A river is a system comprising both the main course and all the tributaries that feed into it; the area that the river system drains is known as the catchment. The main characteristic of rivers is their continuous one-way flow in response to gravity.
Due to variations in physical conditions, such as slope and bedrock geology, rivers are dynamic and may change form several times throughout their course. For example, a fast-flowing mountain stream may develop into a wide, deep and slow-flowing lowland river.
When assessing river characteristics and water quality, it is important to bear in mind that a river comprises not only a main course but also a vast number of tributaries. For example, the main course of Europe's largest river, the Volga, is 3 500 km long. In addition, it receives water from ten tributaries of over 500 km in length and more than 151 000 tributaries of over 10 km in length.
The following pages describe: