Science

Even small dams have severe impact on river ecology

Research shows that they alter rivers and their fish communities drastically

It seems to stand to reason that small dams cause less environmental problems than large ones. But the first study on small hydropower projects in India proves that they cause as severe ecological impacts as big dams, including altering fish communities and changing river flows.

Such hydroprojects, which usually generate less than 25 megawatts of power and consist of a wall that obstructs a river's flow, a large pipe that diverts the collected water to a turbine-driven powerhouse to generate electricity and a canal that releases the water back into the river, are touted to be better than large dams because they submerge fewer regions and barely impact river flow. Such projects receive financial subsidies — even carbon credits — for being ‘greener’.

To see how green such small dams really are, scientists from organisations including Bengaluru’s Foundation for Ecological Research, Advocacy and Learning (FERAL) compared almost 50 kilometres of three river tributaries — over one undammed and two dammed stretches — of the Netravathi river in the Western Ghats of Karnataka.

They studied three zones in detail: above the dam (upstream), in the area between the dam’s wall and the powerhouse, sometimes completely devoid of water (‘de-watered’) and below the powerhouse (downstream). Here, they studied differences in water depth and width, which signify how much habitat is available to the river's denizens, and habitat quality through factors including dissolved oxygen content and water temperatures.

Their results show that changes in water flow in the dammed sections reduced the stream’s depth and width; water in these stretches was also warmer and had lower dissolved oxygen levels. These changes were most evident in the ‘de-watered’ zones and worsened in the dry seasons.

Habitat quality

This decrease in habitat quantity and quality showed in fish diversity too. The team found that un-dammed stretches recorded a higher diversity of fish species, including endemics (species seen only in the Western Ghats).

“The upstream and downstream stretches get disconnected and this impedes the river,” says Suman Jumani, lead author of the study and researcher at FERAL.

Such small hydro-projects cropping up on rivers in the Ghats is a serious worry, she adds, especially because they do not require environmental impact assessments.

“It is not a question of small versus big dams,” says Jumani. “Small dams are not necessarily bad if there are proper regulations in place.”

Regulations could include limiting the number of dams in a river basin or maintaining a minimum distance between dams on the same river stretch.

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Printable version | May 22, 2020 5:05:22 AM | https://www.thehindu.com/sci-tech/science/even-small-dams-have-severe-impact-on-river-ecology/article24122743.ece

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