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==Geology==
The internal drainage patterns of the Mekong are unusual among those of large rivers.<ref name="MRC_2010a" /> Most large river systems that drain the interiors of continents, such as the [[Amazon River|Amazon]], [[Congo River|Congo]], and [[Mississippi River|Mississippi]], have relatively simple [[wikt:dendritic|dendritic]] tributary networks that resemble a branching tree.<ref>{{cite journal |author1=M. Clark |author2=L. Schoenbohm |author3=L. Royden |author4=K. Whipple |author5=B. Burchfiel |author6=W. Zhang |author7=W. Tang |author8=E. Wang |author9=L. Chen |year=2004 |title=Surface uplift, tectonics, and erosion of eastern Tibet from large-scale drainage patterns |journal=[[Tectonics]] |volume=23 |issue=TC1006 |pages=227–234 |doi=10.1029/2002TC001402 |bibcode=2004Tecto..23.1006C |doi-access=free }}</ref>

Typically, such patterns develop in basins with gentle slopes where the underlying geological structure is fairly homogeneous and stable, exerting little or no control on [[river morphology]].<ref name="Twidale2004">{{cite journal |author=C. Twidale|author-link=Charles Rowland Twidale |year=2004 |title=River patterns and their meanings |journal=[[Earth-Science Reviews]] |volume=67 |issue=3–4 |pages=159–218 |doi=10.1016/j.earscirev.2004.03.001 |bibcode=2004ESRv...67..159T}}</ref> In marked contrast, the tributary networks of the [[Salween River|Salween]], [[Yangtze River|Yangtze]], and particularly the Mekong, are complex with different sub-basins often exhibiting different, and distinct, drainage patterns. These complex drainage systems have developed in a setting where the underlying geological structure is heterogeneous and active, and is the major factor controlling the course of rivers and the landscapes they carve out.<ref>{{Cite book |author1=S. K. Tandon |author2=R. Sinha |name-list-style=amp |year=2007 |chapter=Geology of large river systems |pages=7–28 |editor=A. Gupta |title=Large rivers: geomorphology and management |location=London |publisher=[[John Wiley & Sons]] |isbn=978-0-470-84987-3}}</ref>

The elevation of the [[Tibetan Plateau]] during the Tertiary period was an important factor in the genesis of the south-west [[monsoon]],<ref name="CliftPlumb2008">{{cite book |author1=P. D. Clift |author2=A. R. Plumb |name-list-style=amp |year=2008 |title=The Asian monsoon: causes, history, and effects |location=Cambridge, UK |publisher=[[Cambridge University Press]] |isbn=978-0-521-84799-5}}</ref> which is the dominant climatic control influencing the hydrology of the Mekong Basin. Understanding the nature and timing of the elevation of Tibet (and the [[Central Highlands (Vietnam)|Central Highlands]] of Vietnam) therefore helps explain the provenance of sediment reaching the delta and the [[Tonle Sap]] Great Lake today. Studies of the provenance of sediments in the Mekong delta reveal a major switch in the source of sediments about eight million years ago [[Year#SI prefix multipliers|(Ma)]].<ref>{{cite journal |author1=P. D. Clift |author2=A. Carter |author3=I. H. Campbell |author4=M. Pringle |author5=V. Nguyen |author6=C. Allen |author7=C. M. Allen |author8=K. V. Hodges |author9=T. T Mai |year=2006 |title=Thermochronology of mineral grains in the Red and Mekong Rivers, Viet Nam: Provenance and exhumation implications for Southeast Asia |journal=[[Geochemistry, Geophysics, Geosystems]] |volume=7 |issue=10 |pages=1–28 |doi=10.1029/2006GC001336 |bibcode=2006GGG.....710005C |doi-access=free }}</ref><ref>{{cite book |doi=10.1029/149GM14 |chapter=Marine sedimentary evidence for monsoon strengthening, Tibetan uplift and drainage evolution in East Asia |title=Continent-Ocean Interactions within East Asian Marginal Seas |series=Geophysical Monograph Series |date=2004 |last1=Clift |first1=Peter D. |last2=Layne |first2=Graham D. |last3=Blusztajn |first3=Jerzy |volume=149 |pages=255–282 |isbn=0-87590-414-9 }}</ref> From 36 to 8 Ma the bulk (76%) of the sediments deposited in the delta came from erosion of the bedrock in the [[Three Parallel Rivers of Yunnan Protected Areas|Three Rivers Area]]. From 8 Ma to the present, however, the contribution from the Three Rivers Area fell to 40%, while that from the [[Central Highlands (Vietnam)|Central Highlands]] rose from 11 to 51%. One of the most striking conclusions of provenance studies is the small contribution of sediment from the other parts of the Mekong basin, notably the Khorat Plateau, the uplands of northern Laos and northern Thailand, and the mountain ranges south of the Three Rivers area.

