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DAMS on the River Rhône in Switzerland are threatening to turn the
crystal waters of Lake Geneva into a deoxygenated, weed-covered dead zone, say
researchers at the University of Geneva.

Over the past forty years, engineers have built nine large hydroelectric dams
on the tributaries of the Upper Rhône, the main source of water for the
lake—the shoreline of which is known as the Swiss Riviera.

One result has been to disrupt the seasonal flow of the river. The dams
capture the peak summer flows from melting snow and glaciers, and release the
stored water through turbines during the winter to provide electricity.
Crucially, the dams have eliminated the summer floods that were once a feature
of the river and which generated currents that brought badly needed supplies of
dissolved oxygen into the otherwise stagnant depths.

Now biologists Jean-Luc Loizeau and Janusz Dominik have calculated the impact
of the dams on the lake. They have found that the average number of days when
flows into the lake are high enough to generate these “density currents” has
fallen from 55 a year to just 15. As a result, they say, the lake bottom is
being starved of oxygen.

This deoxygenation could soon have unwelcome consequences at the surface,
says Loizeau. Lake Geneva contains a lot of phosphorus locked up harmlessly in
its sediments. But it is only held in place by “a barrier of oxidised sediments
where the sediment meets the water”, he says.

This barrier is maintained by oxygen in the bottom layers of the lake’s
water. Lose this deep-water oxygen, says Loizeau, and the barrier is breached,
allowing phosphorus to pour up into the lake. This over-fertilises the water and
sparks a process called eutrophication, where algae form smelly toxic tides
which suffocate the lake.

“It is important for the whole lake for there to be a minimum of 4 to 5
milligrams of oxygen per litre of water in the lake’s bottom,” says Loizeau.
Anything less and there will be a release of phosphorus. “It is possible, even
probable that this is happening now, but we are not yet sure.”

Worse still, the phosphorus will trigger the growth of plankton, which will,
in turn, consume more oxygen and allow even more phosphorus to escape . The net
result could be a runaway breakdown of the lake’s chemical health. “It is a
feedback loop,” Loizeau says.