Rivers and coasts, Year 7: more practice

Evaporation

提示It happens at the surface of seas, lakes and rivers.

为什么Evaporation is the first step of the water cycle. Warm air carries the vapour upwards.

It condenses into tiny liquid droplets, forming clouds

提示The vapour changes state again.

为什么Condensation is the opposite of evaporation. When droplets grow large enough they fall as precipitation.

The River Severn

提示It flows from Wales into England and out to the Bristol Channel.

为什么The Severn is about 354 km long. The Thames is the longest river wholly in England.

A tributary

提示The larger river is joined by these.

为什么A river's tributaries gather water from across its drainage basin.

A confluence

提示Two rivers become one here.

为什么A tributary joins the main river at a confluence. The word means 'flowing together'.

A watershed

提示Rain falling either side drains into different rivers.

为什么Rain that falls on one side of a watershed flows into one river, and rain on the other side flows into another.

The river drops the material it was carrying

提示It happens when the flow loses energy.

为什么When a river slows down, it can no longer carry its load. Sand, silt and pebbles settle on the bed or banks.

The inside bend

提示The water flows slower on this side.

为什么Slower water has less energy, so it drops its load. Faster water on the outside bend erodes the bank.

Traction

提示They are rolled, not lifted.

为什么Traction moves big boulders in floods. Saltation hops smaller stones. Suspension and solution carry fine material and dissolved minerals.

They knock against each other and the bed and wear away (attrition)

提示Think of stones bumping into one another all the way down.

为什么This process is called attrition. It is one reason that river sediment is finer near the mouth.