How Dams Work

So let’s take a look, a strange
look, at a river. This is going along the
river towards the sea. And this river happens
to have trees that grow up on the riverbank. So you can get an idea how
we’re looking at this. And what we want to do is ask
what happens when we build a dam on this river? And we’re concerned about the
future of a couple of houses that used to be along the river,
one just below the dam right down here, and the other
one up above the dam up here, sitting along the riverbank. Well, they build the dam, and
the dam fills with water, and it doesn’t quite take out
the house up above. But the river’s carrying mud. It’s carrying sediment. And the sediment starts to
deposit out into the lake to fill it with a delta. Now, rivers have
to go downhill. So as the lake is replaced with
the mud, if the river were to hit a perfectly flat
spot like this, you know what it has to do. It has to build up so that
it’s headed downhill. And so as the river builds the
delta out into the lake to fill the lake, why, you have
to bury the house, and that makes the person who
lives there mad. Now, at the other side, it’s
even more interesting. There’s no floods anymore, so
the river loses the ability to carry gimongous rocks,
which might make the homeowner happy. But the water coming out
from the dam is clean. It has no sediment in it. And if there’s sand below the
dam, why, the water will start washing that sand away. And that will do a
number of things. Your house, now, rather than
sitting there right next to the riverbank– you’ll look out some morning and
you’re ready to fall into a giant hole that’s been cut. And you get out there and you
fall in and then you say, oh, no, and you’re very unhappy. You are not nearly as unhappy as
the salmon that are trying to come upstream, because as you
may know, salmon like to get really amorous around
sand and gravel bars. And if the river has washed away
the sand, then you can’t have fun with your honey
and then leave your eggs there to do well. And so you have a big loss in
salmon, as well as getting people’s houses unhappy. And so putting a dam on a river
makes a big difference.

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