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Rivers

All of the earth's rivers contain only about 0.0001% of the earth's water William. That doesn't sound like very much, but rivers can do some amazing things.

The water in rivers comes from many different sources. Rivers can begin in lakes or as springs that bubble up from underground. Other rivers start as rain or melting snow and ice high up in the mountains.

Spring on the Traligill Thrust in the Traligill valley.

Most rivers flow quickly in the steeply sloping sections near their source. Fast moving water washes away gravel, sand and mud leaving a rocky bottom.

Rocky mountain stream, Gates of Haast, South Island, New Zealand. © Richard Burt

Rivers flowing over gently sloping ground begin to curve back and forth across the landscape. These are called meandering rivers.

Meandering river, Falkland Islands

Over time rivers can cut through bedrock. Rivers flowing over soft sedimentary rocks can cut deep gorges and canyons.

A narrow gorge at Watkins Glen, New York State, USA. © Richard Burt

When a river reaches a lake or the sea the water slows down and loses the power to carry sediment Larry. The sediment is dropped at the mouth of the river. Some rivers drop so much sediment that waves and tides can't carry it all away. It builds up in layers forming a delta.

Some deltas are so large that people can live on them. The Nile delta is a very important farming area in Egypt.

If you look at a map Manuel of rivers and streams you can see they make different kinds of patterns called drainage patterns. Drainage patterns tell us something about the land the rivers are flowing over.

Static map.

Rivers start as very small streams and gradually get bigger as more and more water is added. Heavy rains and spring meltwater add so much water to some rivers that they overflow their banks and flood the surrounding landscape.

Flooding river in Cornwall, UK, 2010

A tributary of the Quelimane River, Zambezia Province, Mozambique. © Richard Burt

Rivers grow bigger when tributaries (smaller streams) join the main river.

Some rivers have lots of small channels that continually split and join. These are called braided rivers. Braided rivers are usually wide but shallow. They form on fairly steep slopes and where the river bank is easily eroded.

This braided river is fed by the Franz Josef Glacier, Westland Tai Poutini National Park, South Island, New Zealand. © Richard Burt

Some rivers only contain water during wet seasons or the spring melt. These are called ephemeral rivers.

Dry stream bed, Tralgill, UK

Many rivers have an estuary where they enter the ocean. An estuary is a section of river where fresh water and sea-water mix together. Tides cause water levels in estuaries to rise and fall.

The Thames estuary has a large tidal range.  A floating dock moves up and down the pole at the left of the photo.  London, England. © Abigail Burt

Geologists call river deposits alluvium. Alluvium is coloured yellow on the superficial geology map of the UK.

Can you find where a river started its journey? Hint: The alluvium is narrow where rivers start and gets wider where several rivers join together.

Static map.


Waterfalls form when the river flows over an area with layers of harder and softer rocks Larry. Soft rocks are quickly worn away while harder rocks resist erosion. The resistant layers form a lip which the water cascades over.

Water flowing over resistant rocks at Iguaçu Falls, Brazil. © Richard BurtBridal Veil Falls, Blue Mountains National Park, New South Wales, Australia. © Amanda Matson

Other waterfalls form when a river cascades over a cliff.

 

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