Interlocking Spurs - As the river cuts its deep V-shaped valley in its upper course, it follows the path of the easies trock to erode. Thus it tends to wind its way along, leaving the more resistant areas of rock as interlocking spurs.
Meanders - Meanders occur in the mid course and lower course of the river, where it is beginning to cut laterally as it gets closer to base level. Meanders are basically bends in the river, where the faster water on the outside of the bend has cut into the bank, eroding it and creating a river cliff. At the same time the slow moving water on the inside of the bend deposits its load, building up a shallow slip-off slope. Meanders migrate downstream as they cut through the valley sides.This creates a line of parallel cliffs along the sides of the valley.
Ox-Bow Lakes - In the lower course of the river meanders can become so pronounced that they can form ox-bow lakes. In the lower course, the rapid lateral erosion cuts into the neck of the meander, narrowing it considerably. Eventually, the force of the river breaks through the neck, and as this is the easiest way for the water to go, the old meander is left without any significant amount of water flowing through it. Quickly the river deposits material along the side of its new course, which completely block off the old meander, creating an ox-bow lake.
V-shaped valleys - In the upper course of the river, it cuts rapidly downwards, as the river puts almost all of its energy towards cutting down to base level. This causes the most distinctive river feature, the V-shaped valley. Rocks and other material are washed into the river from the steep valley sides during times of heavy rainfall, adding to the material being carried by the river.
Waterfalls - Waterfalls are perhaps the most spectacular erosional feature of a river. They primarily occur in the upper course of the river. Often a waterfall will form where a band of harder rock lies over a softer one. As the river flows over the edge of the harder, more resistant rock, into its plunge pool, it erodes away the softer rock below, creating an overhang. Once the overhang is big enough the whole thing collapses due to gravity and its own weight. The whole process then occurs again. This means that over time waterfalls will move backwards up the valley, leaving a steep sided gorge in front of them.
Deltas - Deltas occur where a river that carries a large amount of sediment meets a lake or the sea. This meeting causes the river to lose energy and drop the sediment it is carrying. Deltas form two types, called arcuate and birds foot. An arcuate delta is a delta that builds out into the sea, extending the coastline, as the Nile Delta does in Egypt. A bird's foot delta is an extension of this as "fingers"of material form further off the edge of the delta. The delta of the Mississippi river shows these characteristics.
Flood Plain - The Flood Plain is the area of alluvial deposits found beside the river in its lower course. As meanders move slowly down the course of the river they erode away the valley to create a wide valley floor, and they deposit layers of alluvial material on the slip-off slopes. Over time, this builds up into a large flood plain. A very good example is the Canterbury Plains in New Zealand, where many large rivers have contributed to a huge area of alluvial deposition that has become prime agricultural land.
Levees - Levees are naturally formed banks along the sides of a river channel in its lower course, as it flows through the flood plain. They are formed by the river depositing material when it floods. During a flood, the river deposits its heaviest, coarsest material closest to its normal course. Over years this deposition has built up the natural embankments, built of coarse material. Beyond them, the flood plain has been built up of the finer material that was deposited further away from the normal course of the river.
|Upper Course||Mid Course||Lower Course|
|Features||Interlocking spurs, waterfalls, V-shaped valley, gorges.||Meanders, Slip-off slopes, ox-bow lakes.||Deltas, flood plains, levees, meanders, ox-bow lakes.|
|Velocity||Relatively slow moving. Despite areas of fast flowing water, the large amount of material on the river channel bed means that friction will slow the water down.||The water has increased in speed as the channel widens and becomes smoother. Some boulders cause friction to slow it down a little.||The fastest section of the river, as the channel is widest, with very smooth sides, and the greatest volume of water.[/ulist]|