1961.
Jump up ^ Messner; et al. (2002-11-01). "Robust Political Equilibria under Plurality and Runoff Rule" (PDF). Retrieved 2011-06-04.
Jump up ^ Gelms, Ginny (2010-04-26). "Request for City Council Committee Action from the Department of the City Clerk" (PDF). Retrieved 2017-01-17.
External links[edit]
ACE Project: Two Round System
A Handbook of Electoral System Design from International IDEA
Electoral Design Reference from the ACE Project
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River delta
From Wikipedia, the free encyclopedia
Jump to: navigation, search
"Deltas" redirects here. For other uses, see Delta (disambiguation).
Sacramento (California) Delta at flood stage, early-March 2009
A river delta is a landform that forms from deposition of sediment carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water.[1][2] This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot transport away the supplied sediment. The size and shape of a delta is controlled by the balance between watershed processes that supply sediment and receiving basin processes that redistribute, sequester, and export that sediment.[3][4] The size, geometry, and location of the receiving basin also plays an important role in delta evolution. River deltas are important in human civilization, as they are major agricultural production centers and population centers. They can provide coastline defense and can impact drinking water supply.[5] They are also ecologically important, with different species assemblages depending on their landscape position.
Contents
[hide]
1 Formation
2 Types of deltas
2.1 Wave-dominated deltas
2.2 Tide-dominated deltas
2.3 Gilbert deltas
2.4 Tidal freshwater deltas
2.5 Estuaries
2.6 Inland deltas
2.7 Mega deltas
3 Sedimentary structure
4 Examples of deltas
5 Ecological threats to deltas
6 Deltas in the economy
7 Deltas on Mars
8 See also
9 References
10 Bibliography
11 External links
Formation[edit]
Delta forms where river meets lake[6]
River deltas form when a river carrying sediment reaches either (1) a body of water, such as a lake, ocean, or reservoir, (2) another river that cannot remove the sediment quickly enough to stop delta formation, or (3) an inland region where the water spreads out and deposits sediments. The tidal currents also cannot be too strong, as sediment would wash out into the water body faster than the river deposits it. The river must carry enough sediment to layer into deltas over time. The river's velocity decreases rapidly, causing it to deposit the majority, if not all, of its load. This alluvium builds up to form the river delta.[7] When the flow enters the standing water, it is no longer confined to its channel and expands in width. This flow expansion results in a decrease in the flow velocity, which diminishes the ability of the flow to transport sediment. As a result, sediment drops out of the flow and deposits. Over time, this single channel builds a deltaic lobe (such as the bird's-foot of the Mississippi or Ural river deltas), pushing its mouth into the standing water. As the deltaic lobe advances, the gradient of the river channel becomes lower because the river channel is longer but has the same change in elevation (see slope).
As the slope of the river channel decreases, it becomes unstable for two reasons. First, gravity makes the water flow in the most direct course down slope. If the river breaches its natural levees (i.e., during a flood), it spills out onto a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope.[8] Second, as its slope gets lower, the amount of shear stress on the bed decreases, which results in deposition of sediment within the channel and a rise in the channel bed relative to the floodplain. This makes it easier for the river to breach its levees and cut a new channel that enters the body of standing water at a steeper slope. Often when the channel does this, some of its flow remains in the abandoned channel. When these channel-switching events occur, a mature delta develops a distributary network.
Another way these distributary networks form is from deposition of mouth bars (mid-channel sand and/or gravel bars at the mouth of a river). When this mid-channel bar is deposited at the mouth of a river, the flow is routed around it. This results in additional deposition on the upstream end of the mouth-bar, which splits the river into two distributary channels. A good example of the result of this process is the Wax Lake Delta.
In both of these cases, depositional processes force redistribution of deposition from areas of high deposition to areas of low deposition. 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 results in more uniform deposition of sediment on the delta front. 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.
Types of deltas[edit]
Lower Mississippi River land loss over time
Delta lobe switching in the Mississippi Delta, 4600 yrs BP, 3500 yrs BP, 2800 yrs BP, 1000 yrs BP, 300 yrs BP, 500 yrs BP, current
Deltas are typically classified according to the main control on deposition, which is a combination of river, wave, and tidal processes,[9] depending on the strength of each.[10] The other two factors that play a major role are landscape position and the grain size distribution of the source sediment entering the delta from the river.[11]
Wave-dominated deltas[edit]
In wave dominated deltas, wave-driven sediment transport controls the shape of the delta, and much of the sediment emanating from the river mouth is deflected along the coast line.[9] The relationship between waves and river deltas is quite variable and largely influenced by the deepwater wave regimes of the receiving basin. With a high wave energy near shore and a steeper slope offshore, waves will make river deltas smoother. Waves can also be responsible for carrying sediments away from the river delta, causing the delta to retreat.[5] For deltas that form further upriver in an estuary, there are complex yet quantifiable linkages between winds, tides, river discharge, and delta water levels.[12][13]
The Ganges Delta in India and Bangladesh is the largest delta in the world and it is also one of the most fertile regions in the world.
