Water chemistry is responsible for many of the characteristics associated with the "quality" of a river (see the general section on water chemistry). It is a reflection of complex interdependent relationships involving the river, the atmosphere, the surrounding soil and rocks, groundwater, sediments and living systems.

Rivers have been called the "gutters down which flow the ruins of continents". The world's rivers dump 2.25 x 1010 metric tons of dissolved and particulate matter from erosion of the land into the seas every year, thus playing a major role in global biogeochemical cycling. The river, however, is more than just a transporter of materials. It is also a processor of materials as the biota it contains take up, convert, use and release materials that come to them. It is useful to think of a river as an active biological system that metabolizes the organic matter contained within it. The water that arrives at the mouth of a river is far different, both quantitatively and qualitatively, from what was present in the waters nearer the source.