Which characteristic is common in mature rivers?

This is from my own observation. A mature river will have diversified life, both vegetative and animal, there is usually a load of sediment, debris and silt on the rivers bottom, there should be inlets and outlets along its traveled path, with evidence of corrosion on the rivers banks. If it is running through rock you want to see not just the layers of rock but staining from the water on the rock face. Many people say that a mature river will be shallow and wide, what about the Meh Kong, Nile and Niagara? They also say there will be no waterfalls, again I say, what about Niagara, and Angel Falls too? I also check the quality of the wood on the river base. As wood sits in water it can rot or become petrified, petrifaction takes a very long time to achieve.