Editing Mekong (section)

==Drainage basin==
[[File:Chamdo Mekong.png|thumb|Upper Mekong (Za Qu) south of [[Chamdo]].]]
[[File:Phou si - Mekong River - Luang Prabang Laos プーシーの丘、メコン川 ラオス・ルアンプラバーン DSCF6787.jpg|thumb|The Mekong from [[Phou si]]]]
[[File:Nam Ou River confluence in Mekong Laos.jpg|thumb|The confluence of the Mekong and the [[Nam Ou]] Rivers, [[Laos]]]]

The Mekong Basin is frequently divided into two parts: the "upper Mekong basin" comprising those parts of the basin in [[Tibet Autonomous Region|Tibet]], [[Yunnan]] and eastern [[Myanmar]], and the "lower Mekong basin" from [[Yunnan]] downstream from China to the [[South China Sea]].<ref name ="MRC_2005">{{cite web |author=Mekong River Commission |year=2005 |title=Overview of the Hydrology of the Mekong Basin |publisher=MRC, Vientiane, Laos |url=http://www.mekonginfo.org/assets/midocs/0001968-inland-waters-overview-of-the-hydrology-of-the-mekong-basin.pdf |author-link=Mekong River Commission |access-date=21 April 2012 |archive-date=13 July 2018 |archive-url=https://web.archive.org/web/20180713003412/http://www.mekonginfo.org/assets/midocs/0001968-inland-waters-overview-of-the-hydrology-of-the-mekong-basin.pdf |url-status=dead}}</ref> From the point where it rises to its mouth, the most precipitous drop in the Mekong occurs in the upper Mekong basin, a stretch of some {{cvt|2200|km|mi|}}. Here, it drops {{cvt|4500|m|ft||abbr=out}} before it enters the lower basin where the borders of Thailand, Laos, China, and Myanmar come together in the Golden Triangle. Downstream from the Golden Triangle, the river flows for a further {{cvt|2600|km|mi|}} through Laos, Thailand, and [[Cambodia]] before entering the [[South China Sea]] via a complex delta system in [[Vietnam]].<ref name="MRC_2005" />

===Upper basin===
The upper basin makes up 24% of the total area and contributes 15–20% of the water that flows into the Mekong River. The [[catchment]] here is steep and narrow with [[Soil erosion]] being a major problem and as a result of this, approximately 50% of the sediment in the river comes from the upper basin.

In Yunnan Province in China, the river and its tributaries are confined by narrow, deep gorges. The tributary river systems in this part of the basin are small. Only 14 have catchment areas that exceed {{cvt|1000|km2|sqmi|sigfig=1}}, yet the greatest amount of loss of [[forest cover]] in the entire river system per square kilometer has occurred in this region due to heavy unchecked demand for natural resources. In the south of Yunnan, in [[Simao District|Simao]] and [[Xishuangbanna]] Prefectures, the river changes as the valley opens out, the floodplain becomes wider, and the river becomes wider and slower.

===Lower basin===
Major tributary systems develop in the lower basin. These systems can be separated into two groups: tributaries that contribute to the major wet season flows, and tributaries that drain low relief regions of lower rainfall. The first group are left bank tributaries that drain the high rainfall areas of Laos. The second group are those on the right bank, mainly the Mun and Chi Rivers, that drain a large part of northeast Thailand.

Laos lies almost entirely within the lower Mekong basin. Its climate, landscape and land use are the major factors shaping the hydrology of the river. The mountainous landscape means that only 16% of the country is farmed under lowland terrace or upland [[shifting cultivation]].<ref name="MRC_2005" /> With upland shifting agriculture (slash and burn), soils recover within 10 to 20 years but the vegetation does not. Shifting cultivation is common in the uplands of northern Laos and is reported to account for as much as 27% of the total land under rice cultivation.<ref name="MRC_2005" /> As elsewhere in the basin, forest cover has been steadily reduced during the last three decades by shifting agriculture and permanent agriculture. The cumulative impacts of these activities on the river regime have not been measured. However, the hydrological impacts of land cover changes induced by the [[Vietnam War]] were quantified in two sub-catchments of the lower Mekong River basin.<ref>{{cite journal |author1=G. Lacombe |author2=A. Pierret |author3=C. T. Hoanh |author4=O. Sengtaheuanghoung |author5=A. Noble |year=2010 |title=Conflict, migration and land-cover changes in Indochina: a hydrological assessment |journal=Ecohydrology |volume=3 |issue=4 |pages=382–391 |doi=10.1002/eco.166 |bibcode=2010Ecohy...3..382L }}</ref>

