The Long Profile of a River

The Long Profile

The long profile of a river marks its change in gradient and altitude from its source to its mouth. It shows how a river changes throughout its course. Long profiles are concave in shape and depict how a river’s slope becomes gentler as it nears the sea.

Diagram showing the long profile of a river

The long profile also illustrates how from its source, the river’s gradient is steep, but it widens out when it approaches the sea. It is also possible for a long profile to show the river’s knickpoints. These are sudden changes in the river’s gradient and often indicate the presence of landforms such as lakes and waterfalls.

Over time, deposition and erosion will eventually turn the river’s long profile into a graded profile. A profile that is smooth, concave, and without any knickpoints. This graded river’s input and output are in balanced proportions; hence, the river no longer experiences any further erosion and is in equilibrium. However, this is only theoretical and is not really a phenomenon observed in nature.  

The three levels of a river’s course

A river’s course can be separated into three levels: the upper course, the middle course, and the lower course. In each of these courses, the river experiences a change in its energy.

Upper course

The part of a river nearest to its source, the upper course of a river is found high above sea level. It usually flows through valleys with steep sides and narrow channels. The upper course also has an abundance of potential energy that can be later converted into kinetic energy. Common features found in this level are lakes, waterfalls, potholes, and gorges. Additionally, in the upper course, there is mostly vertical erosion. This creates its V-shaped valleys. 

Middle course

In the middle course, the river valley widens and the gradient of the river decreases. Lateral erosion turns the once steep-sided valley into flood plains. Potential energy is converted into kinetic energy and the river experiences an increase in velocity. Meanders and river cliffs will also form. Water from different sources enters the channel, and the channel widens. 

Lower course 

The lower course is the level closest to the river’s mouth. The river’s slope is gentle and nearly flat due to a lack of vertical erosion. The lower course is where the river is widest and deepest. Large amounts of water flow at high velocities. The river has very little potential energy, but has very high kinetic energy. Lateral erosion occurs mainly at meanders. Common features of the lower course include meanders, levees, and floodplains. 

The Cross Profile 

The cross profile, or cross-valley profile, is the cross-section of a river at a certain point in its course. Much like the long profile, the cross profile of a river changes as it transitions from the upper course to the lower course. These differences are also due to changes in a river’s energy.

The three courses in a cross profile

Upper course

The large amount of vertical erosion and little lateral erosion makes the upper course’s channel narrow and deep. At this level, the river has steep, v-shaped valley sides. The river’s efficiency is low, and its waters are turbulent.

Middle course

The increase in lateral erosion has resulted in an increase in width, but the river valley’s depth is yet to change significantly. Floodplains are present on either side of the river, and the valley’s slopes are now not as steep. The river is now more efficient with its energy, and its waters are beginning to settle down.

Lower course  

The river valley is now at its widest, its sides are gentle in slope, and the floodplain has greatly expanded in size. The channel is wider, but it has only become a little deeper. The river is very efficient, and its discharge and velocity have increased. 

The Downstream Increase in Velocity and Discharge

Discharge is defined as the volume of water passing through the river at a certain point. As the river moves downstream from its source to its mouth, there is an increase in both velocity and discharge. Smaller rivers and streams, called tributaries, join the main channel and add to the river’s volume and increase its discharge. 

As the course of a river continues further downstream, its velocity also increases. This is due to the wideness of the channel in the lower courses. The upper course’s channel is narrow and the water is often in contact with the bed and banks. The high amounts of friction in the upper course reduces the water’s velocity. On the other hand, in the lower course, the channel is wide and a smaller proportion of water flowing through it comes in contact with the bed or banks. Less friction in the lower course allows the water to flow faster. Additionally, the water flowing in from tributaries also contributes to this increase in velocity. 

The Efficiency of a River

Hydraulic Radius

A river’s efficiency is measured by its hydraulic radius. A river with a higher hydraulic radius translates into a more efficient river. 

