Seasonal Stream Temperature Changes Following Forest Harvesting

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.