River carrying material is deposited inside reservoirs (where water is stored by dams). With time, this “silt” or “sediment” builds up at the bottom of the reservoir, reducing the amount of water it can actually hold. A study was conducted regarding the same by Researchers Uma Madhuri Mogili and Somil Swarnkar from the Indian Institute of Science Education and Research Bhopal, who looked at nearly 370 large reservoirs — capacity of over 100 million cubic metres — in seven regions, including the Himalayan and Indo-Gangetic Plains. The study has been published in the journal Stochastic Environmental Research and Risk Assessment.
As per the analysis, "significant regional disparities in sedimentation impacts", with most of the regions exhibiting a vulnerability due to "steep terrain, high erosion, and monsoonal sediment inflows".
"Multiple reservoirs exhibit storage capacity losses exceeding 50 per cent, severely undermining their ability to mitigate floods and manage droughts," according to the study.
The reservoirs in the Himalayan region showed the highest loss in yearly storage capacity, further stressing their critical vulnerability, according to the researchers in their study.
The team also projected that by 2050, 10 reservoirs on the east-flowing rivers and seven on west-flowing ones might surpass a 50 per cent loss in storage capacity, "posing a severe threat to water security".
The majority of the reservoirs were found to have become operational between 1939 and 1991 — in line with India's post-independence infrastructure development phase, the authors added. East-flowing rivers beyond Godavari, having a high number of reservoirs (160), are a critical zone in meeting south India's agricultural and industrial water needs, they added. However, west-flowing rivers up to the Narmada were noted to have fewer, but larger reservoirs.
The authors then noted that the lack of reservoirs in the Indo-Gangetic Plains and the Himalayan region reflects the distinct difficulties of each landscape: the latter relies on natural river flows, while the former faces challenging mountain terrain. The authors underlined that these regional variations highlight the necessity of customised water management plans appropriate for India's diverse hydrological and geographical circumstances.
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