Define the term fluvial. What is a fluvial process?

Erosion is a fundamental fluvial process that powerfully shapes our natural world. In basic terms, erosion is the wearing away of the land by the force of moving water. When it comes to rivers, erosion occurs continuously—each time water flows, it takes a little bit of the riverbank or bed with it. This can be seen through several mechanisms such as hydraulic action, where the force of the water breaks rock particles away from the river channel, and abrasion, where the river’s load scours the bed and banks. Abrasion occurs when the eroded materials that the river carries grind against the channel, much like sandpaper on wood.

Another mechanism is attrition, which leads to the river sediment becoming progressively finer downstream as the particles knock against each other and break apart. Additionally, solution or corrosion involves the chemical interaction between the water and the soluble minerals in the rocks, leading to the rock's dissolution. Over time, these erosion processes can have dramatic effects, carving deep valleys and creating features such as waterfalls and canyons.

Once material has been eroded by a river, it must be transported downstream. This is achieved through several different methods. Suspended Load refers to particles that are held in the flow of the river and includes silt and clay that often give rivers their muddy appearance. On the other hand, the Bed Load comprises larger particles that roll, slide, and bounce along the riverbed, a process known as saltation. Lastly, sediment can also be carried in Solution, dissolved within the water itself, largely invisible to the naked eye.

The load a river carries is not consistent; it heavily depends on the velocity of the water, which itself is influenced by factors such as gradient, channel shape, and volume of water. During floods, rivers can transport significantly larger amounts of sediment, contributing to rapid changes in the landscape. Understanding sediment transport is crucial as it contributes to the creation of a variety of fluvial landforms and directly impacts the river's capacity to shape its surrounding environment.

The formation of river channels is a complex process that is governed by the interaction between water flow, sediment carried by the river, and the landscape through which the river travels. Initially, small channels begin to form through a process known as headward erosion, where water flowing overland begins to carve a channel from its head downslope. Various types of channels can develop depending on factors like sediment load, velocity, discharge consistency, and the composition of the riverbed.

Meandering Channels

These are characterized by their sinuous paths and form in areas of soft sediment where water flow erodes the banks laterally.

Braided Channels

They occur when a river contains a high load of sediment relative to its water volume, leading to the river splitting into multiple channels that intertwine with each other.

River channel formation is also deeply influenced by the underlying geology; for example, harder rock can lead to straighter channels, while softer rock promotes meandering. Over time, river channels evolve and adapt to changes in water flow and sediment load, which can lead to the migration of meanders, the creation of oxbow lakes, and deltas where rivers meet larger bodies of water. This dynamic process, coupled with erosion and sediment transport, demonstrates the powerful and ever-changing nature of fluvial processes.