How do natural springs bring water to the surface?

1. Water trickles out of this hillside day and night, with no pipe and no pump. It is a spring. Let us cut the hill open and find out where the water comes from.
2. It starts with rain on the hilltop. This hill is made of sandstone, a rock full of tiny gaps. The rain soaks in and sinks down through the gaps.
3. Deep in the hill the water meets a layer of clay. Clay is packed so tightly that water cannot get through. The water piles up on top of it.
4. The clay layer is tilted. So the water does what water always does: it runs downhill, along the top of the clay, toward the lower side of the hill.
5. Where the clay layer reaches the hillside, the water has nowhere left to go but out. That spot is the spring. From there a stream runs down to the pond.
6. The rain stops, but the spring keeps flowing for weeks. The hill is like a huge sponge that lets its water out slowly. That is why springs run on sunny days.

Tap each part and follow the water from the hilltop to the pond.

• Hilltop. This is where the spring's water goes in. Rain and melting snow soak into the hilltop.
• Sandstone. Sandstone is made of sand grains stuck together, with tiny gaps between them. Water can soak in and move through it.
• Clay layer. Clay is packed so tightly that water cannot pass. It works like a roof inside the hill that steers the water sideways.
• Water on the move. The water creeps through the sandstone along the top of the clay, always toward the lower end.
• Spring. Where the clay reaches the hillside, the water comes out into the daylight. Spring water is usually cool and clear, because the rock has filtered it.
• Stream. The spring is the start of a stream. Many rivers begin as springs high in the hills.
• Pond. The stream fills the pond in the valley. Animals and people have always settled near water like this.

Tilt the clay layer, make it rain and watch the drops. Can you make the spring come out on each side of the hill?

Experiment: The clay layer slopes down to the left. Where will the spring come out?

1. Press Make it rain and watch which side the spring comes out.
2. Tilt the clay layer the other way and watch again.
3. Try the layer flat. What happens now?

What we found: The spring always comes out on the side where the clay layer is lower. Water sinks until the clay stops it, runs downhill along the top of the clay, and comes out where the clay meets the hillside.

Turn on JavaScript to try it yourself.

Look at the clay layer inside the hill. Move the trough to the spot where the spring will come out, then make it rain.

A spring can look like magic: clear water pouring out of the ground with no rain in sight. But every drop of it fell as rain or snow, perhaps weeks or even years ago. It soaked into the ground somewhere higher up and has been traveling slowly through the rock ever since.

Rock is not all the same. Some rocks, like sandstone, are full of tiny gaps that water can move through. Others, like clay, are packed so tightly that water cannot pass. When sinking water reaches a layer like clay, it cannot go down any further. So it flows sideways along the top of that layer, always toward the lower end.

If the layer comes out on a hillside or in a valley, the water comes out there too, and that is a spring. This is why springs are often found partway down a slope, and why several springs sometimes appear in a line along a hill: they all mark the edge of the same hidden layer.

Because the rock stores water like a giant sponge, a spring keeps flowing long after the rain has stopped. It also means that anything spilled on the ground uphill can end up in the spring, so the land above a spring needs looking after. Spring water looks clean, but ask a grown-up before drinking from one. Our model hill is very simple: real hills have many layers, cracks and bends.

Key words

Spring

A place where water from under the ground flows out by itself.

Permeable

Letting water pass through. Sandstone and gravel are permeable.

Impermeable

Not letting water pass through. Clay and solid, uncracked rock are impermeable.

Contact spring

A spring that comes out where a rock that holds water sits on top of one that blocks it.

Discharge

How much water flows out of a spring or along a stream in a given time, for example in liters per second.

Recharge area

The land where rain soaks in to feed a spring or an aquifer.

Check your thinking

1. Where does the water in a spring come from?

   Show the answer

   Rain that soaked into the ground higher up. All spring water began as rain or snow that soaked into the ground and traveled through the rock.

2. A clay layer inside a hill slopes down toward the east. On which side will the spring be?

   Show the answer

   The east side. Water runs downhill along the top of the clay, so it comes out at the layer's lower end.

3. Why does a spring keep flowing after the rain has stopped?

   Show the answer

   The rock stores water and lets it out slowly. The hill is like a giant sponge. It soaks up rain and releases it over weeks.

4. A short clay layer high in a hill ends before it reaches the hillside. What happens to the water running along it?

   Show the answer

   It drops off the end and sinks on to the next impermeable layer. A spring needs an outlet at the surface. Inside the hill the water just keeps sinking through permeable rock until another layer stops it.

5. Someone spills oil on the ground at the top of a hill. Why might that matter for a spring lower down?

   Show the answer

   Whatever soaks into the hilltop can come out at the spring. The spring's water went in at the hilltop, so pollution there can travel with it.