The concept of a river's long profile and its overall shape

A river's long profile represents the path it takes from its source in the uplands to its endpoint, where it meets the sea, a lake, or merges with a larger river. This profile illustrates how the river's elevation and gradient change over its course, providing insight into its behaviour and characteristics at different stages.

Key features of a river's long profile

• Overall shape - The long profile typically forms a smooth, concave curve.
• Elevation changes - The river begins at a high elevation above sea level in upland regions and descends progressively towards its destination.
• Gradient variation - The slope is steep and often irregular in the upper reaches, while it flattens out in the lower reaches.

Changes in river channel characteristics from upstream to downstream

As a river flows from its source to its mouth, the characteristics of its channel undergo significant transformations. These changes are influenced by the river's energy, the landscape it traverses, and the volume of water it carries.

Characteristics increasing downstream

• Discharge - The volume of water flowing through the river increases as tributaries join the main channel, contributing more water.
• Occupied channel width - The river channel becomes broader due to erosion and the increased volume of water.
• Channel depth - The river cuts deeper into the landscape, creating a more pronounced channel as it gains energy and volume.
• Average velocity - The speed of the water flow rises as the river encounters less resistance in smoother, wider channels.
• Load quantity - The total amount of material carried by the river grows as it picks up sediment from erosion along its path.

Characteristics decreasing downstream

• Load particle size - Sediment becomes smaller and more rounded due to attrition, where particles collide and wear down over time.
• Channel bed roughness - The riverbed becomes smoother as rough, jagged rocks are eroded or covered by finer sediment.
• Slope angle (gradient) - The steepness of the river's slope reduces as it moves from upland to lowland areas, resulting in a gentler flow.

Factors influencing variations in the long profile

The smoothness and shape of a river's long profile are not always consistent due to natural and human-induced features in the landscape. These factors can create irregularities or interruptions in the river's descent.

Influences on long profile variations

• Hard rock outcrops - Resistant rock layers crossing the river's path can create steep drops or irregularities in the profile.
• Natural lakes - These can act as temporary storage areas for water, disrupting the continuous descent of the river.
• Reservoirs - Human-made structures like dams alter the natural flow, creating flat sections in the profile where water is held back.

The impact of these changes on river processes and sediment load

The transformations in river channel characteristics and long profile have a direct effect on the processes occurring within the river and the nature of the sediment it transports.

Effects on river processes

• Erosion and transportation - In upstream areas, the steep gradient and rough bed lead to high-energy erosion, cutting deep V-shaped valleys. As the river moves downstream, the increased discharge and velocity enhance its capacity to transport larger quantities of material.
• Deposition - In downstream regions, the gentler gradient and reduced energy result in the deposition of finer sediment, contributing to the formation of wider floodplains.

Effects on sediment load

• Size and shape - Upstream, the load consists of larger, more angular particles due to limited attrition. As the river progresses downstream, constant collisions reduce particle size, resulting in smaller, smoother sediment.
• Quantity - The amount of load increases downstream as the river erodes more material from its banks and bed, and receives contributions from tributaries.