Water flowing through a river perform three kinds of work:
Which of this three processes are at any time predominant depends on the local conditions. The river always tries to adjust on the existing conditions. In general, erosion is predominant in the headwater areas, transport in the middle reaches and deposition in the lower reaches of the respective watershed .
By these processes rivers are an important agent in wearing down the landscapes in all parts of the world. This is object of research in fluvial morphology.
Start studying river processes by learning about fluvial erosion.