Rivers and coasts

The source

HintThe opposite end of its journey from the mouth.

The whyA 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

HintThe same thing that fogs a cold window in a warm room.

The whyWarm 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

HintA side road feeding the motorway, but made of water.

The whyThe 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

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

The whyA 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

HintThe sea uses the stones it already carries as sandpaper.

The whyThis 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

HintFalling water drills a hollow in the rock where it lands.

The whyThe 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

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

The whyThe 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

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

The whyThe 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

HintNothing soaks into a hard surface, so the rain runs straight off it.

The whyTarmac 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

HintWorking with the landscape instead of building against it.

The whyThese 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

HintThey stop the beach being dragged along the shore by the waves.

The whyLongshore 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

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

The whyA 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.