Chapter 2 - 3

03 Geography

Now listen to a lecture related to the topic in step 1. Fill in the blanks of the note diagram below with keywords or key phrases used in the lecture.

M: So basically, a dem restricts the normal flow of water and has a lot of negative side effects.

Sometimes, in order to reverse these side effects of dams, we’ll release a lot of water from the dam at one time − in other words, we’ll flood the river.

But rather than solving the problems introduced by the dam, this flooding process is actually extremely bad for the river.

Since dams kind of collect the silt and don’t let it run downstream beyond the dam, the river below the dam doesn’t get many of the nutrients.

Some think flooding the river would introduce more silt into the river, but it doesn’t really fix this. Sure, flooding lets a bunch of silt out from the dam, but the increased flow of the river just carries the silt away and, in some cases, the larger flow of the river… it can actually remove more silt from the bed of the river below the dam.

That’s clearly not good for the river, since silt has a lot of nutrients important to life in and along the river.

Flooding the river also destroys natural sandbars. At first, you’d think that flooding the river would restore natural sandbars − after all, a large flow of water does carry lots of sand.

However, actual tests have shown that it just doesn’t work like this. Once again, we have an unusually large amount of water running down the river at once, and it turns out that sandbars aren’t restored, but in many cases, they’re totally wiped out.

See, the flow of the river is so great that the sandbars just get worn down during the flooding process and rather than restoring sandbars and the habitat, it actually wears them down significantly.

Again, clearly not good.

Finally, flooding the river also destroys the plants along its banks. We might assume that this would help establish plants along the river’s banks.

At first, it seems like this could occur − I mean, you’re moistening the soil along the bank and stuff − but the reality of the situation is that it just doesn’t work like that.

In fact, it’s quite the opposite − what basically happens is that the river just uproots a lot of plants already established along the bank and does nothing to help establish new ones.

Looking at the actual results of these flooding procedures, it’s pretty apparent that flooding the river below a dam actually harms the river quite a bit.

From removing silt to eroding sandbars to uprooting plants on the river’s bank, artificial flooding just ends up hurting rivers.