Rivers play a dual and contradictory role in the political state.
Since the earliest civilizations, some rivers have united people more than they separated them.
In early Mesopotamia, the Tigris and Euphrates rivers were highways of internal trade, commerce, and communication.
Ease of transportation was crucial when riverine states in this region were in the process of formation, for this, in no small degree influences the extent of the political domain.
The same was also true of the NILE in early EGYPT.
The RHINE (in Europe), and the MISSISSIPPI (in the UNITED STATES), IRRAWADDY (in MYANMAR), Menam Chao Phraya (in THAILAND), Mekong (in VIETNAM and CAMBODIA), and Hwang Ho (in CHINA) play key roles as national unifiers today.
About one-fifth of the world's political boundaries are rivers.
The actual boundary lines of rivers follow along either a bank or the mid-channel of a stream.
Most river boundaries are of the mid-channel type in order to assure shared usage by adjoining political units.
Lakes, for the same reason, tend to have divisional boundaries.
The Canada-U.S.
boundary divides the Great Lakes and the CALIFORNIA-NEVADA boundary does the same for Lake Tahoe.
The CASPIAN SEA (actually a lake) in Central Asia, Lake VICTORIA in Africa, and the Bodensee (Lake Constance) in the ALPS are prominent international lakes.
Rivers in particular do not make perfect boundaries.
They give the illusion of permanence on a map- a trait valued by boundary makers-but stream courses do change.
For instance, the Mississippi, which for most of its length is a U.S.
interstate boundary line, has varied its course frequently, leaving parts of the left bank on the right side of the line and vice versa.
The international wanderings of the RIO GRANDE have made the problem of unstable river courses famous.
An 1853 survey drew the boundary between the U.S.
state of TEXAS and MEXICO down the middle of the Rio Grande.
The first of a series of disputes came in the wake of floods in 1864, which caused a change in the river's course that left a chunk of 630 acres (about 1 square mi or 2.6 square km) of land north of the river.
Several other wanderings resulted in loses or gains of land for both countries in the ensuing years.
As the region became more populated, control of the boundary was more difficult.
In 1884, the two countries agreed that the boundary should follow the abandoned river channel whenever the river changed course.
This meant that the area transferred from one bank to the other would remain under the sovereignty of the original state.
However, this policy diminished the function of the river as a boundary.
In 1905, in order to protect the integrity of the river as the boundary, the governments agreed to exchange land cutoff by the river, but only if the land area or the number of people living there was sufficiently large.
Otherwise, the river channel would remain the boundary.
A permanent commission was also set up to determine exchanges.
In the 1960s, the two governments finally stabilized the channel with concrete.
Land claim problems along the border are rare today.
The stabilization of the rivers like the Rio Grande is exceptional.
Around the world, meandering rivers create potential boundary problems such as the Rio Grande did.
Other disputes evolve around repositioning boundaries so that rivers become boundaries.
The list below contains examples of rivers and lakes that are the bases of recent boundary disputes.
Amazon and Maranon rivers (ECUADOR and PERU) Armur River (China and RUSSIA) Atrak River (KYRGYZSTAN and TAJIKISTAN) Belesa-Mareb-Setit Rivers (ERITREA and ETHIOPIA) Caspian Sea (AZERBAIJAN, IRAN, KAZAKHSTAN, Russia, and TURKMENISTAN) Congo/Zaire River (Democratic Republic of the CONGO and Republic of the Congo) Essequibo River (GUYANA and VENEZUELA) Ganges-Brahmaputra-Meghna rivers (India and China) La Plata River (BRAZIL and URUGUAY, and Brazil and PARAGUAY) Lake Chad (CAMEROON, CHAD, NIGER, and NIGERIA) Lake Malawi (MALAWI and TANZANIA) Lake Tanganyika (Democratic Republic of the Congo, ZAMBIA, and Tanzania) Linyanti River (BOTSWANA and NAMIBIA) Maroni River (SURINAME and FRENCH GUIANA) Mekong River (LAOS and Thailand) New River (Guyana and Suriname) Orange River (Namibia and SOUTH AFRICA) Sara and Una rivers (BOSNIA-HERZEGOVINA and SERBIA, and HUNGARY and SLOVAKIA)#