The geophysical characteristics of the drainage basin and the biochemical and physical environment of the river affect the concentration of dissolved oxygen. These factors are embodied in the fundamental equation of continuity that describes the oxygen balance. The variation of the fresh-water flow and cross-sectional area is included, as well as the various sources and sinks of oxygen: natural and artificial aeration, the photosynthetic contribution, bacterial and algae respiration, carbonaceous and nitrogenous oxidation, and benthal deposits. Application of the basic equation is summarized by two general cases: the first in which bacterial respiration and spatial profiles are significant and the second in which the algae activity and the temporal changes are predominant. The equations provide technological functions for these cases to assess water quality and pollution and to determine the effect of many natural or artificial changes in the stream environment.