Rivers and coasts

The source

提示The opposite end of its journey from the mouth.

为什么A river begins high up, often at a spring or a boggy upland, and ends at its mouth where it meets the sea or a lake. Everything in between is its course.

Condensation

提示The same thing that fogs a cold window in a warm room.

为什么Warm air rises, cools, and the vapour it carries collects around tiny specks of dust to form cloud. When the droplets grow heavy enough they fall as precipitation.

A tributary

提示A side road feeding the motorway, but made of water.

为什么The place where it joins is the confluence. All the land drained by a river and its feeders is the drainage basin, and the ridge dividing one basin from the next is the watershed.

deposition

提示The opposite of wearing away: the river lets go of its load.

为什么A river erodes hardest when it is fast and full, and lets go as it slows - on the inside of a bend, across a flood plain, or at the mouth where it meets the sea.

Abrasion

提示The sea uses the stones it already carries as sandpaper.

为什么This does most of the work of cutting a wave-cut notch at the foot of a cliff. Hydraulic action forces air into cracks, and attrition rounds the pebbles as they knock together.

A plunge pool

提示Falling water drills a hollow in the rock where it lands.

为什么The falling water and the rocks it carries swirl at the base, deepening the hollow and undercutting the hard band above. When the overhang collapses the waterfall retreats upstream, leaving a gorge.

The outside

提示The water on that side has furthest to travel round the curve.

为什么The fast outer bend erodes into a river cliff while the slow inner bend deposits a slip-off slope of sand and shingle. In time the neck narrows until the loop is cut off as an oxbow lake.

A stack

提示Once the roof of the arch falls, a lonely tower is left offshore.

为什么The sequence runs crack, cave, arch, stack, stump. Old Harry Rocks in Dorset and the Needles on the Isle of Wight were shaped this way.

Faster

提示Nothing soaks into a hard surface, so the rain runs straight off it.

为什么Tarmac is impermeable, so almost all the rain becomes surface run-off and arrives in the channel at once. Trees catch rain on their leaves and their roots help it sink in, spreading the flow over days.

Soft engineering

提示Working with the landscape instead of building against it.

为什么These schemes slow water down where it falls - planting woodland, restoring meanders, letting washlands flood on purpose. They are cheaper and help wildlife, but a city centre still needs hard defences.

Groynes

提示They stop the beach being dragged along the shore by the waves.

为什么Longshore drift shifts sand steadily along a coast, and these hold it in place so the beach itself absorbs wave energy. The cost is that the coast further along is starved of sand and erodes faster.

A U-shaped valley

提示Ice is far broader than a stream, so it grinds out the sides as well as the floor.

为什么A river cuts a narrow V; a glacier is wide and heavy, so it planes the sides back and flattens the bottom. Snowdonia and the Lake District are full of them, cut in the last Ice Age.