In the upper course, the river channel is typically shallow and narrow, often characterized by a rough bed due to large, angular bedload. The surrounding valley sides are usually steep, forming a distinctive V-shaped valley.
Water velocity in the upper course is generally low despite the steep gradient, primarily because a large proportion of the water is in contact with the rough bed and banks, leading to high levels of friction. This friction reduces the efficiency of water flow.
The dominant geomorphological process in the upper course is vertical erosion, where the river cuts downwards into its bed. This downward cutting, combined with weathering and mass movement on the valley sides, contributes to the formation of the V-shaped valley.
The middle course features a river channel that is deeper and wider than in the upper course, with valley sides that are gentler in slope. The bedload material becomes smaller and more rounded due to attrition during transport.
Water velocity generally increases in the middle course compared to the upper course, as the channel becomes more efficient. This is because a smaller proportion of the water is in contact with the bed and banks, resulting in lower levels of friction.
Lateral erosion becomes the dominant process in the middle course, meaning the river erodes sideways into its banks rather than primarily downwards. This lateral erosion contributes to the widening of the valley and the development of meanders.
In the lower course, the river channel is at its deepest and widest, flowing across a broad, flat floodplain. The bedload consists mainly of fine sediment and alluvium, which are easily transported.
Water velocity is typically at its highest in the lower course, despite the very gentle gradient, due to the significantly reduced friction from a smooth, deep, and wide channel. This efficient flow allows for the transport of large volumes of fine material.
While some lateral erosion continues, deposition becomes the dominant process in the lower course, especially during periods of reduced energy or when the river enters a larger body of water. This leads to the formation of floodplains, levees, and deltas.
The discharge (volume of water flowing per unit time) and velocity of a river are critical factors determining its energy and capacity for erosion and transportation. Higher discharge and velocity generally lead to increased erosive power.
Friction plays a significant role in modulating river velocity; rough, shallow, and narrow channels in the upper course create more friction, reducing flow efficiency. Conversely, smooth, deep, and wide channels in the lower course reduce friction, increasing velocity.
The size and angularity of bedload change along the river's course, influencing friction and erosional processes. Large, angular bedload in the upper course contributes to abrasion and high friction, while fine, rounded sediment downstream reduces friction and is more easily transported or deposited.
Geology and slope also profoundly affect river characteristics; resistant rock types slow down erosion, while steeper slopes enhance vertical erosion. Climate, altitude, and vegetation cover can indirectly influence discharge and weathering rates, further shaping the river's profile.
It is crucial to distinguish between the valley shape and the river channel shape when describing river characteristics. The valley refers to the broader landform encompassing the river, while the channel is the specific path the water flows through.
The transition from vertical erosion in the upper course to lateral erosion in the middle course, and finally to deposition in the lower course, is a fundamental principle of river geomorphology. This shift is driven by changes in gradient, energy, and channel efficiency.
Understanding the interrelationship between a river's characteristics and its processes is vital: for instance, a steep gradient and high friction in the upper course lead to vertical erosion and a V-shaped valley, while a gentle gradient and low friction in the lower course promote deposition and floodplain formation.
While the gradient decreases downstream, the overall velocity of the river generally increases from source to mouth. This seemingly counter-intuitive trend is explained by the reduction in friction as the channel becomes wider, deeper, and smoother, allowing water to flow more efficiently.
