What are the two main processes involved in fluvial landforms?

The two main processes are erosion (the wearing away of the land) and deposition (the settling of sediment as water slows down).

What is the difference between a V-shaped valley and a U-shaped valley?

A V-shaped valley is typically formed by the vertical erosion of a youthful river in mountainous areas, whereas a U-shaped valley is often formed by glacial erosion widened by river action.

How is an oxbow lake formed?

An oxbow lake is formed when a meander becomes very sharp and eventually gets cut off from the main river channel during a flood, leaving a crescent-shaped lake behind.