Edgardo M. Latrubesse

LARGE RIVERS OF THE WORLD

GEOMORPHOLOGY, HYDROLOGY, ENVIRONMENTAL IMPACTS AND MANAGEMENT

Geomorphologists and hydrologists, have neither agreed upon how large a river needs to be to be considered “large”, or have identified objective criteria to categorize such rivers but the main criteria for identifying large rivers, have been a large drainage basin, and a high mean annual discharge.

Obviously drainage area is a helpful parameter to define large rivers. To study modern geomorphic processes, however, discharge is a key variable because channel and floodplain processes are strongly related to fluvial processes. Several fluvial systems with extensive drainage areas, for example, are located in dry regions and, thus, are often not considered large rivers.

The Global Commission on Continental Paleohydrology (GLOCOPH) Large Rivers Working Group, for example generally considers large rivers to have a mean annual discharge (Qmean) >1000 m3/s). Into this context,  two major areas of the planet concentrates the bigger proportion of the largest rivers: the global tropics and Siberia.

The role of large rivers draining continents is very relevant. For example, the 10 largest fluvial basins of the world (Amazon, Congo, Nile, Mississippi, Parana, Obi, Yenisei, Lena, Niger, and Amur) are responsible for draining nearly 21% of the
emerged land of our planet.

While the large Siberian basins are characterized by a very low population density, tropical rivers are crucial  because they concentrate good part of the gross domestic product of large countries like the Parana River in Argentina and Brazil, the Ganges-Brahmaputra in India, the Ayeyarwady in Myanmar, the Indus River in Pakistan, and the Mekong River in several countries of South East Asia. The same strategic role for commerce and roles play the Mississippi in the USA and the Yangtze River in China.

The environmental importance of tropical rivers is also outstanding. Large tropical rivers hold the highest biodiversity of fluvial fauna and alluvial vegetation and many of them, particularly those in Southeast Asia, are among the most hazardous systems for floods in the entire world. Tropical rivers draining mountain chains such as the Himalaya, the Andes and insular Southeast Asia are also among the most heavily sediment loaded rivers and play a key role in both the storage of sediment at continental scale and the transference of sediments from the continent to the Ocean at planetary scale. The large Siberian rivers are fundamental for the Arctic system but also extremely vulnerable to climate change.

Largest rivers in water discharge

Large Rivers of the World. (Qmean-=mean annual discharge; Qesp= specific discharge as normalized per basin area; ,Qsed= mean annual sediment load; Seed. Yield= tons of sediments per unit of drainage area (km2). Data from Latrubesse, E. (2015) Quaternary megafans, large rivers and other avulsive fluvial systems: a potential “who is who” in the geological record. Earth Science Reviews 146, 1-30.
Anabranching pattern of the Solimões (Amazon) River, Brazil. (picture by E. M. Latrubesse)

Large Rivers & Channel patterns
Classification of alluvial channel patterns as
a continuum from straight to meandering to
braided rivers has been adapted widely in fluvial
geomorphology and only slightly changed ever
since. For decades, meandering and braided
patterns have been the focus of fluvial research.
However, anabranching multichannel patterns
dominate the largest rivers of the world
(Latrubesse , E. 2008, Patterns of Anabranching channels: the ultimate end-member adjustments of mega-rivers. Geomorphology, 101, 130-145.).

Large rivers in the Anthropocene
The number of Anthropogenic stressors on large number of the large rivers is extreme. It includes river engineering such as large-scale damming, irrigation and flood defences; extreme and accelerate land use changes (LULC); hydro-geomorphological change; pollution, introduction of non-native species; sediment mining; destruction/modification of riparian system and related aquatic ecosystems, and extinctions of species.
Geopolitical conflicts among countries for water use and basin management also increase the environmental vulnerability of rivers as many of the larges fluvial basins spread on more than one country.
Reference: Best. 2019. Anthropogenic stresses on the world’s big rivers. Nature Geosciences

Dams
The impacts produced on rivers by humans in the twentieth and twenty-first centuries are dramatic and >45 000 dams above 15m retain 96 500 km3 of water, or about 15% of the total annual global river runoff. Dams also trap the sediments transported by the rivers  toward the Oceans and many of the floodplains and deltas of the world are suffering a dramatic starvation of sediment and nutrients. Hundreds of new dams are planned for construction, mainly in countries of Asia, Africa and Latin America, and megadams such as the ones in the Madeira basin and Belo monte in Xingu River, both tributaries of the Amazon, were recently constructed. More than 800 hundreds dams are planned in the Cerrado biome of Brail where some of the largest fluvial basins of south of the continent are such as Tocantins, Parana, Araguaia, and Sao Francisco.  In the Mekong basin. Currently approximately 18% of the total electric energy is produced by hydroelectric power.

If a man fails to honor the rivers, he shall not gain the life from them. — (Anonymous)