Study traces shrinking of Yamuna over 225 years in Delhi with 1799 map, reports 89 pc drop in volume

New Delhi, Jul 15 (PTI) The map of a wider, freer Yamuna from 1799 has opened a window into the river's past, helping researchers reveal the toll exacted by time, human intervention and urbanisation over more than 200 years in Delhi in a first-of-its-kind study. The research found that Yamuna flowing through Delhi has narrowed by about 68 per cent and its discharge -- the volume of water flowing through it -- has dropped by around 89 per cent since the late 18th century. The study, accessed by PTI, was carried out by researchers from the Department of Geology, University of Delhi, and the Indian Institute of Science Education and Research (IISER), Bhopal. They reconstructed the river's past using an archival map prepared by Upjohn in 1799 and preserved in the National Archives of India, along with historical maps and modern satellite images. The findings have been published in the paper, 'Two Centuries of Hydrogeomorphic Changes: Width-Discharge Dynamics of the Urbanised Yamuna River in Delhi'. "People have talked about changes in the Yamuna river in the Delhi stretch, but no one has talked about the changes in its discharge in the stretch on this timescale," Professor Vimal Singh, one of the researchers, said. The researchers said the 1799 map captured the Yamuna before any barrages were built across the river, offering a rare glimpse of its natural state. They found that the average bankfull width -- the width of the river when it is full but not overflowing its banks -- has reduced from about 658 metres in 1799 to around 210 metres in 2024. Using this width, the researchers estimated that the river's discharge has fallen from about 30,000 cubic metres per second in 1799 to roughly 3,900 cubic metres per second in 2024. The study said these changes took place during a period when Delhi's population grew from around 2.5 lakh in the early 19th century to nearly 2.15 crore in 2024, while a series of barrages, canals and embankments were built to divert water and regulate the river. According to the study, the first major intervention came with the British-built Tajewala barrage in 1873, followed by the Okhla barrage in 1874, the Wazirabad barrage in 1959 and the ITO Barrage in 1966-67. The study said these structures diverted large volumes of water upstream, fundamentally changing the river's natural flow through Delhi. The study also found that between 1912 and 2024, around 45 square kilometres of Delhi's floodplains -- the low-lying land beside the river that naturally stores floodwater -- became disconnected because of embankments built to protect the city from floods. Many of these areas were later converted for farming and urban development. Another striking finding was the sharp decline in channel bars, which are the naturally formed sandy islands within the river. Their total area fell from about 20 square kilometres in 1985 to just four square kilometres in 2020, with many being altered or converted into agricultural land. The researchers said these changes have weakened the river's natural ability to deal with floods. Floodplains normally allow excess water to spread out during heavy rainfall, but embankments and urban development now confine much of the flow within the river channel, increasing flood levels. They cited the July 2023 floods in Delhi as an example. Around 20,076 cubic metres of water per second was released upstream during the 1978 floods, which was nearly double the 10,187 cubic metres per second released during the 2023 floods. Yet, the river reached a higher water level in Delhi in 2023 because the river has become narrower and more constrained over time, researchers said. To reconstruct the Yamuna's transformation, the researchers compared the 1799 map with maps from 1893, 1924 and 1955, besides analysing Landsat satellite images from 1986 onwards and Sentinel-1 radar imagery. They manually measured the river's width and used an established relationship between river width and discharge to estimate how much water the Yamuna carried during different periods. Radar imagery was used during the monsoon because it can capture the river through cloud cover. The researchers noted that the historical maps have some limitations because mapping techniques from that period are not fully documented, and the 1799 map does not show channel bars. However, they said these records provide the best available evidence for reconstructing Yamuna's past over two centuries. The researchers pointed out that Delhi occupies just 0.05 per cent of India's land area but is home to nearly 1.5 per cent of the country's population, putting disproportionate pressure on the river. Drawing on historical records, census data and population projections, the study said Delhi had a population of around 1.5 lakh in 1630 during the reign of Mughal emperor Shah Jahan, which grew to about 2.5 lakh in the early 19th century. The city's population stood at around four lakh in 1901, rose to about 9.2 lakh by the 1940s and jumped to 17.44 lakh in the 1950s following Independence and Partition, before reaching a projected 2.15 crore in 2024. The researchers noted that the Yamuna river flows for about 50 kilometres through Delhi, one of the most heavily modified stretches of the Himalayan river. Around 22 kilometres of this stretch are controlled by the Wazirabad and Okhla barrages, while 18 municipal drains discharge into the river. The researchers concluded that human interventions, rather than natural changes, have transformed Yamuna in Delhi into a much narrower, lower-flow river that is less resilient to extreme weather events. PTI SGV PRK