Tide-dominated deltas[edit]
Erosion is also an important control in tide dominated deltas, such as the Ganges Delta, which may be mainly submarine, with prominent sand bars and ridges. This tends to produce a "dendritic" structure.[14] Tidal deltas behave differently from river- and wave-dominated deltas, which tend to have a few main distributaries. Once a wave- or river-dominated distributary silts up, it is abandoned, and a new channel forms elsewhere. In a tidal delta, new distributaries are formed during times when there is a lot of water around – such as floods or storm surges. These distributaries slowly silt up at a more or less constant rate until they fizzle out.[14]
Gilbert deltas[edit]
A Gilbert delta (named after Grove Karl Gilbert) is a specific type of delta formed from coarse sediments, as opposed to gently-sloping muddy deltas such as that of the Mississippi. For example, a mountain river depositing sediment into a freshwater lake would form this kind of delta.[15] [16] While some authors describe both lacustrine and marine locations of Gilbert deltas,[15] others note that their formation is more characteristic of the freshwater lakes, where it is easier for the river water to mix with the lakewater faster (as opposed to the case of a river falling into the sea or a salt lake, where less dense fresh water brought by the river stays on top longer).[17]
G. K. Gilbert himself first described this type of delta on Lake Bonneville in 1885.[17] Elsewhere, similar structures occur, for example, at the mouths of several creeks that flow into Okanagan Lake in British Columbia and forming prominent peninsulas at Naramata (49°35′30″N 119°35′30″W / 49.59167°N 119.59167°W / 49.59167; -119.59167), Summerland (49°34′23″N 119°37′45″W / 49.57306°N 119.62917°W / 49.57306; -119.62917), or Peachland (49°47′00″N 119°42′45″W / 49.78333°N 119.71250°W / 49.78333; -119.71250).
Tidal freshwater deltas[edit]
A tidal freshwater delta[18] is a sedimentary deposit formed at the boundary between an upland stream and an estuary, in the region known as the "subestuary".[19] Drowned coastal river valleys that were inundated by rising sea levels during the late Pleistocene and subsequent Holocene tend to have dendritic estuaries with many feeder tributaries. Each tributary mimics this salinity gradient from their brackish junction with the mainstem estuary up to the fresh stream feeding the head of tidal propagation. As a result, the tributaries are considered to be "subestuaries". The origin and evolution of a tidal freshwater delta involves processes that are typical of all deltas[4] as well as processes that are unique to the tidal freshwater setting.[20][21] The combination of processes that create a tidal freshwater delta result in a distinct morphology and unique environmental characteristics. Many tidal freshwater deltas that exist today are directly caused by the onset of or changes in historical land use, especially deforestation, intensive agriculture, and urbanization.[22] These ideas are well illustrated by the many tidal freshwater deltas prograding into Chesapeake Bay along the east coastline of the United States. Research has demonstrated that the accumulating sediments in this estuary derive from post-European settlement deforestation, agriculture, and urban development.[23][24][25]
Estuaries[edit]
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 Saint Lawrence River and the Tagus estuary.
Inland deltas[edit]
Okavango Delta
In rare cases the river delta is located inside a large valley and is called an inverted river delta. Sometimes a river divides into multiple branches in an inland area, only to rejoin and continue to the sea. Such an area is called an inland delta, and often occurs on former lake beds. The Inner Niger Delta and Peace–Athabasca Delta are notable examples. The Amazon has also an inland delta before the island of Marajó.
In some cases, a river flowing into a flat arid area splits into channels that evaporate as it progresses into the desert. Okavango Delta in Botswana is one well-known example.
Mega deltas[edit]
The generic term mega delta can be used to describe very large Asian river deltas, such as the Changjiang (Yangtze), Pearl, Red, Mekong, Irrawaddy, Ganges-Brahmaputra, and Indus.