Loss of forest cover in the Thai areas of the lower basin has been the highest of all the lower Mekong countries over the past 60 years. On the [[Khorat Plateau]], which includes the [[Mun River|Mun]] and [[Chi River|Chi]] tributary systems, forest cover was reduced from 42% in 1961 to 13% in 1993.<ref name="MRC_2005" /> Although this part of northeast Thailand has an annual rainfall of more than {{cvt|1000|mm|in|sigfig=1}}, a high evaporation rate means it is classified as a semi-arid region. Consequently, although the Mun and Chi basins drain 15% of the entire Mekong basin, they only contribute 6% of the average annual flow.<ref name="MRC_2005" /> Sandy and saline soils are the most common soil types, which makes much of the land unsuitable for wet rice cultivation. In spite of poor fertility, however, agriculture is intensive. Glutinous rice, maize, and cassava are the principal crops.<ref name="MRC_2005" /> Drought is by far the major hydrological hazard in this region.<ref name="MRC_2005" />
[[File:Mekong floating homes.jpg|thumb|Floating homes on the Mekong, Cambodia]]

As the Mekong enters Cambodia, over 95% of its flows have already joined the river.<ref name="MRC_2005" /> From here on downstream the terrain is flat and water levels rather than flow volumes determine the movement of water across the landscape. The seasonal cycle of changing water levels at [[Phnom Penh]] results in the unique "flow reversal" of water into and out of the Great Lake via the [[Tonle Sap River]]. Phnom Penh also marks the beginning of the delta system of the Mekong River. Here the mainstream begins to break up into an increasing number of branches.<ref name="MRC_2005" />

In Cambodia, wet rice is the main crop and is grown on the flood plains of the Tonle Sap, Mekong, and [[Bassac River|Bassac]] (the Mekong delta distributary known as the Hậu in Vietnam) Rivers.<ref name="MRC_2005" /> More than half of Cambodia remains covered with mixed evergreen and deciduous broadleaf forest, but forest cover has decreased from 73% in 1973 to 63% in 1993.<ref name="MRC_2005" /> Here, the river landscape is flat. Small changes in water level determine the direction of water movement, including the large-scale reversal of flow into and out of the Tonle Sap basin from the Mekong River.<ref name="MRC_2005" />
[[File:Mekong Delta river -a.jpg|thumb|upright|Mekong Delta, Vietnam]]

The [[Mekong Delta]] in Vietnam is farmed intensively and has little natural vegetation left. Forest cover is less than 10%. In the [[Central Highlands (Vietnam)|Central Highlands]] of Vietnam, forest cover was reduced from over 95% in the 1950s to around 50% in the mid-1990s.<ref name="MRC_2005" /> [[Agricultural expansion]] and [[population pressure]] are the major reasons for [[land use]] and landscape change. Both [[drought]] and [[flood]] are common hazards in the Delta, which many people believe is the most sensitive to upstream hydrological change.<ref name="MRC_2005" />

===Water flow along its course===
'''Table 1: Country share of Mekong River Basin (MRB) and water flows'''<ref name="MRC_2005" />
{| class="wikitable"
|-
|-
| || China || Myanmar || Laos || Thailand || Cambodia || Vietnam || Total
|-
| Basin area (km<sup>2</sup>) || 165,000 || 24,000 || 202,000 || 184,000 || 155,000 || 65,000 || 795,000
|-
| Catchment as % of MRB || 21 || 3 || 25 || 23 || 20 || 8 || 100
|-
| Flow as % of MRB || 16 || 2 || 35 || 18 || 18 || 11 || 100
|}

By taking into account hydrological regimes, physiography land use, and existing, planned and potential resource developments, the Mekong is divided into six distinct [[Reach (geography)|reaches]]:<ref name="MRC_2005"/>
[[File:2009-08-30 09-03 Luang Prabang 020 Mekong.jpg|thumb|The Mekong in Laos]]

''Reach 1: Lancang Jiang or Upper Mekong River in China''. In this part of the river, the major source of water flowing into the river comes from melting snow on the [[Tibetan plateau]]. This volume of water is sometimes called the "Yunnan component" and plays an important role in the low-flow hydrology of the lower mainstream. Even as far downstream as Kratie, the [[Yunnan]] component makes up almost 30% of the average dry season flow. A major concern is that the ongoing and planned expansion of dams and reservoirs on the Mekong mainstream in Yunnan could have a significant effect on the low-flow regime of the lower Mekong basin system.<ref name="MRC_2005"/><ref name=NYTimes2015-03-30>{{cite news |url=https://www.nytimes.com/2015/03/31/opinion/the-price-of-damming-tibets-rivers.html |title=The Price of Damming Tibet's Rivers |work=The New York Times |author=Michael Buckley |date=30 March 2015 |page=A25 |archive-url=https://web.archive.org/web/20150331103802/http://www.nytimes.com/2015/03/31/opinion/the-price-of-damming-tibets-rivers.html |archive-date=31 March 2015 |access-date=1 April 2015 |url-status=live |quote=Other plans call for diversion of water from the Brahmaputra, Salween and Mekong – all rivers that cross national boundaries.}}</ref><ref>{{cite journal |last1=Yeophantong |first1=Pichamon |title=China's Lancang Dam Cascade and Transnational Activism in the Mekong Region: Who's Got the Power? |journal=Asian Survey |volume=54 |issue=4 |doi=10.1525/as.2014.54.4.700 |year=2014 |pages=700–724}}</ref>

