By Bhanu Sridharan

Bengaluru, Jun 28 (IANS/Mongabay): Most rivers in South India begin as small streams in forested mountains. The Western Ghats are filled with such streams. As they flow downhill they join other streams and gain speed, depth and width to become large rivers like the west-flowing Nethravathi and the east-flowing Kaveri (Cauvery) rivers.

Suman Jumani, a freshwater ecologist and doctoral student, is fascinated by these streams. "Hill streams are really fast flowing clear streams. They gush through rocks and boulders and are highly oxygenated," she said. "And they support some really nice fish assemblages."

But this natural occurrence of streams may be altered, according to the researcher, because of small dams that have been built or are being planned across the Western Ghats.

Small Hydropower Projects (SHP) are simply small dams that produce smaller quantities of electricity. They are called run-of-the-river projects because the design of the dam is believed to incorporate the flow of the river and not alter it drastically.

But does small mean harmless? Jumani and her fellow ecologists from four Bengaluru based research and conservation organisations set out to understand how two small dams, in the Sakaleshpur taluka of the Karnataka Western Ghats, affected streams and the fish that depended on them, by comparing them to an undammed stream in the same region.

Explaining the rationale for the study, Jumani, who was with the Foundation for Ecological Research Advocacy and Learning (FERAL) at that time, said: "The basic premise is that the run-of-the-river projects don't alter river flows or rivers very significantly. What we wanted to see was how the river was physically affected."

Misconception about small dams being more "green" than large dams

Traditional large hydroelectric dams need a large reservoir of water, which is used to operate turbines that produce electricity. SHPs (in India anything less than 25 MW capacity is considered small) on the other hand don't need a large reservoir of water, so no land needs to be submerged.

Typically, a dam or weir is built upstream and water is diverted towards a downstream powerhouse where it rotates turbines to produce electricity. The water is then released back into the original stream bed.

This has led to the belief that SHPs are green energy sources, akin to solar power or wind power. To encourage more private players to build SHPs, the Indian government offers energy companies huge subsidies and has waived norms such as Environmental Clearances and Environmental Impact Assessments (EIA), even when the dams are built in forest areas.

Although SHPs don't need huge amounts of submerged land, they do divert water away from their natural course to operate turbines. Jumani and her colleagues found that this caused the dammed streams to change shape.

Typically, a stream gets wider and deeper as it flows down the mountain. This area covered with water is called the wetted width and wetted depth. In the undammed stream the wetted width increased from around nine metres upstream to 19 metres downstream and the depth from 50 centimetres to 80 centimetres.

But the dammed stream actually became narrower as it flowed down the mountain, towards the barrier, going from seven metres to just two metres. The depth went from 30 centimetres to mere three centimetres near the dam.

"Basically it narrowed down to a trickle," Jumani said.

The researchers then turned to water quality in the stretches near the dam. "Because the water was so reduced in volume, it was getting heated much faster," said Jumani. Warmer water also meant lower oxygen levels.

Since the most fundamental requirement for fish is water, this change in habitat quantity and quality had direct implications.

Small dams make fish habitats unsuitable for survival

So, what happens when a stream is shallower, warmer and has reduced oxygen levels? Jumani explains that hill stream fish have evolved to live in fast flowing, oxygen rich waters. So, the impact of the dams was really apparent on the fish communities.

The typical hill stream fish that Jumani spoke of stuck to the undammed streams. Specialist, endemic species like the blue spotted hill trout (Barilius bakeri) and the endangered Deccan mahseer (Tor khudree) - much loved by local communities as well as recreational fishers - were almost missing in the dammed streams.

These hill fish species were instead replaced by smaller generalist fish species like the Malabar danio (Devario malabaricus). "The local fishers call it "crap fish"," said Jumani, loosely translating from Kannada.

These findings are not surprising according to Kim Birnie-Gauvin, a freshwater ecologist and doctoral student from the Technical University of Denmark. Birnie-Gauvin who is not connected to this study, reported similar impacts of small dams affecting fish species like the brown trout (Salmo trutta) in Europe.

"Barriers (dams) completely alter the habitat upstream with slow-moving water," she said. "These fish are typically present in areas of high gradient, where the slope is steep. Barriers essentially remove that slope, so the fish find themselves in unsuitable habitat."

Both researchers pointed out that apart from altering habitat, small dams also impact migrating fish.

Every year, during the rains the Deccan mahseer travels upstream, fighting the current to reach the headwaters of streams and lays eggs in cooler waters. It is literally an uphill task, but if the mahseer spawns lower down the hill streams, the eggs would be at greater risk of drying out in warmer waters, Jumani explained.

Shallower upstream waters also ensure that the juveniles of these fish then grow peacefully without the threat of a larger predator.

But with small dams in the way, there may be no choice for the mahseer. The stretches near the dam where the stream is diverted can have little to no water when the mahseer begins its journey. "The dam essentially fragments the river," said Jumani. "Even when there is water, there is also a 10-15 feet wall (the dam) that the fish face. I don't think a mahseer can climb so high," she quipped.

In some countries SHPs do have openings for fish to move through, but Birnie-Gauvin believed they were inadequate. "Fish have trouble finding the outlet of a barrier during downstream movements because their flow is altered and slower," she explained about fish in Europe.

In any case, the small dams being planned in the Western Ghats, have no such outlets, pointed out Jumani.

Jumani, however is not entirely against small dams. "I think SHPs can be integrated into existing river projects," she said. "There are a lot of large dams in our country. The infrastructure is already built. The environmental cost has already been paid. But it makes no sense to do this in pristine rivers."