The statistical technique of analyzing data collected at regular intervals to reveal periodic components of the data is described by reference to actual records. The data chosen for illustration include tide height in a river; biochemical oxygen demand and dissolved oxygen in the same river; discharged salt into a river system and its relation to the hardness of the water; and the discharge of sewage into a river. Annual, biannual, and other cycles are demonstrated in the data. The limitations of the technique, the sources of error resulting from methods of data collection and choice of parameters for use in the statistical analysis are discussed. It is emphasized that interpretation of the results must be based on an understanding of the natural process. The book is primarily intended for use in mathematical modeling of environmental parameters in studies of water pollution of rivers and estuaries. There is a short bibliography. (AL)