The hydraulic radius can be obtained by dividing the channel’s cross-sectional area by its wetted perimeter.

Water coming into contact with the wetted perimeter creates friction. This causes an increase in energy loss and reduces the river’s velocity. A larger hydraulic radius means that there is a smaller proportion of water that comes in contact with the wetted perimeter. In turn, the decrease in friction also reduces energy loss, and the river’s discharge and velocity increases. 

Channel Roughness

The natural features of a channel, such as its roughness, also have an effect on its velocity and discharge. Smooth, narrow, and deep channels have larger hydraulic radiuses and are more efficient than channels that are rough, wide, and shallow. Protruding banks and large, jagged rocks on the river bed increase a channel’s wetted perimeter. This leads to an increase in friction, and decrease in the river’s efficiency, velocity, and discharge. 

A channel’s turbulence is directly proportional to its roughness. A rougher channel leads to more turbulent water flow. This increase in turbulence results in an increase in erosion, and the river picks up more material than if it were not turbulent. 

The upper course of a river is where the channel is the roughest. Water flow is turbulent, and the river loses great amounts of energy to friction. In this level, the river’s gradient is at its steepest, and its velocity and discharge are at its lowest. 

On the other hand, the lower course of a river is where there is the least channel roughness. The lower course has smoother riverbanks and riverbeds, and water flow is non-turbulent. This leads to less friction, and less energy loss. A river’s velocity and discharge are greatest at the lower course. 

River Landforms 

Landforms created by fluvial erosion

Waterfalls 

A waterfall is a river’s steep descent over a rocky ledge and into a plunge pool below. Waterfalls are formed in areas where the river flows from soft rock to hard rock. The river erodes the soft rock quicker than the hard rock creates a step in the riverbed. This also results in the creation of a hard ledge which the river falls over. 

As it approaches the waterfall, a river’s velocity increases, which in turn leads to an increase in erosion. The rush of water and the sediment that it carries cascades over the waterfall, and erodes the plunge pool at its base. Whirlpools created by the water crashing from above further erodes the rock of the plunge pool. 

Over time, the waterfall will also begin to recede. The erosion at its base wears away the rock behind the waterfall, creating a cave-like rock shelter. Eventually, the rocky ledge will tumble down into the pool below, and the waterfall retreats many metres upstream. Thus, the waterfall’s erosion process begins anew.

Rapids

Rapids are areas of fast-flowing water in a river. They are found in the parts of a river with a somewhat steep gradient. Due to this gradient, turbulent water rushes over hard, uneven rock. 

Similar to waterfalls, the softer rocks at the riverbed are eroded faster than harder rocks. This differential erosion eventually wears away the softer rocks and only leaves the hardier rocks. These stronger rocks disturb the flow of the stream in breaks that create small waterfalls. 

Potholes 

Potholes are cylindrical, bowl-shaped, or irregular hollows found on the riverbed. They are made by the abrasive grinding of sediment in a circular motion. Potholes are usually deeper than they are wide, and have spiral grooves on the inside surface.

Meanders

Found in the middle and lower levels, meanders are sweeping curves and U-bends in the course of a river. They are formed in alluvial materials and freely change and adjust their shape in relation to the alluvial valley’s slope. Meanders usually occur in a series. They are formed by a combination of the processes of erosion and deposition.

Meanders develop in parts of a river that alternative between even-spaced areas of deep water and shallow waters. Pools are deeper and more efficient, thus the water that flows through it has more energy.

The channel increases in velocity and turbulence when passing through pools. The flow of water will then twist and coil, developing corkscrew, helicoidal currents. These spiralling currents then run down the river and increase erosion. This erosion makes the pools even deeper, and eroded material is deposited on the inside of the succeeding bend. Overall, this process creates the meanders’ winding, twisting, asymmetrical shape.

Oxbow Lakes

Oxbow lakes are U-shaped lakes that are formed when a large meander is cut off from the river. Erosion and deposition create a new channel to cut through, and the river finds a shorter course. The former meander is separated from the channel and becomes an oxbow lake beside the river.