''Reach 2: [[Chiang Saen]] to [[Vientiane]] and [[Nong Khai]]''. This reach is almost entirely mountainous and covered with natural forest although there has been widespread slash and burn agriculture. Although this reach cannot be termed "unspoiled", the hydrological response is perhaps the most natural and undisturbed of all the lower basin. Many hydrological aspects of the lower basin start to change rapidly at the downstream boundary of this reach.<ref name="MRC_2005"/>

On 19 July 2019 this reach of the river dropped to its lowest level in a century. Officials are particularly concerned as July is in the wet season, when mainstream flows are abundant historically. Locals are blaming low water on the newly constructed [[Xayaburi Dam]], as it enters its test phase prior to the start of commercial operation in October 2019.<ref>{{cite news |title=Mekong River in Golden Triangle drops to lowest level in a century |url=https://www.thaipbsworld.com/mekong-river-in-golden-triangle-drops-to-lowest-level-in-a-century/ |access-date=21 July 2019 |work=Thai PBS |date=20 July 2019}}</ref>

''Reach 3: Vientiane and Nong Khai to [[Pakse]]''. The boundary between Reach 2 and 3 is where the Mekong hydrology starts to change. Reach 2 is dominated in both wet and dry seasons by the [[Yunnan]] Component. Reach 3 is increasingly influenced by contributions from the large left bank tributaries in Laos, namely the [[Nam Ngum]], [[Nam Theun]], Nam Hinboun, [[Xe Bang Fai River|Se Bang Fai]], Se Bang Hieng and Se Done Rivers. The [[Mun River|Mun]]-[[Chi River|Chi]] river system from the right bank in [[Thailand]] enters the mainstream within this reach.<ref name="MRC_2005"/>

''Reach 4: Pakse to [[Kratié (city)|Kratie]]''. The main hydrological contributions to the mainstream in this reach come from the [[Se Kong]], [[Se San]], and Sre Pok catchments. Together, these rivers make up the largest hydrological sub-component of the lower basin. Over 25% of the mean annual flow volume to the mainstream at Kratie comes from these three river basins. They are the key element in the hydrology of this part of the system, especially to the [[Tonle Sap]] flow reversal.<ref name="MRC_2005"/>

''Reach 5: Kratie to [[Phnom Penh]]''. This reach includes the hydraulic complexities of the Cambodian floodplain, the Tonle Sap and the Great Lake. By this stage, over 95% of the total flow has entered the Mekong system. The focus turns from hydrology and water discharge to the assessment of water level, over- bank storage and flooding and the hydrodynamics that determine the timing, duration and volume of the seasonal flow reversal into and out of the Great Lake.<ref name="MRC_2005"/>

''Reach 6: Phnom Penh to the [[South China Sea]]''. Here the mainstream divides into a complex and increasingly controlled and artificial system of branches and canals. Key features of flow behaviour are tidal influences and salt water intrusion. Every year, 35–50% of this reach is flooded during the rainy season. The impact of road embankments and similar infrastructure developments on the movement of this flood water is an increasingly important consequence of development.<ref name="MRC_2005"/>

Table 2 summarises the mean annual flows along the mainstream. The mean annual flow entering the lower Mekong from China is equivalent to a relatively modest {{cvt|450|mm|in}} depth of runoff. Downstream of Vientiane this increases to over {{cvt|600|mm|in}} as the principal left bank tributaries enter the mainstream, mainly the Nam Ngum and Nam Theun. The flow level falls again, even with the right bank entry of the Mun-Chi system from Thailand. Although the Mun–Chi basin drains 20% of the lower system, average annual runoff is only {{cvt|250|mm|in|0}}. Runoff in the mainstream increases again with the entry from the left bank of the Se Kong from southern Laos and [[Se San]] and Sre Pok from Vietnam and Cambodia.
[[File:CanThoFloatingMarket.jpg|thumb|Floating market, Cần Thơ, Mekong delta]]
[[File:CauKhi SongTien BinhDai BenTre VN.jpg|thumb|[[Cầu khỉ]] (monkey bridge) and small ''nước mắm'' (fish sauce) workshop on the bank of the [[Tiền River]] (branch of Mekong), Binh Dai District, [[Ben Tre Province]], Vietnam]]
[[File:PovertyHamlet Mekong@BenTre Vietnam.jpg|thumb|Hamlet, [[Tiền River]], Binh Dai District, [[Ben Tre Province]], Vietnam]]

