River length - the distance from the source to its mouth. It is measured along the geometric axis of the channel, and for large rivers along the fairway.

Determining the length of a river is not an easy task, requiring knowledge of the places where the river begins and ends, as well as accurate measurements of the river length between these points. Because of this, many river lengths can only be determined with some approximation. Difficulties in determining the beginning of a river can occur because of the large number of tributaries. Of all the tributaries, the one that begins at the farthest point from the mouth is considered the beginning of the river, giving the river its maximum total length. In practice, however, the name of this most distant tributary is often not the same as the name of the river itself. In this case, we are not talking about the length of the river, but the length of the total watercourse. Seasonal changes in the channel also complicate the calculation of the total length. The mouth of a river is often an estuary that gradually widens and opens into the ocean, which also makes it difficult to accurately determine the end of a river. Some rivers do not have an estuary, but gradually lose volume and eventually evaporate - thus their endpoints also depend on the season.

Determining the length of a river was often determined by the accuracy of its map. After the mastery of space photography, mapping became easier, but questions still remained - the length depends on the selected arm in the delta or how the length is calculated over the lake.

The longest rivers in the world (lengths of river systems including all tributaries):

Amazon - 7100 km (Brazil, Peru, Bolivia, Colombia, Ecuador, Venezuela, Guyana);

Nile - 6670 km (Burundi, Egypt, Kenya, Congo, Rwanda, Sudan, South Sudan, Tanzania, Uganda, Eritrea, Ethiopia);

Mississippi-Missouri - 6,420 km (USA, Canada)

Yangtze - 6,300 km (PRC)

Yellow River - 5,500 km (PRC)