What can physical indicators tell you about a river?

Question

like a rivers stage? what else?

Answer 1

If it is above flood stage, it either has been raining too heavily in its watershed or else snow is melting very fast or a combination of the two.  If it is low and hardly flowing, the watershed is suffering from a drought.  If the fish are dead or dying, there is too much pollution in the water.  If it is full of mud and debris, the volcano up the way just erupted.