Dam It: Part 2
When what was river becomes land
Last time, I outlined the effects of dams on the areas immediately surrounding them. Things that happen when land is covered by water, and the watery side turns into dry land.
Those effects are easy to guess at. You probably thought of some of them already. But what about further downstream? The little-known facts, about what else a dam does, are what we’ll be going into this week.
When you think of the weight of a dam, you’d think of the walls, to hold the water in, and the strong foundation to keep it stable. But that’s not all.
Making a dam also involves collecting a whole lot of water. That water gets shifted from the riverbed onto the reservoir, and adds itself to the relatively tiny weight of the actual dam.
And water is heavy. Very heavy. In fact, it weighs about a kg per litre. And with multiple cubic kilometres of water in any place, something seems bound to happen. And sometimes, it does.
The Earth’s crust is like a jigsaw of pieces, pieces that are constantly grinding and shifting and pushing against one another. And at the boundaries of these pieces — or fault lines, to use the technical term — there is a tremendous amount of pressure. It the kind of pressure that formed the Himalayas, Mount Kilimanjaro, the Pacific Ring of Fire.
In some places, those fault lines become dormant. And they often stay dormant with no signs of tension for decades, even centuries.
But when you combine a potential tension hotspot with the weight of 1,000,000,000,000 litres of water—that’s a billion tonnes !— then things tend to go awry.
Awry, that is, in the form of an earthquake.
In December 1967, the Koyna Dam in Maharashtra is said to have triggered an earthquake of magnitude 6.6. In September 1993, the Latur Dam, just ten metres high, caused an earthquake that read 6.2 on the Richter scale.
Both were rated “Severe” and the damage was widespread. The dams stood. Will others in the future be as lucky?
This point is rather more obvious than the last, but often overlooked.
When you dam a river, you’re stopping the water from flowing downstream. Depending on the case, either all or most of the water is contained. That also means that either little or none of the water that should have been flowing downstream makes it across.
Rivers are an important source of irrigation and fulfil millions of peoples’ everyday water needs. Rivers create jobs: fishing doesn’t end at the fisherman — after him comes processing, transport, export, wholesale, retail. Rivers are foundations for entire ecosystems and societies.
Take away the river and you lose all the rest.
‘Delta’ is the term for the place where the river meets the sea, so named because it usually looks like the Greek letter ‘Δ delta’. Here, the river widens, flattens, and slows down to a crawl before it reaches the sea. Here, the steady, powerful flow of water keeps the sea at bay. And here is where, if there is a dam, the river fails to reach.
And then? There’s nothing left to keep the seawater in the sea!
There’s a natural process that occurs every summer in most deltas: the salty water floods the plains of the delta, up to several kilometres inland. Come rainy season, the river flows back, washing the salts back out of the soil. But, with a dam blocking the flow of the river all year round, the salt water stays put — all year round as well.
This is bad news for all the animals and organisms that require this cyclical back and forth of freshwater and salt water . With high and ever-increasing saltiness, the quality and quantity of their habitat slowly dwindles.
This is bad news for humans, too. Often, delta plains are farmed for their high water content and fertility. High salinity means that this can no longer happen.
In some cases, when land is too high for the seawater to flow in, the land doesn’t get salty. It simply dries up. But this is no better than salinization — in fact, it could even be worse.
Water has volume. Obvious, I know — but stay with me for a second. That volume doesn’t disappear when water leaves the surface. It occupies the same amount of space underground as well.
In places with a high water table, like in a delta, this is crucial. The underground water is responsible for much of the height of the land. And so, when the water evaporates, the land sinks down. Entire plains can, quite literally, lose decimetres in height.
This, in turn, leaves them more vulnerable to saltwater floods.
Some prominent examples of this are the Nile delta and the Sacramento river delta. In both places, they’ve had to build walls to keep the seawater out.
Kind of like a dam in reverse — but without the benefits.
Dams also trap sediments that flow along with the river: bits of rock and soil that are eroded by weathering, and then carried along by the flowing water. In the Ganga river, for example, 403 to 602 thousand tonnes make their journey down to the sea, every year.
Why is this important? Well, silt is a major player in the maintenance of the coastline, as all of it is deposited across the delta and near the mouth. It balances the erosion, the washing away of the ravine and the valley that happen on a near constant basis.
But when sediments remain trapped at the dam, there’s nothing to counter the erosion of river banks, beds and the coastline. That could compromise the water table, decrease variety in ecosystems and habitats, and impact wider systems of sedimentation and erosion.
We don’t know how much further these effects extend. Some of it may be good, and some of it may be bad.
The upsides and downsides of dams have been likened to a balance sheet.
Electricity to heat a million households.
Ecology left out in the cold.
Freshwater for millions of people.
Kilometres-square submerged below.
Irrigation for a million farmers.
Field deserted for two hundred thousand.
Clean water for a million.
Bare ground for another.
Is it worth it?
Dam It: ‘Deserting Rivers’ is the second of a two-part series outlining the downsides of large damming projects. Check out the first part, if you haven’t yet!
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