1. Using GIS for River Classification
River Classification is a cost-effective way of identifying rivers which are similar in their flow, sediment, nutrients, and biota
Classification is used by management agencies to set appropriate management plans
For example, they might try to manage a mountain river so that it is a good salmon fishing area, but that would not be a sensible goal for lowland streams which are fundamentally unsuitable for salmon

2. Using GIS
IF you have a soundly-based set of rules for classifying rivers on the basis of mapped information, THEN GIS is a good way to implement classification
We have mapped classes at the river reach scale (link in a network)
The class of a river depends on the properties of the catchment draining to that reach, e.g. average temperature in the catchment