Fluvial Landforms · RiverLab

Fluvial erosion: V-shaped valleys

The crust lifts a block of highland; rain gathers into rills, rills into rivers. In the upper course the slopes are steep and the water fast, so downcutting dominates, and the river also extends towards its sources by headward erosion, its tributaries spreading like the branches of a tree.

As the bed is cut deeper, the valley sides keep collapsing under gravity, giving a cross-section shaped like the letter V: a V-shaped valley. A hard rock layer resists erosion, so where the river crosses it a waterfall forms; the soft rock beneath is scoured out, the hard rock above breaks off, and the waterfall slowly retreats upstream.

Turn on the rain and watch the valley network carve itself; use the erosion / deposition overlay to see where the land is being eroded; drag the section to cut the terrain open like a geological block diagram. How to read contour maps is shown in Contour 3D.

Alluvial fan: the mountain front

In the mountains a river carries a great deal of sediment. Once it leaves the mountain front, the ground suddenly flattens and the channel, no longer held by the valley, widens; the flow slows sharply, its carrying power drops, and the sediment is laid down.

The channel keeps switching across the growing deposit, spreading sediment layer by layer into an alluvial fan with its apex at the mountain front. Large gravel settles first and stays at the fan apex; fine sand is carried further and settles at the fan toe — this is the sorting of fluvial deposits.

Release a flash flood and watch the fan spread lobe by lobe; after the water recedes, open the grain-size overlay and see how coarse and fine are arranged.

River bends: concave and convex banks

As a river enters the flat middle and lower course, downcutting weakens and lateral erosion grows. Once the channel has a bend, inertia throws the flow towards the outside; the outer bank, the concave bank, has deep, fast water and keeps being eroded back, while on the inner convex bank the water is shallow and slow and sediment builds a point bar.

The concave bank cuts in further and the convex bank builds out further, so the channel grows ever more winding into meanders. That is why harbours on a bend are usually built on the concave bank (deep water) and sand is dug from the convex bank.

Release a stream of dye and watch it race along the concave bank; open the erosion / deposition overlay: red is the concave bank, blue the convex bank. Here the channel is moved by a model of bank erosion and deposition; the water flow is computed for real.

Meander cutoff and oxbow lake

As the meanders grow, two neighbouring loops come closer and closer until only a narrow strip of land separates them — the meander neck. In a flood the water spills over and breaks through the neck, and the river takes the short cut.

The abandoned loop is cut off from the new channel and becomes a lake shaped like an ox yoke — an oxbow lake. With no flowing water to feed it, it slowly fills with sediment and plants until it becomes a marsh and then a hollow.

Send a flood and watch the river cut through; as time passes, watch the oxbow lake silt up.

Delta: where the river meets the sea

Where a river enters the sea the mouth is flat and the seawater holds the flow back, so the current slows sharply and sediment is laid down in large amounts. Coarse grains form a bar near the mouth; fine grains spread with the water far out to sea.

As sediment keeps accumulating, the land grows out into the sea and the channel splits into several distributaries, forming a roughly triangular landform: a delta. The Yellow River carries an exceptional load of sediment, and its delta is one of the fastest-growing pieces of land in China.

If sea level falls, the river's base level drops; the river cuts down again and its former floodplain is left above the channel as a river terrace. Try dragging the sea-level slider.

Real rivers: from the Yarlung Tsangpo to the Yellow River mouth

This is real terrain, from Copernicus DEM GLO-30 30 m elevation data (terrain only — no borders or place names).

Yarlung Tsangpo Grand Canyon: the river cuts through the eastern Himalaya, carving one of the deepest gorges in the world — compare lesson 1's V-shaped valley. Qiankun Bay on the Yellow River: on the Loess Plateau the Yellow River is sunk deep into the rock, its winding channel carved into a gorge — these are incised meanders; compare lessons 3 and 4. Yellow River delta: sediment piles up at the mouth and the shoreline advances into the Bohai Sea — compare lesson 5.

The view is vertically exaggerated; adjust it with the slider.

In-class testing

Look back over the six topics and answer the multiple-choice questions below.