Sedimentary structure[edit]
The formation of a delta is complicated, multiple, and cross-cutting over time, but in a simple delta three main types of bedding may be distinguished: the bottomset beds, foreset/frontset beds, and topset beds. This three part structure may be seen in small scale by crossbedding.[15][26]
The bottomset beds are created from the lightest suspended particles that settle farthest away from the active delta front, as the river flow diminishes into the standing body of water and loses energy. This suspended load is deposited by sediment gravity flow, creating a turbidite. These beds are laid down in horizontal layers and consist of the finest grain sizes.
The foreset beds in turn are deposited in inclined layers over the bottomset beds as the active lobe advances. Foreset beds form the greater part of the bulk of a delta, (and also occur on the lee side of sand dunes).[27] The sediment particles within foreset beds consist of larger and more variable sizes, and constitute the bed load that the river moves downstream by rolling and bouncing along the channel bottom. When the bed load reaches the edge of the delta front, it rolls over the edge, and is deposited in steeply dipping layers over the top of the existing bottomset beds. Under water, the slope of the outermost edge of the delta is created at the angle of repose of these sediments. As the foresets accumulate and advance, subaqueous landslides occur and readjust overall slope stability. The foreset slope, thus created and maintained, extends the delta lobe outward. In cross section, foresets typically lie in angled, parallel bands, and indicate stages and seasonal variations during the creation of the delta.
The topset beds of an advancing delta are deposited in turn over the previously laid foresets, truncating or covering them. Topsets are nearly horizontal layers of smaller-sized sediment deposited on the top of the delta and form an extension of the landward alluvial plain.[27] As the river channels meander laterally across the top of the delta, the river is lengthened and its gradient is reduced, causing the suspended load to settle out in nearly horizontal beds over the delta's top. Topset beds are subdivided into two regions: the upper delta plain and the lower delta plain. The upper delta plain is unaffected by the tide, while the boundary with the lower delta plain is defined by the upper limit of tidal influence.[28]
Examples of deltas[edit]
The Ganges/Brahmaputra combination delta, which spans most of Bangladesh and empties into the Bay of Bengal, is the world's largest delta.
The St. Clair River delta, between the Canadian province of Ontario and the U.S. state of Michigan, is the largest delta emptying into a body of fresh water.
Other rivers with notable deltas include:
Ayeyarwady (Irrawaddy)
Danube
Ebro
Fly River
Godavari
Indus
Kaveri
Krishna
Lena
Mackenzie
Mekong
Mississippi
Niger River
Nile
Ob
Orinoco
Otter Point Creek
Paraná
Po
Rhine
Rhône
Sacramento–San Joaquin
Tigris–Euphrates river system
Volga
Yangtze
Yenisei
Yellow River (also known as Huanghe)
Yukon River
The Ebre River delta at the Mediterranean Sea
Ecological threats to deltas[edit]
Human activities, such as the creation of dams for hydroelectric power or to create reservoirs can radically alter delta ecosystems. Dams block sedimentation, which can cause the delta to erode away. The use of water upstream can greatly increase salinity levels as less fresh water flows to meet the salty ocean water. While nearly all deltas have been impacted to some degree by humans, the Nile Delta and Colorado River Delta are some of the most extreme examples of the ecological devastation caused to deltas by damming and diversion of water. Construction, irrigation, and land alteration have impacted delta formation. As humans have altered surface roughness, runoff, and groundwater storage, studies have shown river delta retreat. However, historical data documents show that during the Roman Empire and Little Ice Age (times where there was considerable anthropogenic pressure), there were significant sediment accumulation in deltas. The industrial revolution has only amplified the impact of humans on delta growth and retreat.[29]
Deltas in the economy[edit]
Ancient deltas are a benefit to the economy due to their well sorted sand and gravel. 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.[30] Not all sand and gravel quarries are former deltas, but for ones that are, a lot of the sorting is already done by the power of water.
As lowlands often adjacent to urban areas, deltas often comprise extensive industrial and commercial areas as well as agricultural land. These uses are often in conflict. The Fraser Delta in British Columbia, Canada, includes the Vancouver Airport and the Roberts Bank Superport and the Annacis Island industrial zone, and a mix of commercial, residential and agricultural land. Space is so limited in the Lower Mainland region, and in British Columbia in general, which is very mountainous, that the Agricultural Land Reserve was created to preserve agricultural land for food production.
Deltas on Mars[edit]
Researchers have found a number of examples of deltas that formed in Martian lakes. Finding deltas is a major sign that Mars once had large amounts of water. Deltas have been found over a wide geographical range. Below are pictures of a few.[31]
Delta in Ismenius Lacus quadrangle, as seen by THEMIS.
Delta in Lunae Palus quadrangle, as seen by THEMIS.
Delta in Margaritifer Sinus quadrangle as seen by THEMIS.
