Introduction to Rivers

Where the river begins… The start of a river is called its source. Large portions of the world's largest rivers begin in hollows in the land. Water drips in from the surrounding soil to allow the formation of a tiny flow of water called a seep.

As the river flows… From its source, a river flows downhill. This is because of the Earth's gravity. Small trickles of water join up to form a stream. The amount of water in the stream increases steadily as more streams, called tributaries, join it. Eventually the stream becomes big enough to be a river.

Drainage Patterns A drainage basin is all the land that supplies a river and its tributaries with water. If you could look down on a river from above, you would see that it branches. This is called a drainage pattern. The shape of the pattern depends on rocks, soil, climate and the changes made to the river.

Radial and Dendritic Drainage Radial drainage happens when streams flow down from a central high point, such as a mountain top. Other rivers, such as the Amazon, form a pattern like the branches of a tree. This is called dendritic drainage.

River Erosion Corrasion is the grinding of rock fragments carried by the river against the bed and banks of the river. This action both widens and deepens the channel. Attrition is the knocking of rock fragments in the water against one another. The fragments are broken into smaller pieces and become smoother along the process.

River Erosion Solution is the process by which river water reacts chemically with soluble minerals in the rocks and dissolves them.  Hydraulic action is the breaking down of rocks and dragging them away from the bed and banks by the force of the running water itself. When water from a fast moving stream enters cracks in a rock, the force breaks up the force into pieces.

Transporting along the channel… A river uses its energy to carry or transport eroded materials such as mud, sand, boulders and dissolved materials. These materials are called its load. Rivers transport their load by four processes. These processes are traction, saltation, suspension and solution.

Saltation Large particles such as gravel and coarse sand are lifted and dropped along the river, so they bounce along the river bed in a series.

Traction Larger particles like pebbles and boulders roll and slide along the river bed.

Solution Dissolved materials containing minerals like calcium and sodium are carried in the water.

Suspension Smaller particles such as clay, silt and fine sand are carried along without contact with the river bed. Materials carried in suspension usually forms the greatest part of the total river bed.

Waterfalls A waterfall is where a river falls steeply over a band of hard rock. It is formed through a thousand-year process in which a river flows over hard rock followed by soft rock, leaving a small step of hard rock sticking up into the river. Over thousands of years, the soft rock is worn away until the river falls from a great height.

The middle river The amount of water and material in the river increases as more tributaries join in. The bed widens, its slope becomes gentler, and the water flows much more slowly as the river flows out of the mountains. The path of the middle river is always changing as it cuts sideways into the land and starts to deposit its load.

Meanders Loops, called meanders, and oxbow lakes are typical features of the middle river. During a storm, meanders stop river water flowing easily. This causes water to build up in places, and may lead to flooding.

As it reaches the sea… Most rivers end their journey when they flow into a sea. The river slows down and deposits the sediment it is carrying. Heavy grains of sand and gravel drop to the bottom while lighter particles of fine silt and clay are carried further out to sea. Eventually, the sediment spreads out to form a new piece of land with gently sloping sides. This is called a delta.