ABSTRACT: In this study a set of equations was developed which can be used to separate the time varying effects from observed dissolved oxygen (DO) data. A steady state DO profile thus derived allows a reasonable stream stimulation such that both the model and the data used in its formulation do not contain DO due to biological activities. Biological DO production and consumption are complex phenomena. By excluding these highly variable processes, this method simplifies stream DO modeling considerably. The net oxygen input due to these processes exist only part of the day, but, in the stream waste assimilative capacity analysiis and waste load allocation, one would focus his attention on critical condition. Hence, unless the change of stream ecology is the main concern, it is desirable to formulate a stream water quality model without this time varying term.