Rivers originate in mountainous or hilly tracts, flow through a lowland and finally reach the sea. Those rivers, which maintain the flow throughout the year, are known as perennial rivers.
The course of a river from its source to its mouth is divided into three sections i.e. upper, middle and lower courses. In the upper course, as the water rushes down the steep slope, maximum corrosive action takes place. Such deepening of the river channel produces gorges and canyons. The steep-valley slopes get weathered gradually to assume a V-shape.
The valley gets deepened more rapidly and differential rates of erosion along the river bed lead to the formation of rapids and waterfalls. The occurrence of a hard resistant rock across the path of a river may cause a waterfall due to rapid erosion of less resistant rock downstream.
In the middle course, the path of the river is less steep. Here the volume of water is much greater and the valley gets widened, forming a broad valley floor. The river channel develops broad sweeping curves in the level plains called meanders. During floods, water overflows the channels and covers the entire plain, submerging a vast area. When floods subside, sediments called alluvium gets deposited. The plains on either side of the channel are called flood plains, as they get submerged during floods.
In the lower course, the valley floor has an extremely gentle slope and hence the river is unable to transport all the sediments. Deposition is the prominent process here, which obstructs the channel and the river is divided into many distributaries. The lower course has many such distributaries of various sizes spread over a large area of alluvial plain. This vast alluvial plain is called the delta, as it resembles the Greek letter . The river deltas contain fertile alluvium e.g. river Nile in Egypt and the east-flowing rivers of the Peninsular India like the Godavari, the Krishna and the Kaveri have deltas along the coast due to the gentle slope in the lower course.
Contrarily, the west-flowing rivers have no deltas, as they flow rapidly along the steep western slope and deposit the alluvium on the sea floor. They (The Narmada and The Tapi), in fact, enter the sea by deep, narrow channels called estuaries.
This is where the river starts.
This is where the river is still young and quite small, often up in the hills. You might find steep valleys and waterfalls in the upper course.
This is where two rivers meet.
This is a smaller river that flows into a bigger river
This is when the river gets larger- the valleys may be wider and the slopes more gentle.
A Meander is when the river flows in S-shaped bends.
This is a crescent-shaped lake which used to be part of the main river but which has been cut off.
This is normally in the lower course of the river, and where the land either side of the river is very flat; when there is a lot of rain, the river may overflow and flood the land either side.
The Lower course of the river is when it approaches the sea. The river is at its widest, and it often goes through flat land (the flood plain).
The river mouth is where the river meets the sea- it may be very wide (an Estuary) or split into smaller channels (called a Delta).