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Unlock This CourseRivers are like nature's sculptors, constantly carving and shaping the landscape as they flow from source to mouth. Understanding how rivers work means looking at their profiles - both from the side (long profile) and from above looking down (cross profile). These profiles tell us the story of how a river has developed over thousands of years.
Key Definitions:
The long profile shows how a river's gradient changes from source to mouth. It typically forms a smooth, curved line called a concave profile. The river is steepest at its source in the mountains and gradually becomes gentler as it approaches the sea. This happens because the river has more energy to erode vertically in the upper course but loses energy as it flows downstream.
Every river can be divided into three main sections, each with distinct characteristics and landforms. Understanding these courses helps us predict what processes will be dominant and what landforms we might find.
The upper course is where the river begins its journey, typically in mountainous or hilly areas. Here, the river has tremendous energy due to the steep gradient, but it's narrow and shallow with relatively little water volume.
Steep gradient, narrow and shallow channel, large angular rocks on the bed, fast-flowing water with white water rapids and waterfalls.
Vertical erosion dominates, creating deep V-shaped valleys. Hydraulic action and abrasion are the main erosion processes.
V-shaped valleys, waterfalls, rapids, gorges, interlocking spurs and boulder-strewn river beds.
In the middle course, the river has gained more water from tributaries and the gradient begins to level out. The river starts to develop more sideways movement as well as downward cutting.
Moderate gradient, wider and deeper channel, mixture of rock sizes, alternating pools and riffles.
Both vertical and lateral erosion occur. Transportation becomes more important as the river carries more sediment.
Meanders begin to form, river cliffs and slip-off slopes and wider valley floors start to develop.
The lower course is where the river approaches the sea. The gradient is very gentle and the river is wide, deep and slow-moving. Deposition becomes the dominant process.
Very gentle gradient, wide and deep channel, fine sediment on the bed, slow-flowing water.
Deposition dominates as the river loses energy. Some lateral erosion continues on the outside of meander bends.
Large meanders, oxbow lakes, floodplains, levees, deltas and estuaries.
The River Tees perfectly demonstrates the three river courses. In its upper course in the Pennine Hills, it creates the spectacular High Force waterfall (21m high) and flows through a narrow V-shaped valley. In the middle course around Barnard Castle, it begins to meander and the valley widens. In the lower course near Middlesbrough, it flows slowly through a wide floodplain before entering the North Sea through an industrial estuary.
The cross profile shows how the shape of a river valley changes along its course. This shape is determined by the balance between erosion and the resistance of the underlying rock.
V-shaped valleys dominate because vertical erosion is much stronger than lateral erosion. The valley sides are steep and narrow, with the river occupying most of the valley floor. Weathering and mass movement on the valley sides contribute sediment to the river.
The valley begins to widen as lateral erosion increases. The valley floor becomes broader and the river starts to develop a small floodplain. The valley sides become less steep as the river cuts sideways as well as downwards.
Wide, flat-bottomed valleys with extensive floodplains characterise the lower course. The river channel may only occupy a small portion of the valley floor, with the rest being fertile floodplain that floods during high discharge periods.
The graded profile theory explains how rivers develop their characteristic concave long profile over time. A graded river is one that has achieved a balance between erosion, transportation and deposition.
Rivers naturally work towards creating a smooth, concave profile where they have just enough energy to transport their sediment load. This doesn't mean the river stops changing - it means the river has found a balance where it can efficiently move water and sediment from source to mouth.
Factors affecting the graded profile:
The River Thames demonstrates a classic graded profile. From its source in the Cotswolds at 110m above sea level, it flows 346km to the North Sea. The upper course around Lechlade has a gradient of about 1:1000, while near London it's only 1:10000. The profile shows the characteristic concave shape, steeper in the upper course and very gentle in the lower course through London.
Human activities can significantly alter natural river profiles, sometimes with unintended consequences for both the river system and surrounding communities.
Dams create artificial base levels, causing rivers to deposit sediment upstream and erode more actively downstream. This can lead to the formation of new waterfalls and changes in the river's natural profile.
Straightening rivers and lining channels with concrete increases flow velocity and can cause increased erosion downstream. This disrupts the natural balance that creates the graded profile.
Urban development increases surface runoff, leading to higher peak discharges. This can cause increased erosion and change the river's ability to transport sediment, affecting both long and cross profiles.
Sometimes rivers that have achieved a graded profile are 'rejuvenated' - given new energy to erode vertically again. This usually happens due to sea level changes or tectonic uplift.
When a river is rejuvenated, it cuts down into its former floodplain, leaving the old floodplain as a terrace above the new river level. These terraces are evidence of past river levels and climate changes. The River Thames has several terraces that show how it has cut down over thousands of years.