The last glacial period came to an abrupt end about 19,000 years ago (19 [[Year#SI prefix multipliers|ka]]) when sea levels rose rapidly, reaching a maximum of about {{cvt|4.5|m|ft}} above present levels in the early Holocene about 8 ka.<ref name="Tamuraetal2004">{{cite journal |author1=T. Tamura |author2=Y. Saito |author3=S. Sotham |author4=B. Bunnarin |author5=K. Meng |author6=S. Im |author7=S. Choup |author8=F. Akiba |year=2009 |title=Initiation of the Mekong River Delta at 8 ka: Evidence from the sedimentary succession in the Cambodian lowland |journal=[[Quaternary Science Reviews]] |volume=28 |issue=3–4 |pages=327–344 |doi=10.1016/j.quascirev.2008.10.010 |bibcode=2009QSRv...28..327T}}</ref> At this time the shoreline of the [[South China Sea]] almost reached Phnom Penh and cores recovered from near Angkor Borei contained sediments deposited under the influence of tides, and salt marsh and mangrove swamp deposits.<ref name="Tamuraetal2004" /> Sediments deposited in the Tonle Sap Great Lake about this time (7.9–7.3 ka) also show indications of marine influence, suggesting a connection to the South China Sea.<ref>{{cite journal |author=D. Penny |year=2006 |title=The Holocene history and development of the Tonle Sap, Cambodia |journal=[[Quaternary Science Reviews]] |volume=25 |issue=3–4 |pages=310–322 |doi=10.1016/j.quascirev.2005.03.012 |bibcode=2006QSRv...25..310P}}</ref> Although the hydraulic relationships between the Mekong and the [[Tonle Sap]] Great Lake systems during the [[Holocene]] are not well understood, it is clear that between 9,000 and 7,500 years ago the confluence of the Tonle Sap and the Mekong was in proximity to the South China Sea.

The present river morphology of the Mekong Delta developed over the last 6,000 years.<ref name="MRC_2010a" /> During this period, the delta advanced {{cvt|200|km|mi}} over the continental shelf of the South China Sea, covering an area of more than {{cvt|62500|km2|mi2}}. From 5.3 to 3.5&nbsp;ka the delta advanced across a broad embayment formed between higher ground near the Cambodian border and uplands north of [[Ho Chi Minh City]]. During this phase of its development the delta was sheltered from the wave action of long-shore currents and was constructed largely through fluvial and tidal processes.<ref>{{cite journal |author1=T. K. Ta |author2=V. L. Nguyen |author3=M. Tateishi |author4=I. Kobayashi |author5=S. Tanabe |author6=Y. Saito |year=2002 |title=Holocene delta evolution and sediment discharge of the Mekong River, Southern Viet Nam |journal=[[Quaternary Science Reviews]] |volume=21 |issue=16–17 |pages=1807–1819 |doi=10.1016/S0277-3791(02)00007-0 |bibcode=2002QSRv...21.1807T}}</ref> At this time the delta was advancing at a rate of {{cvt|17|to|18|m|ft}} per year. After 3.5&nbsp;ka, however, the delta had built out beyond the embayment and became subject to wave action and marine currents. These deflected deposition south-eastwards in the direction of the [[Cà Mau Peninsula]], which is one of the most recent features of the delta.

For much of its length the Mekong flows through bedrock channels, i.e., channels that are confined or constrained by bedrock or old alluvium in the bed and riverbanks.<ref name="MRC_2010a" /> [[geomorphology|Geomorphologic]] features normally associated with the [[alluvial]] stretches of mature rivers, such as [[meander]]s, [[oxbow lake]]s, cut-offs, and extensive [[floodplains]] are restricted to a short stretch of the mainstream around [[Vientiane]] and downstream of [[Kratié (city)|Kratie]] where the river develops alluvial channels that are free of control exerted by the underlying bedrock.

The Mekong basin is not normally considered a seismically active area as much of the basin is underlain by the relatively stable continental block. Nonetheless, the parts of the basin in northern [[Laos]], northern [[Thailand]], [[Myanmar]] and China do experience frequent earthquakes and tremors. The magnitude of these earthquakes rarely exceeds 6.5 on the [[Richter magnitude scale]] and is unlikely to cause material damage.<ref>{{cite journal |author=C. H. Fenton, P. Charusiri & S. H. Wood |year=2003 |title=Recent paleoseismic investigations in northern and western Thailand |journal=[[Annals of Geophysics]] |volume=46 |issue=5 |pages=957–981 |hdl=2122/998}}</ref>{{Page needed|date=December 2015}}