Water flowing through a river perform three kinds of work (see figure):
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 (Bradshaw and Weaver 1995; Marcinek 1997).
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.
You’ll get an introduction to fluvial morphology in German language if you follow this hyperlink.
Start studying river processes by learning about fluvial erosion.