'''Table 2: Lower Mekong Mainstream annual flow (1960 to 2004) at selected sites.'''<ref name="MRC_2005"/>
{| class="wikitable"
|-
! Mainstream site !! Catchment area (km<sup>2</sup>) !! Mean annual flow !! !! !! as % total Mekong
|-
| || || Discharge m<sup>3</sup>/s || Volume km<sup>3</sup> || Runoff (mm) ||
|-
| [[Chiang Saen District|Chiang Saen]] || 189,000 || 2,700 || 85 || 450 || 19
|-
| [[Luang Prabang]] || 268,000 || 3,900 || 123 || 460 || 27
|-
| [[Chiang Khan District|Chiang Khan]] || 292,000 || 4,200 || 133 || 460 || 29
|-
| [[Vientiane]] || 299,000 || 4,400 || 139 || 460 || 30
|-
| [[Nong Khai]] || 302,000 || 4,500 || 142 || 470 || 31
|-
| [[Nakhon Phanom]] || 373,000 || 7,100 || 224 || 600 || 49
|-
| [[Mukdahan]] || 391,000 || 7,600 || 240 || 610 || 52
|-
| [[Pakse]] || 545,000 || 9,700 || 306 || 560 || 67
|-
| [[Stung Treng]] || 635,000 || 13,100 || 413 || 650 || 90
|-
| [[Kratié (town)|Kratié]] || 646,000 || 13,200 || 416 || 640 || 91
|-
| Basin Total || 760,000 || 14,500 || 457 || 600 || 100
|}
Flows at Chiang Saen entering the lower basin from [[Yunnan]] make up about 15% of the wet season flow at [[Kratié (city)|Kratie]]. This rises to 40% during the dry season, even this far downstream. During the wet season, the proportion of average flow coming from Yunnan rapidly decreases downstream of Chiang Saen, from 70% to less than 20% at Kratie. The dry season contribution from Yunnan is much more significant. The major portion of the balance comes from Laos, which points to a major distinction in the low-flow hydrology of the river. One fraction comes from melting snow in China and [[Tibet]] and the rest from over-season catchment storage in the lower basin. This has implications for the occurrence of drought conditions. For example, if runoff from melting snow in any given year is very low, then flows upstream of [[Vientiane]]-[[Nong Khai]] would be lower.<ref name="MRC_2005"/>

In a large river system like the Mekong, seasonal flows can be quite variable from year to year. Although the pattern of the annual hydrograph is fairly predictable, its magnitude is not. The average monthly flows along the mainstream are listed in Table 3, providing an indication of their range and variability from year to year. At [[Pakse]], for example, flood season flows during August would exceed {{cvt|20000|m3/s|gal/s}} nine years out of ten, but exceed {{cvt|34000|m3/s|gal/s}} only one year in ten.<ref name="MRC_2005"/>

'''Table 3: Mekong Mainstream monthly discharge, 1960–2004''' (m<sup>3</sup>/s).<ref name="MRC_2005"/>
{| class="wikitable"
|-
! Month !! Chiang Saen !! Luang Prabang !! Vientiane !! Nakhon Phanom !! Mukdahan !! Pakse !! Kratie
|-
| Jan || 1,150 || 1,690 || 1,760 || 2,380 || 2,370 || 2,800 || 3,620
|-
| Feb || 930 || 1,280 || 1,370 || 1,860 || 1,880 || 2,170 || 2,730
|-
| Mar || 830 || 1,060 || 1,170 || 1,560 || 1,600 || 1,840 || 2,290
|-
| Apr || 910 || 1,110 || 1,190 || 1,530 || 1,560 || 1,800 || 2,220
|-
| May || 1,300 || 1,570 || 1,720 || 2,410 || 2,430 || 2,920 || 3,640
|-
| Jun || 2,460 || 3,110 || 3,410 || 6,610 || 7,090 || 8,810 || 11,200
|-
| Jul || 4,720 || 6,400 || 6,920 || 12,800 || 13,600 || 16,600 || 22,200
|-
| Aug || 6,480 || 9,920 || 11,000 || 19,100 || 20,600 || 26,200 || 35,500
|-
| Sep || 5,510 || 8,990 || 10,800 || 18,500 || 19,800 || 26,300 || 36,700
|-
| Oct || 3,840 || 5,750 || 6,800 || 10,200 || 10,900 || 15,400 || 22,000
|-
| Nov || 2,510 || 3,790 || 4,230 || 5,410 || 5,710 || 7,780 || 10,900
|-
| Dec || 1,590 || 2,400 || 2,560 || 3,340 || 3,410 || 4,190 || 5,710
|}

There is little evidence from the last 45 years of data of any systematic changes in the hydrological regime of the Mekong.<ref name="MRC_2005"/>