On September 23, 2025, Kolkata woke up to a deluge that had submerged its roads, neighbourhoods and Durga Puja pandals following hours of intense, uninterrupted rainfall.
The biggest crisis was that despite attempts to flush out the floodwater and bright sun throughout the day, the city remained inundated for several hours. At least 10 people lost their lives, many being electrocuted.
This is not a common civic crisis in Kolkata, which is in a flood-prone region, receiving heavy rainfall every year.
A section of experts said the flood was caused by the overflowing River Hooghly, while many residents termed the situation a government failure.
To get to the bottom of the problem, it is important to reckon with the history of the riverine and drainage system of the city. The potential ticking public health hazard calls for immediate action on restoring these flows.
Kolkata was a complex wetland with good water connectivity to the Sundarbans. The British settled here because the west was already occupied by other European colonial powers, and they wanted to exploit the waterways for trade.
Canals were constructed and linked from the river to the wetlands in Kolkata for transport and drainage. Some creeks were converted into canals, leaving the landscape with many broken rivers. These broken rivers and lost creeks, such as the Bidyadhari, Matla, Adi Ganga, Jamuna, have reduced the capacity to drain water in the region.
The illustrations created by artist Koustabh Chakraborty are from the book Towards Sustainable Flows (2024) by Jaladarsha.
The topographical features of Kolkata are derived principally from the interactions of the rivers from the Gangetic system and the sea. The major slopes have been from north to south; superimposed on these, are slopes towards the east.
Originally, canals were constructed for navigational routes. Due to the eastward slope of the land, the canals used to carry the wastewater to the salt water lakes in the east and from there to the Bay of Bengal through the River Bidyadhari. Numerous ponds stored large quantities of rainwater keeping the risk of water-logging minimal. As the population of Kolkata grew with urbanisation, the initial canals were inadequate as a natural drainage system. The then government devised elementary measures to drain wastewater and sewage to the Hooghly which was against the natural slope of the land.
In 1829, the Circular Canal was built on the recommendation of the “Improvement Committee” (1827). This was inadequate for flood management when floodwater flowed in from both River Hooghly and the saltwater lakes during heavy rains.
Thus, steam-powered lockgates and sluice gates were recommended to regulate the flow. Several pumping stations were proposed to flush out floodwater into the canals, rivers and saltwater lakes as early as in 1857.
This archaic water engineering system has been the basis of flood management over all the years till 2000. Post-2000, foreign aid, loans and water infrastructure improvement programmes looked at maintaining and repairing these canals and building new pumping stations and repairing the city’s sewers.
However, citizens complained of the poor maintenance of the canals during the same period, prompting the National Green Tribunal to take cognizance of the matter and issue orders to remedy the situation.
Civil society organisations and independent experts have been engaged in evidence-based research to identify waterbodies in and around the city that can be revived to improve drainage. Jaladarsha, the team that developed the above illustrations, studies the collective system of waterscapes in the city to understand its system of transportation and discharge of waste into East Kolkata Wetlands (EKW) and Hooghly, and the relationship local communities have to these waterbodies.
In 2024, the team conducted a spot-wise observation of this water network. Their team found that a large part of the Circular Canal from ward 13 and 14, a major part of Stormwater Flow and Dry Weather Flow channel, Tolly Nullah, Tolly-Panchannagram and Bagjola canal were stagnant. Rampant plastic garbage dumping and irregular maintenance have choked the canals, interrupting waterflow.
These canals finally flow towards the EKW, considered the ‘kidneys’ of the city, which follows a natural eutrophication process of treating sewage water from the city. Rapid urbanisation has led to a shrinkage in the capacity of these wetlands. A major portion of the Salt Lake area is concretised, adding to the problem.
The population density in Kolkata is 24,000 persons per square kilometre, according to the national census of 2011 census. This creates the greatest pressure on urban ecology, in terms of land and water use, and solid waste management.
The interconnected waterbodies are Kolkata’s sponges. A greater planning focus on conserving them can ensure the city isn’t crippled by severe rainfall events that are projected to become common as Earth warms.
Sayantoni Datta and Souvik Das are researchers with Jaladarsha, a team of researchers, artists, community process workers leading water and climate justice in Kolkata and the Sunderbans. Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth.