When did humans start to influence rivers? A contribution to the Anthropocene debate

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• Professor Martin Gibling, Department of Earth and Environmental Sciences, Dalhousie University
• Monday 13 February 2023, 18:00-19:00
• Tilley Lecture Theater, Department of Earth Sciences.

If you have a question about this talk, please contact Buck Blake.

The term Anthropocene, first used informally by Paul Crutzen and Eugene Stoermer in 2000, has been widely adopted to indicate how profoundly humans have modified the Earth system. A proposal is being discussed to define a geological Anthropocene Epoch, commencing around 1950. Through the 20th century, the proliferation of big dams, irrigation projects and industrial pollutants have profoundly affected rivers worldwide. However, humans have always lived along rivers and began to modify them much earlier in the Quaternary, as shown by evidence from sedimentology and archaeology.

The use of fire, perhaps by 1.6 Ma in Africa, may have been the first anthropogenic influence on river landscapes. Indigenous use of fire in hunting may have affected Australian vegetation by 45,000 years ago (45 ka), although this is controversial. Hunter-gatherers collected and processed grain on floodplains and uplands before 30 ka, but the rise of agriculture and associated deforestation greatly enhanced sediment runoff, creating river “legacy sediments” that date back to nearly 8 ka in New Guinea. Preceded by cultivation of wild cereal species, domestication of wheat, barley and other founder crops after about 10.7 ka led to organised agriculture on river plains across the Near East by 9.8 ka. Rice was domesticated by 9 ka in China, with widespread ricefield irrigation after 7 ka, and maize and squash were cultivated in Mexico by about 9 ka. Animal husbandry affected river landscapes through grazing, transport routes along rivers, and ploughing. Cattle, sheep, goats and pigs were domesticated by 10 ka, yaks by 7 ka, donkeys and horses after 7 ka, and water buffalo by 5 ka.

Settled communities along rivers go back to at least 14 ka in the Near East, with Jericho founded at a spring around 10 ka. Large riverside cities developed after about 7.4 ka in the empires of Mesopotamia, Egypt, and the Indus Valley. The growth of cities was associated with urban supply systems and irrigation canals, soon leading to salination of floodplain soils in Mesopotamia. By 3 ka, principles of groundwater extraction were well understood, bringing subsurface water to fields. Floodplain clays were worked for pottery after around 20 ka, for ochre by around 15 ka, and for bricks by around 9.5 ka. River boats and infrastructure were in use early in the Holocene, and papyrus has been harvested from rivers since at least 5 ka.

Following early brushwood and earth dams, the Egyptian Pharaohs built the first stone-faced dam in 4.5 ka, and the Romans constructed 45 large dams in the Middle East. Chinese emperors engaged in enormous engineering schemes along the Yellow River over the past several thousand years. Water mills and ponds affected river sediments and depleted fish stocks across Europe in the Middle Ages.

In summary, anthropogenic activities influenced rivers in many ways from the latest Pleistocene onward, intensifying through the Holocene. Such profound modifications were diachronous and left their mark not only in the geological record but also on biological communities and human cultures. It seems appropriate to represent a transformative change over this long period as an Anthropocene Event (Gibbard et al., 2021), allowing the term to be applied to a range of disciplines and societal applications.

Gibbard, P.L. et al., 2021, A practical solution: the Anthropocene is a geological event, not a formal epoch. Episodes, Nov. 15, 2021.

This talk is part of the Sedgwick Club talks series.

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• Department of Earth Sciences seminars
• Sedgwick Club talks
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