No new water flows in or out of oxbow lakes. They are considered still water lakes. Eventually, its water will evaporate, drying up the lake. Often, oxbow lakes become swamps or bogs.

Landforms created by fluvial deposition

Braiding

Braiding is when a river splits up into smaller, winding channels that eventually reunite to form a single channel. When a river carries large amounts of sediment, either it slows down or the sediment is deposited into the channel. 

Floodplains

Floodplains are generally flat areas of land on the sides of a river. Floodplains extend from the riverbank to the outer edges of the valley. 

A floodplain has two main parts: the floodway and the flood fringe. The floodway is the main channel of the river. On the other hand, the flood fringe stretches from the floodway’s outer banks and to where the valley begins to rise as bluffs. Floodplains vary in size, some are very narrow while others can reach great widths. Floodplains are also areas teeming with biodiversity.

Levees

A levee is a raised wall near the banks of a river that blocks the flow of water. Levees may be natural or artificial. Natural levees are created when rivers overflow onto their banks and deposit sediment across the floodplain. Over time, the heavy sediment settles near the banks of the river, builds up, and creates a natural barrier between the channel and the floodplain.

Deltas

Deltas are wetlands created by the deposition of sediment where the river meets another body of water such as a lake, sea, or another river. Upon reaching its mouth, a river slows down, and the sediment it had been carrying begins to settle on the riverbed. Eventually, the deposited sediment, called alluvium, builds up and rises above sea level and blocks the mouth of the river. To reach the other body of water, the channel then branches out into smaller streams, all part of a distributary network. Deltas hold great importance in wetland ecosystems.

Rejuvenation

Rejuvenation is the lowering of a river’s base level caused by a sudden drop in sea level or a rise in ground level. The disturbance causes a river’s potential energy to rise. There will also be an increase in vertical erosion, amplifying the riverbed’s erosion rate. Rejuvenation extends a river’s long profile. As a result, a knick point will mark where the old long profile and the new long profile meet.  

Landforms related to rejuvenation

River Terraces

The increase in vertical erosion creates river terraces. These were previously floodplains, but now reside above the current flood levels.

Incised or entrenched meanders

Also a product of vertical erosion, incised meanders are deep meanders that cut into hard rock. These winding valleys have steep sides that have deepened and widened over long periods of time. One famous example of such a phenomenon is the Grand Canyon in the United States of America.

Frequently Asked Questions

What is the long profile of a river?

The long profile of a river represents the changes in elevation from its source to its mouth. It shows the river’s gradient, which affects its flow and erosion processes.

How does the long profile change as a river flows downstream?

Typically, the gradient decreases as the river flows downstream, resulting in smoother and wider channels. This leads to changes in water velocity and sediment transport.

What are knickpoints, and how do they influence the long profile of a river?

Knickpoints are abrupt changes in slope along a river’s course, often caused by tectonic activity or differential erosion. They can lead to waterfall formation and downstream adjustments.

How does the long profile of a river influence its erosional and depositional features?

The long profile affects the river’s ability to erode its channel and transport sediment. Steeper gradients lead to more erosion, while gentler slopes promote deposition.

What is base level, and how does it relate to the long profile of a river?

Base level is the lowest point to which a river can erode its channel. It can be local (e.g., a lake or sea) or ultimate (sea level). Rivers adjust their profiles to reach base level.

References

Cite/Link to This Article

  • "The Long Profile of a River". Geography Revision. Accessed on March 28, 2024. https://geography-revision.co.uk/a-level/physical/the-long-profile-of-a-river/.

  • "The Long Profile of a River". Geography Revision, https://geography-revision.co.uk/a-level/physical/the-long-profile-of-a-river/. Accessed 28 March, 2024.

  • The Long Profile of a River. Geography Revision. Retrieved from https://geography-revision.co.uk/a-level/physical/the-long-profile-of-a-river/.