Rivers and coasts, Year 10: river profiles, processes and landforms of deposition (River landscapes option)

long

提示It describes the whole length of the journey, not a slice across it.

为什么The long profile runs along the river; a cross profile cuts across it at one point. Geographers use the long profile to show how gradient changes, and cross profiles to show how the channel and valley change shape.

It starts steep and becomes gentler

提示Sketch it and the curve looks like a playground slide.

为什么Near the source the river drops quickly through the hills. Further down the slope eases, and near the mouth the river is almost level. Drawn as a graph, the long profile is a smooth curve that is concave (it sags in the middle).

The channel becomes wider and deeper

提示Tributaries keep joining, so there is more and more water to hold.

为什么In the upper course the channel is narrow and shallow, and full of large rocks. Tributaries add water all the way down, and the bigger river erodes a bigger channel. In the lower course the channel is at its widest and deepest.

Interlocking spurs

提示From downstream they overlap like the teeth of a zip.

为什么A small upper-course river does not have the energy to cut through hard rock, so it flows around it. As it cuts downwards, the ridges it has gone around are left standing on each side in turn. Looking along the valley, they seem to lock together.

Very wide and almost flat

提示Think of the land you see from a train crossing a lowland river.

为什么In the lower course the river erodes sideways, not downwards, and its meanders swing across the valley floor. Floods spread sediment over the level ground. The result is a broad flood plain with only low valley sides far apart.

Hydraulic action (also called hydraulic power)

提示The first word names the fluid-driven brakes and lifts on machines.

为什么No stones are needed: the power of the moving water does the work. It is strongest where the current is fastest, such as on the outside of a bend and at the foot of a waterfall. It is one of four kinds of river erosion, with abrasion, attrition and solution.

It is dissolved in the water

提示Think of sugar stirred into tea.

为什么River water is slightly acidic and slowly dissolves rocks such as chalk and limestone. That is erosion by solution. The minerals are then carried along invisibly in the water, which is transport by solution. You cannot see this part of the load.

vertical

提示The opposite of horizontal.

为什么In the upper course the river is far above sea level and flows down a steep slope, so it cuts downwards. This gives a narrow, steep-sided, V-shaped valley. Lower down, the river erodes mostly sideways.

Lateral erosion

提示Think of a crab's way of walking.

为什么In the middle and lower courses a river has less height to lose, so more of its energy goes into cutting sideways. This is strongest on the outside of bends, where the current is fastest. It widens the valley floor and makes meanders grow.

traction; suspension

提示The first is also the grip of a tyre on the road; the second is what a bridge hangs by.

为什么A river moves its load in four ways. Boulders are rolled along the bed, pebbles and sand bounce along it (saltation), fine particles are held up in the flow, and dissolved minerals are carried in solution. The fine particles are what turn a river brown after rain.

It slows down and loses the energy needed to carry it

提示The water meets something that is hardly moving.

为什么A river needs energy to carry sediment, and its energy depends on how fast it flows. It also slows, and so deposits, on the inside of bends, in shallow water and when its volume falls after a flood.

A natural levee

提示Five letters, from the French verb meaning "to raise".

为什么Each time the river overflows, it drops sediment beside the channel. Over many floods the banks build up higher than the flood plain behind them. A higher bank lets the channel hold more water before it floods again.

The largest, heaviest sediment

提示Ask which particles need the most energy to keep moving.

为什么As water leaves the channel it spreads out, becomes shallow and slows sharply. It can no longer carry coarse sand and gravel, so these fall at once, on the bank. Fine silt and clay stay in the water longer and settle further out across the flood plain.

alluvium

提示A Latin-looking word beginning with "all" and ending like "aquarium".

为什么Layer after layer of this silt builds the flood plain upwards. It is rich in minerals and holds water well, which is why flood plains make good farmland even though they flood.

Meanders moving sideways across the valley floor (lateral erosion)

提示Think about what the river's bends do to the land on either side over centuries.

为什么The outside of each bend cuts into the edge of the valley, and the bends slowly shift position. Over a long time they wear back the valley sides and leave a broad, level floor. Floods then cover it with silt.

Mudflats

提示Wading birds feed here at low tide, and your boots would sink in.

为什么When the water stops moving, even the finest silt and clay settle out. They build wide, level banks that are covered at high tide and uncovered at low tide. Plants may later colonise them to make salt marsh. They are an important habitat for birds.