Probable delta in Eberswalde crater, as seen by Mars Global Surveyor. Image in Margaritifer Sinus quadrangle.
See also[edit]
Wetlands portal
Alluvial fan
Avulsion (river)
Estuary
Levee
Nile delta
Regressive delta
References[edit]
Jump up ^ Miall, A. D. 1979. Deltas. in R. G. Walker (ed) Facies Models. Geological Association of Canada, Hamilton, Ontario.
Jump up ^ Elliot, T. 1986. Deltas. in H. G. Reading (ed.). Sedimentary environments and facies. Backwell Scientific Publications, Oxford.
Jump up ^ Blum, M.D.; Tornqvist, T.E. (2000). "Fluvial responses to climate and sea-level change: a review and look forward". Sedimentology. 47: 2–48. doi:10.1046/j.1365-3091.2000.00008.x.
^ Jump up to: a b Pasternack, Gregory B.; Brush, Grace S.; Hilgartner, William B. (2001-04-01). "Impact of historic land-use change on sediment delivery to a Chesapeake Bay subestuarine delta". Earth Surface Processes and Landforms. 26 (4): 409–427. doi:10.1002/esp.189. ISSN 1096-9837.
^ Jump up to: a b Anthony, Edward J. (2015-03-01). "Wave influence in the construction, shaping and destruction of river deltas: A review". Marine Geology. 361: 53–78. doi:10.1016/j.margeo.2014.12.004.
Jump up ^ "How a Delta Forms Where River Meets Lake". 2014-08-12. Retrieved 2017-12-12.
Jump up ^ "Dr. Gregory B. Pasternack – Watershed Hydrology, Geomorphology, and Ecohydraulics :: TFD Modeling". pasternack.ucdavis.edu. Retrieved 2017-06-12.
Jump up ^ Slingerland, R. and N. D. Smith (1998), "Necessary conditions for a meandering-river avulsion," Geology (Boulder), 26, 435–438.
^ Jump up to: a b Galloway, W.E., 1975, Process framework for describing the morphologic and stratigraphic evolution of deltaic depositional systems, in Brousard, M.L., ed., Deltas, Models for Exploration: Houston Geological Society, Houston, Texas, p. 87– 98.
Jump up ^ Perillo, G. M. E. 1995. Geomorphology and Sedimentology of Estuaries. Elsevier Science B.V., New York.
Jump up ^ Orton, G.J.; Reading, H.G. (1993). "Variability of deltaic processes in terms of sediment supply, with particular emphasis on grain size". Sedimentology. 40: 475–512. doi:10.1111/j.1365-3091.1993.tb01347.x.
Jump up ^ "Dr. Gregory B. Pasternack – Watershed Hydrology, Geomorphology, and Ecohydraulics :: TFD Hydrometeorology". pasternack.ucdavis.edu. Retrieved 2017-06-12.
Jump up ^ Pasternack, Gregory B.; Hinnov, Linda A. (October 2003). "Hydrometeorological controls on water level in a vegetated Chesapeake Bay tidal freshwater delta". Estuarine, Coastal and Shelf Science. 58 (2): 367–387. doi:10.1016/s0272-7714(03)00106-9.
^ Jump up to: a b Fagherazzi S., 2008, Self-organization of tidal deltas, Proceedings of the National Academy of Sciences, vol. 105 (48): 18692–18695,
^ Jump up to: a b c Characteristics of deltas. (Available archived at [1] – checked Dec 2008.)
Jump up ^ Bernard Biju-Duval, J. Edwin Swezey. "Sedimentary Geology". Page 183. ISBN 2-7108-0802-1. Editions TECHNIP, 2002. Partial text on Google Books.
^ Jump up to: a b "Geological and Petrophysical Characterization of the Ferron Sandstone for 3-D Simulation of a Fluvial-deltaic Reservoir". By Thomas C. Chidsey, Thomas C. Chidsey, Jr (ed), Utah Geological Survey, 2002. ISBN 1-55791-668-3. Pages 2–17. Partial text on Google Books.
Jump up ^ "Dr. Gregory B. Pasternack – Watershed Hydrology, Geomorphology, and Ecohydraulics :: Tidal Freshwater Deltas". pasternack.ucdavis.edu. Retrieved 2017-06-12.
Jump up ^ Pasternack, G. B. 1998. Physical dynamics of tidal freshwater delta evolution. Ph. D. Dissertation. The Johns Hopkins University, 227pp, 5 appendices.
