Temperature modification in streams as a result of hypolimnitic-discharges from impoundments was investigated from both an analytical viewpoint and a case study. A one-dimensional mathematical-analysis of stream temperatures as affected by hypolimnitic discharges is outlined and the interrelationships of stream depth, flow velocity, discharge temperature, and normal stream temperatures are discussed. There are three types of temperature effects that may occur depending on the time of year and the characteristics of the impoundment. Compared to those temperatures which would have occurred in the absence of the impoundment, maximum diurnal temperatures are reduced at all points in the downstream channel only when the discharge temperature is lower than the normal minimum diurnal temperature. A technique is presented by which the general temperature changes which are likely to occur as a result of hypolimnitic discharges may be estimated. Although the technique has not been tested sufficiently to judge its general merit, it provides a reasonable description of the changes observed in Mill Creek. (Author)