The ceremony in Washington, President Roosevelt pressed the golden key to set in motion the

giant cataract of Boulder Dam hundreds of miles away.

Twelve great valves opened to unleash twelve cascades of water at the rate of 30,000 tonnes

a minute.

Boulder Dam not only irrigates the drylands of California and the southwest, she drives

two million horsepower generators for electric power.

Standing at just over 221 metres tall, the Hoover Dam or Boulder Dam, as it was then known,

was the largest dam in the world when it was completed in 1936.

Straddling the Colorado River in the Western United States, this marvel of engineering came

to symbolise modernity and growth in the face of the Great Depression.

Dam's can control floods, provide irrigation and generate power in the form of hydroelectricity.

Today, there are around 3,700 hydropower dams planned or under construction, mostly in developing

countries.

Globally, dams now supply 71% of renewable energy.

But not everyone is a fan of the dam.

With opposition, citing environmental impacts, the displacement of communities and significant

costs.

So just how cost-effective and sustainable are large dams?

Bent fluvia is BT Professor of Major Program Management at Syed Business School at the University

of Oxford.

He was involved in one of the biggest studies of mega-tams around the world, comparing

projected costs with actual costs with some interesting results.

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