This section of "Fresh Water Ecology" on The.Environment talks about how a river is classified. Rivers are classified by their speed and by the number of tributaries they have.

Speed

Rivers classified by speed are either lentic or lotic. A slow moving river is classified as lentic and a fast moving river is classified as lotic.

Tributaries

Classifying a river by its tributaries is a little confusing at first, but is easy after it has been explained. Rivers are classified from one and up. A river classified as one (1) has no tribuarites, and is probably very small. For a river to move up a class, another river of the same class most join it.

When a river starts, it is a one (1). It becomes a two (2) when the river joins with another class one river (1&1=2).

If a river of a lower class joins a river, then the river will remain the same class. For example if a class one (1) river joined our class two (2) river the river would remain a class two (2) river (1&2=2).

For a river to move up a class it must be joined by another river of the same class. For our river, that means that another class two (2) river must join it. Then it will become a class three (3) river (2&2=3).

Another way for a river to move up in class is for it to run into a river of a higher class. For you river, that means that a class four (4) or higher river must join it. When this happens, the new river takes the highest class. (3&4=4, (3&5=5, 3&6=6, etc)

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