Jump up ^ Pasternack, Gregory B.; Hilgartner, William B.; Brush, Grace S. (2000-09-01). "Biogeomorphology of an upper Chesapeake Bay river-mouth tidal freshwater marsh". Wetlands. 20 (3): 520–537. doi:10.1672/0277-5212(2000)020<0520:boaucb>2.0.co;2. ISSN 0277-5212.
Jump up ^ Pasternack, Gregory B; Brush, Grace S (2002-03-01). "Biogeomorphic controls on sedimentation and substrate on a vegetated tidal freshwater delta in upper Chesapeake Bay". Geomorphology. 43 (3–4): 293–311. doi:10.1016/s0169-555x(01)00139-8.
Jump up ^ Pasternack, Gregory B.; Brush, Grace S. (1998-09-01). "Sedimentation cycles in a river-mouth tidal freshwater marsh". Estuaries. 21 (3): 407–415. doi:10.2307/1352839. ISSN 0160-8347.
Jump up ^ Gottschalk, L. C. (1945). "Effects of soil erosion on navigation in upper Chesapeake Bay". Geographical Review. 35: 219–238. doi:10.2307/211476.
Jump up ^ Brush, G. S. (1984). "Patterns of recent sediment accumulation in Chesapeake Bay (Virginia-Maryland, U.S.A.) tributaries". Chemical Geology. 44: 227–242. doi:10.1016/0009-2541(84)90074-3.
Jump up ^ Orson, R. A.; Simpson, R. L.; Good, R. E. (1992). "The paleoecological development of a late Holocene, tidal freshwater marsh of the upper Delaware River estuary". Estuaries. 15 (2): 130–146. doi:10.2307/1352687.
Jump up ^ D.G.A Whitten, The Penguin Dictionary of Geology (1972)
^ Jump up to: a b Robert L. Bates, Julia A. Jackson, Dictionary of Geological Terms AGI (1984)
Jump up ^ Hori, K. and Saito, Y. Morphology and Sediments of Large River Deltas. Tokyo, Japan: Tokyo Geographical Society, 2003
Jump up ^ Maselli, Vittorio; Trincardi, Fabio (2013-05-31). "Man made deltas". Scientific Reports. 3. doi:10.1038/srep01926. ISSN 2045-2322. PMC 3668317  . PMID 23722597.
Jump up ^ Mineral Information Institute. 2011. Sand and Gravel. http://www.mii.org/Minerals/photosandgr.html
Jump up ^ Irwin III, R. et al. 2005. An intense terminal epoch of widespread fluvial activity on early Mars: 2. Increased runoff and paleolake development. Journal of Geophysical Research: 10. E12S15
Bibliography[edit]
Renaud, F. and C. Kuenzer 2012: The Mekong Delta System – Interdisciplinary Analyses of a River Delta, Springer, ISBN 978-94-007-3961-1, doi:10.1007/978-94-007-3962-8, pp. 7–48
KUENZER C. and RENAUD, F. 2012: Climate Change and Environmental Change in River Deltas Globally. In (eds.): Renaud, F. and C. Kuenzer 2012: The Mekong Delta System – Interdisciplinary Analyses of a River Delta, Springer, ISBN 978-94-007-3961-1, doi:10.1007/978-94-007-3962-8, pp. 7–48
Ottinger, M.; Kuenzer, C.; LIU; Wang, S.; Dech, S. (2013). "Monitoring Land Cover Dynamics in the Yellow River Delta from 1995 to 2010 based on Landsat 5 TM". Applied Geography. 44: 53–68. doi:10.1016/j.apgeog.2013.07.003.
External links[edit]
The Wikibook Historical Geology has a page on the topic of: Deltas
Wikimedia Commons has media related to River deltas.
Louisiana State University Geology – World Deltas
http://www.wisdom.eoc.dlr.de WISDOM Water related Information System for the Sustainable Development of the Mekong Delta
Maria Chiara Tosi (Ed.) (2012) Delta Landscape – A monographic study on the River Po Delta (Italy)
Wave-dominated river deltas on coastalwiki.org – A coastalwiki.org page on wave-dominated river deltas
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values: বদ্বীপ – Bangla

values: Дэльта – Belarusian (Taraškievica orthography)

values: Делта (география) – Bulgarian

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Floddelta – Danish
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da
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Delta – Estonian
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Delta – Extremaduran
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Delta – Uzbek
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Delta – Portuguese
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Suisto – Finnish
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Floddelta – Swedish
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values: ดินดอนสามเหลี่ยมปากแม่น้ำ – Thai

values: Річкова дельта – Ukrainian

values: دہانہ – Urdu

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