A quantitative evaluation of stream temperature alterations due to a commercial forest harvesting practice and a research treatment is presented. Summer maximum stream temperatures averaged I C higher in the commercial clearcut and 9 °C higher in the clearcut-herbicided watershed than in the forested control. The largest average monthly temperature increase on the commercial clearcut (2.2 C) occurred during April; on the clearcut-herbicided basin it occurred during June (10.5 C). Significant changes in stream temperature were observed on both watersheds as early as February and as late as November.Changes in minimum stream temperatures are presented in detail along with the impact on diel temperature fluctuations. Changes in the stream temperature regimes of the clearcut watersheds from the headwaters to the mouth of the watersheds are also given. Potential impacts of the stream temperature alterations on aquatic ecosystems are summarized in relation to stress limits for brook trout and other organisms.

Exposing headwater streams to direct solar radiation by removing forest cover has the potential to cause drastic changes in streamwater temperature regimes. A study was conducted to evaluate the maximum potential impacts and to evaluate the effectiveness of management practices used to control these detrimental effects. The control watershed approach was utilized. A clearcut‐herbicide experiment on a small, headwater stream increased maximum stream temperatures as early as February and as late as November. The average monthly maximum stream temperature increase was 4.4°C. Stream temperatures above 21°C occurred nearly every day during the summer. Stream temperatures above 25°C were recorded as early as May. The highest stream temperature recorded was 32°C. On an adjacent forested watershed, stream temperatures rarely exceeded 20°C; the highest recorded temperature was 22°C. Minimum stream temperatures on the clearcut‐herbicided watershed increased an average of 2°C during the summer months, but were as much as 3.9°C lower during the fall and winter months. Diurnal fluctuations in stream temperature were also increased. Diurnal fluctuations as high as 17°C occurred on the clearcut‐herbicided watershed compared with only 4°C on the forested watershed. On an adjacent commercially clearcut watershed, where a buffer zone was left along the perennial stream channels, only slight changes in stream temperature were observed. The average monthly maximum stream temperature increase was <1°C; the highest temperature recorded was 23°C. Minimum temperatures remained generally unchanged.