# How Dams Work

How a gravity dam holds back a reservoir, and how its water drives a turbine to make electricity and passes floods safely\.

By R S

Topic: Engineering

[Open the interactive Jig](https://ask.jig.com/j/ufnXTmdJC3jJ)

## Transcript

### 1. A wall that holds a river

A dam is a wall built across a river\. It holds the water back and makes a lake called a reservoir\. This model is cut open at the front so you can see inside\. Drag to turn it and look around\.

Scroll or pinch to move closer\.

### 2. Deeper water pushes harder

Water pushes on the wall, and the push grows the deeper you go\. Only the depth matters, not how wide or long the lake is\. That is why a dam is thin at the top and very thick at the bottom\.

The blue arrows show the push of the water\. A longer arrow means a stronger push\.

### 3. Too heavy to move

This kind of dam is a gravity dam\. It is so heavy that its own weight, pressing down into the rock, keeps the water from sliding it or tipping it over\.

Hoover Dam in the USA is 221 metres tall\. It is about 200 metres thick at the base, roughly two football fields, but only 14 metres thick at the top\.

### 4. Into the pipe

Low in the wall, an opening called the intake lets water into a huge pipe called a penstock\. The water falls down through the dam and gets faster as it drops\. Open the gate to let it in\.

The height the water falls is called the head\. More head means more energy\.

### 5. The spinning turbine

At the bottom, the rushing water hits the curved blades of a turbine, a kind of water wheel, and spins it round\. Then the water flows out into the river below\.

The turbine and the generator above it share one long shaft, so they always turn together\.

### 6. Spinning makes electricity

The shaft turns a generator\. Inside it, magnets spin past coils of copper wire, and that makes electricity\. Power lines carry it away to homes and towns\.

A generator does not store electricity\. It makes it only while it spins\.

### 7. Letting floods pass

After heavy rain the lake rises\. Water must never spill over the top of the dam, so gates on the spillway open and let the extra water rush safely down a chute\.

### 8. The river carries on

The water is not used up\. It flows back into the river below the dam\. Only its energy was borrowed on the way down, and the rain and the river fill the lake again\.

Not every dam is a gravity dam\. Arch dams curve into the canyon walls, and embankment dams are huge banks of earth and rock\.