RIVERS AND VALLEYS
by Mary and Leanne

Rivers shape a lot of the Earth's surface.  They work like a file cutting down into the land and carrying bits of it away.  A river basin is part of the landscape that is low in the centre.  Its highest part is in the mountains where the river starts as a tiny stream, and its lowest part is near the sea or lakes where the river ends. 

The young river starts on land which is really steep and a long way from the sea.  It is usually quite small but it goes very fast because of the steep land. It can break down big rocks and carry them away because it is going very fast.  It forms a V shaped valley. This is called the headwater tract of the river and it will also have pools, waterfalls and rapids along the river as it flows towards the flatter land. 

A middle aged, or mature river runs on land that is closer to the ocean and is not as steep. This part of the river is called the middle tract.   The river does not go as fast and it is bigger because lots of little rivers usually flow into it.  Because it is slower, it erodes more slowly and breaks down smaller rocks than the young river, and it also cannot carry as much rock and dirt  as it flows along.  It has a flatter shaped valley because it has started to drop the biggest rocks it can no longer carry.  It will form meanders which are big bends in the rivers.  Sometimes the bends in the meanders will get cut off and form a billabong, or ox bow lake.  There are lots of billabongs alongside the Murray River near Albury.

An old river is close to the sea.  This is called the coastal tract.  It usually carries a lot of water but it goes slowly because the land is flat.  It cannot carry  the dirt and rocks that it used to and it drops these.   Right near the ocean where it is moving most slowly, a river will sometimes split into lots of small streams as it drops the sand and dirt it has been carrying.  This triangular shape is called a delta.  The valley an old river flows across is very flat and has a wide flood plain.  When the river floods it leaves behind lots of rich soil that it has carried from higher up in the hills.  Flood plains are often used for growing crops and pastures because of the rich soil on them.

Click HERE to see our model of a river and its valleys.