Yamuna’s shrinking floodplains and increasing flood vulnerability in Delhi are the result of changes that have unfolded over more than two centuries, according to a study published in April this year in the Journal of the Geological Society of India by researchers from the University of Delhi (DU) and the Indian Institute of Science Education and Research (IISER), Bhopal.Reconstructing the river’s evolution using historical maps and satellite imagery, the study noted that barrages, embankments, and rapid urbanisation have altered the Yamuna’s natural seasonal flow and its ability to accommodate high flows during heavy rainfall.The study has estimated that rapid urbanisation has narrowed the river’s channel by about 68%, reduced its long-term channel-maintaining, or formative discharge by nearly 89%, and disconnected around one-third of its floodplains.Formative discharge is crucial, as it provides the flow a river needs to naturally shape its channel, remove accumulated sediment, and maintain its capacity to safely accommodate heavy rains.“These changes in river width, discharge, and land use… have reduced the natural capacity of the river to adjust to extreme events,” the authors wrote.How the study was carried outThe study, authored by researchers Tanya, Sampat Kumar Tandon, Kumar Gaurav, and Vimal Singh, analysed a 50-km stretch of the Yamuna flowing through Delhi using historical maps dating back to 1799, Survey of India records, and satellite imagery up to 2024. By tracing changes in the river’s width over time, the researchers reconstructed its hydrogeomorphic evolution, meaning how evolving landforms dictate water flow, and estimated changes in its formative discharge. In doing so, they studied the long-term average flow that shapes a river channel over decades.Also Read | From the Opinions Editor: In Yamuna floodplains, elsewhere, green action is hurting the poorKey findingsThe construction of the Tajewala, Wazirabad, and Okhla barrages has been a key driver of the Yamuna’s altered flow regime, according to the study. It noted that nearly 90% of the Yamuna’s flow is diverted at the Hathnikund Barrage upstream, leaving the Delhi stretch of the river with very little freshwater during the lean season. The study called the Delhi stretch “an ideal example of an anthropogenically modified river reach.”Story continues below this adComparing floodplain maps from 1912 with present-day data, the researchers estimated that around 45 sq km, roughly one-third of the river’s floodplains in Delhi, have been disconnected by embankments over the last century.These disconnected floodplains, the study said, have since seen agriculture, settlements, and infrastructure development, reducing the river’s natural space to spread during floods.The study also documented a sharp decline in sediment deposits that form within the river. Their total area has shrunk from around 20 sq km in 1985 to about 4 sq km in 2020, with much of it converted for agriculture and other uses.The implications became evident during the July 2023 floods. Although the flood carried less water than the 1978 flood, the Yamuna reached a higher water level in Delhi. According to the study, embankments and urban development have progressively constricted the river corridor, preventing floodwaters from spreading laterally across floodplains and forcing them into a narrower channel instead. “The consequences of these developments have reduced the natural buffer capacity of the Yamuna River corridor… thereby increasing the vulnerability to flooding events,” the paper stated.“The river requires a channel-maintenance discharge,” said Vimal Singh, faculty member at DU’s Department of Geology and one of the study’s authors. He stressed that the study estimates the river’s long-term flow responsible for maintaining the channel’s shape, not the river’s discharge in recent years. Singh said that ensuring environmental flows remains essential for sustaining aquatic ecosystems, but added that restoring the Yamuna can no longer be viewed solely through the lens of river flow.Story continues below this ad“River ecology is now much more complex because livelihoods are dependent on the river,” he said.He added that upgrading sewage treatment plants, increasing the use of treated wastewater instead of potable water wherever feasible, improving the condition of major drains such as the Najafgarh (Sahibi) drain, and reducing the growing dependence of Delhi’s expanding population on the Yamuna for freshwater would all be important measures for improving the river’s health.RecommendationsThe authors recommended that a “long-term hydrogeomorphic perspective” is necessary to understand how the river transitioned into its current “degraded situation.”While the study does not make specific policy recommendations, it cautioned that further constricting the river through embankments and continued encroachment on floodplains could increase flood vulnerability by reducing the Yamuna’s natural buffer capacity.It also called for floodplains to be recognised as dynamic systems rather than spaces to be engineered or reclaimed for development.