forest ecosystems today are continually under pressure from a variety of special interests including timber operations. a goal of the ontario ministry of natural resources to aid natural ecosystems by providing forest management guidelines is to assist foresters with timber operations. the timber management guidelines for the protection of fish habitat were developed to mitigate the harmful influences of timber operations on fish habitat and water quality. knowledge of a stream’s thermal regime is important for assessing the effectiveness of the area of concern when applying the guidelines. currently, the lack of this information in the hearst mnr district led to the attempted use of a stream thermal stability model. the model was developed to allow users to collect only a single stream temperature at 16oohr on a day when temperatures exceeded 24.5°c. fifty-one stream segments were studied and temperature data were collected with automated hobo8 loggers. the thermal stability model proved unsuccessful as the model could not be calibrated for this district.