21.5 million people, one shrinking river: what 200 years did to Delhi’s Yamuna

Delhi’s population has grown from around 250,000 people in the early 19th century to roughly 21.5 million today. Over the same 200 years, a new study finds, the Yamuna river running through the city has moved in almost the exact opposite direction, with its average width shrinking by nearly 68% and its estimated discharge falling by 89%.

The study, carried out by geology researchers at Delhi University and the Indian Institute of Science Education and Research (IISER), Bhopal, and published in the Journal of Geological Society of India, reconstructed 200 years of change along the 50-km Delhi stretch of the river using historical maps dating back to 1799, old topographic surveys, satellite imagery and river-width analysis. They found the river’s average width fell from around 658 metres in 1799 to about 210 metres now, while discharge dropped from nearly 30,000 cubic metres per second to around 3,900 cubic metres per second.

Professor Vimal Singh of Delhi University’s geology department, one of the authors, linked the decline directly to the city’s growth. As Delhi expanded, he said, land along the river that only floods for around 15 days to a month each year began to be treated as available space rather than part of the river’s natural floodplain. ‘People started seeing this as vacant land and started encroaching upon it,’ Singh said, adding that a series of barrages built from the 1870s onward, along with embankments, diverted water and left little flow downstream.

The researchers found that roughly a third of Delhi’s floodplains have become disconnected from the river over the past century, and that the area covered by river islands and channel bars fell from about 20 square kilometres in 1985 to around 4 square kilometres by 2020, pointing to a major shift in how sediment moves through the river system.

The population-river contrast also showed up in recent flood data. During the 2023 floods, Delhi recorded its highest-ever water level even though the peak discharge was lower than during the 1978 flood, which researchers say happened because embankments and encroachment prevented the water from spreading sideways, forcing it to rise within a narrower channel instead.

Along with Singh, the study was authored by professors Sampat Kumar Tandon and Tanya of Delhi University’s geology department and Kumar Gaurav of IISER Bhopal. The authors caution that older historical maps vary in reliability, with some drawn for tourism rather than scientific purposes, so their figures should be read as estimates rather than exact historical measurements.

Image credit: Wikimedia Commons/by Goutam1962