Features of a River

Rivers change their character from source to mouth — fast and narrow in the upper course, wider and slower in the middle course, and broad and winding in the lower course. Each section is shaped by erosion and deposition processes.

What Is a River?

A river is a natural flowing body of fresh water that moves under gravity from higher land downhill to the sea, a lake, or another river. Rivers are one of the most powerful forces shaping Earth's landscape through erosion and deposition.

Key river terms:

Source: Where the river begins (usually in mountains or hills)
Tributary: A smaller river or stream that flows into the main river
Confluence: Where two rivers meet
Mouth: Where the river reaches the sea or a lake
Drainage basin: The total area of land drained by a river and its tributaries

The Long Profile of a River

A river can be divided into three sections — upper, middle, and lower course — each with distinct characteristics.

Upper Course

Near the source, on steep hillsides.
River is narrow and fast-flowing.
High energy: mainly erosion (cutting downward — vertical erosion).
Features: V-shaped valley, waterfalls, rapids, interlocking spurs.

Middle Course

River has more water from tributaries; slopes become gentler.
Lateral erosion (sideways) increases — the valley widens.
Both erosion and deposition occur.
Features: wider valley, meanders (bends), small floodplain begins.

Lower Course

Near the mouth; very gentle slope.
River is wide and slow.
Mainly deposition — sediment is dropped.
Features: wide floodplain, large meanders, oxbow lakes, delta or estuary at the mouth.

Note

The River Nile (about 6650 km) was long considered the world's longest river. The Amazon in South America (about 6400 km) is often given second place, though some measurements now suggest the Amazon may be longer. The Amazon, however, carries far more water — about 20% of all the fresh water that flows into the world's oceans.

River Features Explained

V-shaped valley: Cut by vertical erosion in the upper course — the river cuts deep and the valley sides are steep.

Waterfall: Forms when a river flows over a band of hard rock onto softer rock below. The soft rock erodes faster, leaving the hard rock protruding. The backwash undercuts the hard rock until it collapses, and the waterfall retreats upstream.

Meander: A bend in a river. Water flows fastest on the outside of the bend (where it erodes — forming a river cliff) and slowest on the inside (where it deposits sediment — forming a slip-off slope).

Oxbow lake: Formed when a meander bends so much that the river breaks through the narrow neck of land, cutting off the loop and leaving a curved lake.

Tip

You can work out where energy is going in a river by thinking about the speed. Fast water has high energy and uses it to erode. Slow water has low energy and drops (deposits) the sediment it was carrying. This is why the lower course has wide floodplains built up from deposited material.

Real world: The Rhine — upper course to mouth

The Rhine begins high in the Swiss Alps (upper course) — fast, narrow, and boulder-strewn. By the time it flows through Germany, it has slowed and widened (middle course) with meanders and floodplains used for farming. When it reaches the Netherlands, it spreads into a massive delta before emptying into the North Sea (lower course) — the port of Rotterdam sits on this delta and is Europe's busiest port.

Real world: Niagara Falls — a retreating waterfall

Niagara Falls on the US–Canada border is a classic example of a waterfall formed by hard and soft rock. The hard dolomite cap rock sits on softer shale and sandstone. Water undercuts the softer rock, and the hard cap collapses — causing the falls to retreat upstream by about 1 metre per year. Niagara has retreated about 11 km from its original position since the last ice age.

Real world: The Danube's lower course and floodplain

The Danube flows through 10 countries and has distinct upper, middle, and lower sections. In its lower course through Romania and Bulgaria, it flows slowly across a broad floodplain, depositing rich sediment. This flat, fertile land supports enormous wheat and sunflower farms. The Danube then spreads into a vast delta — the Danube Delta in Romania, a UNESCO World Heritage Site and one of Europe's largest wetlands.

Tip

Where do you see this in real life? Look at the Thames, Rhine, or Amazon on Google Maps in satellite view — you can trace the wide meanders in the lower course and find oxbow lakes cut off from the main river.

Practice Problems

Explain why V-shaped valleys are found in the upper course but wide flat floodplains are found in the lower course.

Show answer

In the upper course, the river flows steeply downhill with high energy, eroding vertically (cutting downward into the rock). This creates a deep, narrow V-shaped valley. In the lower course, the gradient is gentle, the river flows slowly and loses energy. It deposits the sediment it is carrying across the flat valley floor, building up a wide floodplain.

How does an oxbow lake form from a meander?

Show answer

As a meander bends more and more, the neck of land between the two curves of the bend gets narrower. During a flood, the river flows in a straight line across the neck of land, cutting through it. The old meander bend is left separated from the main river, becoming a curved oxbow lake. The cut-off ends are sealed by deposited sediment.