Delhi’s 2023 Yamuna flood hit a record high with less water than 1978: here’s why

When the Yamuna flooded Delhi in 2023, the river reached its highest recorded water level in the city’s history, even though the actual volume of water flowing through was lower than during the devastating 1978 flood. A new study by geologists from Delhi University and the Indian Institute of Science Education and Research (IISER), Bhopal, says the explanation lies not in how much water the river carried, but in how little room it now has to carry it.

Published in the Journal of Geological Society of India, the study found that the Yamuna’s average width in Delhi has shrunk by nearly 68% over the past 200 years, from around 658 metres in 1799 to about 210 metres today, while its estimated discharge has dropped by 89%, from nearly 30,000 cubic metres per second to around 3,900 cubic metres per second since the late 18th century. With embankments and floodplain encroachment hemming the river in, the researchers say, even a smaller volume of water is now forced to stay confined within a much narrower corridor, pushing water levels higher during floods than they would otherwise be.

To trace this change, the researchers reconstructed 200 years of the river’s history along its 50-km Delhi stretch, drawing on historical maps from 1799, old topographic surveys, satellite imagery and river-width analysis. They found that a string of barrages, starting with the British-era Tajewala and Okhla barrages in the 1870s and followed by the Wazirabad, ITO and Hathnikund barrages, have diverted water upstream and reduced flow through the city.

Professor Vimal Singh of Delhi University’s geology department, one of the study’s authors, said the river’s shrinking floodplain reflects decades of encroachment enabled by its own flood pattern: since major flooding now happens for only around 15 days to a month each year, the rest of the floodplain came to be seen as spare land. ‘People started seeing this as vacant land and started encroaching upon it,’ he said, describing the pattern as one playing out with rivers ‘all over the world,’ not just the Yamuna.

The study also found that about a third of Delhi’s floodplains have become disconnected from the river over the last century, and that river islands and channel bars shrank from roughly 20 square kilometres in 1985 to about 4 square kilometres by 2020 — changes the authors describe as evidence that the Yamuna has shifted into a ‘lower flow’ regime driven by human intervention rather than natural climatic shifts.

Besides Singh, the research was conducted by professors Sampat Kumar Tandon and Tanya of Delhi University’s geology department and Kumar Gaurav of IISER Bhopal, who note that their historical estimates carry some uncertainty given the varying accuracy of old maps.

Image credit: Wikimedia Commons/by Goutam1962