NARRATOR：

Listen to part of a lecture in an environmental science class.

FEMALE PROFESSOR：

Now, there's growing interest these days in generating electricity from renewable energy sources, right? from developing wind farms, to tapping into an underground source of geothermal energy.

And when you're considering a new project, it's important to look at the costs, as well as benefits, of developing that energy source.

Let me give you an example of the kind of thing I'm talking about.

There's currently a lot of interest in harnessing the power of the oceans—of the ocean tides, that is...the movement of huge amounts of water, which causes the water level of oceans to rise and fall.

The idea is that if we can harness that tidal energy, it'd be a great, clean, renewable energy source.

One place where this tidal energy can be harnessed is at a shallow body of water, such as an estuary.

Now, can anyone tell us what an estuary is? Yes, Ted.

MALE STUDENT：

An estuary's where a river enters the ocean... the fresh water meets the ocean water.

Sometimes it's covered in water; sometimes it's not...

FEMALE PROFESSOR：

Some parts of the estuary—as the tides go in and out... but other parts are always submerged.

Now, estuaries are an ideal place to try to capture energy from changes in tides because, well, there's an exceptionally large difference between the water level at high tide and at low tide.

All that movement of water generates a lot of energy.

And one way to harness that energy is by building a structure called a barrage there.

A barrage is basically a large low dam that's built across an estuary.

When the tides go in and out, the moving water flows through tunnels in the barrage.

So you have huge amounts of water trying to flow through these relatively small tunnels...and that turns turbines that generate electricity.

Now, these estuaries are important because of their high level of biological productivity.

They're home to lots of birds, fish, and other marine life.

So when you propose to construct a barrage, you have lots of issues to consider.

For example, it would change the existing water levels in the estuary...since a lot of water is getting held up by the barrage, the incoming tides won't go as high... but they wouldn't be as low during low tide, either.

This might help prevent flooding, but it would also affect the mud flats—those areas of mud that normally are exposed when the tide recedes.

MALE STUDENT：

But don't lots of birds rely on exposed mud flats for food? I mean, don't they eat tiny animals that live in the mud?

And what about those tiny animals?

What would happen to them if the mud flats were endangered?

FEMALE PROFESSOR：

So you're seeing some of the potential problems with a barrage.

But consider this...

Right now the water in an estuary is very cloudy.

The tidal currents are constantly churning up the sediments that rivers deposit in the estuary.

But a barrage would reduce the tidal currents, so a lot of that sediment would settle to the floor of the estuary; it wouldn't get stirred up so much.

The water would be clearer... allowing more sunlight to reach deeper into the water... which might lead to more food for birds and other animals...attracting new wildlife to the area.

So it's a complicated environmental picture.

MALE STUDENT：

Have they tried this anywhere... built a barrage?

FEMALE PROFESSOR：

Yes, there are several in operation.

There's one in France.

They have to be careful there about how they turn on the turbines, because they create currents and waves that can affect boats.

But I haven't read about any major ecological problems.

In fact, the fishing is supposed to have improved...plus there's even more bird life.

But some of the barrages we're considering now would be much larger than that one.

There's been one proposed for the estuary of Great Britain's Severn River...one of the largest estuaries in the world.

It would be 16 kilometers long... just to give you an idea, the barrage in France is less than a kilometer.

Outside of environmental concerns, such a project would be hugely expensive.

And that's another argument against barrages—well, such large ones anyway.

Critics say that it would be better to use that money for something else, such as improving the energy efficiency of buildings.

You could make a lot of buildings more efficient with all that money, and that would reduce the need for electricity.