Chapter 6: RIVER

A river is a natural stream of water which flows in a channel towards a mouth or lake or another river. Rivers have two banks (left and right bank). They have a bed over which the water flows. Rivers always have a source, from where they start.

River banks
Rivers have two banks - the right bank and the left bank. If you stand in a river and face the way the water is flowing, the right bank will be on your right hand side and the left bank will be on you left hand side.

River bed
The river bed is the bottom of the river. It can be made of stones, pebbles, boulders, rock and mud. The water flows over the river bed. You can see the river bed in streams, and in the upper course of a river, because the water is shallower. In the middle and lower courses of a river, you cannot see the river bed because the water is much deeper.

River channel
The river channel is the course for the river water. It can either be a straight channel like a canal, or a meandering channel. The channel gives the river its shape.

River System
Due to gravity, water always flows downhill from its source. As a river flows, it travels along a path known as its course. The course of a river may be divided into three parts - the upper, middle and lower courses. Rivers never have a straight course from source to mouth. Their course is always bendy.

1. The upper course is found in the mountains and hills where the river rises from its source. Often waterfalls and rapids are also found in this course. The river is usually fast flowing in the upper course. There are lots of stones and boulders for the water to flow over. The river starts as a stream in the upper course and flows through V-Shaped valleys. Only a small amount of water is transported. Also, the gradient or slope of the river is usually steep.

Here, landforms such as waterfalls and gorges could be found in the upper course. Waterfalls are sudden, steep, vertical flows of fast-flowing water falling from great heights. Waterfalls are normally formed in two ways - through the erosion of rocks of different resistance or through faulting. Gorges are formed when a river flow through an area with vertical resistant rocks by its sides. Therefore, causing it to only erode its channel vertically. Also, V-shaped, narrow and steep-sided valleys could be found in the upper course.

2. The middle course is where the river starts to become wider and deeper. The gradient of the slope is usually not as steep as the upper course. The river starts to meander or bend in the middle course. The river channel is wider as tributaries may join the main river.

In the middle course, landforms such as meanders and oxbow lakes could be found. Meanders are curves or bends along a river which are usually found in the middle or lower course of a river. Meanders are formed by a combination of the processes of erosion, transportation and deposition along a river. Oxbow lakes are generally temporary features. As it cut off from the main river, the oxbow lakes will eventually dry up due to evaporation and encourage the growth of vegetation. An oxbow lake is from with constant erosion along the outer banks of two consecutive meanders, the two meanders move closer to each other, forming a loop. Over time, the loop becomes more distinct and is separated by a narrow neck of land. This neck gets narrower with continual erosion and deposition. As deposition and erosion continue to take place along the inner and outer banks respectively, the two meanders eventually meet. Also, valley with wider sides are found in the middle course.

3. The lower course is where the river becomes its widest and deepest. Also, the gradient is very gentle and the river channel is also wider than the in the middle and upper course. This course is found closest to the sea where the river has its mouth. The flat area of land by the river banks is known as a floodplain. Farmers grow crops there. Sometimes a river can also have an estuary or a delta as its mouth.

Floodplains and levees are landforms that could be found at the lower course of the river. A river may overflow its banks when there is a heavy rain. As a result, land on either side of the river is flooded. As the floodwater spreads over a wide range, the water loses speed. When the water recedes, the river deposits the material it is carrying on the land, leaving behind alluvium on the river banks and beds. The coarse sendiments are deposited closer to the river's edge as they weigh more. Finer particles are deposited further from the river. The deposited material, builds up on both sides of the river as more and more floods occur. over time, wide and flat plains called floodplains are formed on either side of the river. The heavier and coarser materials nearer to the river form raised banks called levees. Also, exceptionally wide valleys could be found in the lower course.

Waterfall

Waterfalls are sudden , steep , vertical flows of fast flowing waters falling from great heights. A waterfall is usually found in the upper course of the water when the gradient is steep. Waterfalls are normally formed in two ways. They may be formed through the erosion of rocks of different resistance or through faulting.

Gorges

When a river flows through an area with resistant rocks by its sides, it can only erode its channel vertically. Over time, a deep,narrow and steep sided valley known as gorge is formed. Gorges are normally found also in the upper course of the river.

Valley

Erosions by rivers also creates landforms called valleys. A valley is a low lying area between hills or mountain. Valleys have different shapes.the shape of a valley depends on how the river erodes the land over which it flows.

Meanders

Meanders are formed by combination of the processes of erosion, transportation and deposition along a river. Meanders are curves or bends along a river which are usually found in the middle or lower course of a river, or where the floor of a vally is broad and the gradient is gentle.

Oxbow lakes

Meanders can become so distinct that they develop into another feature known as the oxbow lakes. With constant erosion along the outer banks of two consecutive meanders, the two meanders move closer to each other forming a loop. Over time, the loop become more distinct and is seperated by a narrow neck of land.