Landforms - Deltas.

Landforms

Deltas

When large amounts of the suspended load is carried by rivers and ends up along the sea, it is then transported along the coast by waves and tidal currents. But sometimes the waves and tides are not strong enough to take it away, so the sediment is deposited at the mouth of the river. This forms a delta.

The deposits split the river into several channels, called distributaries. These spread the sediments over a wide area, giving the delta distinctive shapes.

Upland Areas

In upland areas the long profile of the river is generally steeper and uneven. Harder, more resistant rocks can cause rapids and waterfalls. Waterfalls are often also found where tributary streams enter larger valleys that have been deepened by glaciers in the past, known as hanging valleys.

Many rivers have their source in upland areas where precipitation is high and evapotranspiration low. Also where the soil is thin and has little vegetation. Rocks are often impermeable.

Most water moves down the slopes as overland or throughflow, making lag times shorter. When the rain falls, river discharge can increase very quickly.

Since the course of the river is steep and rocky, the flow will be turbulent with sufficient energy to move larger loads. The rocks then roll around the bed, as they get more rounded, becoming bedload.

The river uses its load to make vertical ...

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Many rivers have their source in upland areas where precipitation is high and evapotranspiration low. Also where the soil is thin and has little vegetation. Rocks are often impermeable.

Most water moves down the slopes as overland or throughflow, making lag times shorter. When the rain falls, river discharge can increase very quickly.

The river uses its load to make vertical erosion. In arid areas and areas of limestone this can create a gorge. But in mountain areas in the British Isles the steep valley sides become unstable.

Weathered rock rumbles downhill into the river and the slopes become less steep and form a V-shaped valley. At the same time the river also uses lateral erosion to make interlocking spurs.

Lowland Areas

As rivers approach the sea in lowland areas, the vertical erosion is slower. But river velocity discharge is higher than in the upland areas, and so rivers are able to transport larger amounts of fine sediment than in upland areas.

As the river moves from side to side in its channel, it causes lateral erosion. The load helps to erode the banks by corrasion. Erosion is greater on the outer part of the bend; this is where the river has the most energy.

In time meanders develop with river cliffs marking the zones of undercutting. Deposition takes place in areas with the least energy and point bars develop from it. At the same time the valley is being widened and the V-shape opens out with a flat valley floor.

Most of these changes take place when the river is at bank-full stage. If the meanders become extreme then the river cuts through the ‘neck’ of the meander and follows a straighter course once again, this leaves the abandoned meander as an ox-bow lake.

Floodplains

A river may overflow its channel and flood the flat valley floor when it has very high discharge. This is known as a floodplain.

The river would carry a large suspended load. As the water floods out of the channel there is a sudden loss of energy, and so the large amounts of sediment are deposited on the bank forming levées. Further deposits are left across the floodplain, every flood adding another layer. These deposits are called alluvium.