IMPACTS OF MODERN TIMBER HARVESTING ON WATER TEMPERATURE IN NORTHERN IDAHO HEADWATER CATCHMENTS

Concerns regarding the impacts of modern timber harvesting on stream water temperature maxima emphasize the need for an increased understanding of temperature patterns in headwater areas. A network of water temperature recorders was installed in the Mica Creek Experimental Study Area in north Idaho to investigate the relationships between forest treatments and stream temperatures. Sensors were placed in 1st-order non-fish bearing control reaches, non-fish bearing harvested reaches, and downstream into 2nd and 3rd-order fish bearing reaches of the stream network. Treated watersheds consisted of 50% canopy removal by modern clearcut methods and selective cut practices. Harvesting followed the current Idaho Forest Practices Act regulations. The Stream Protection Zone (SPZ) for non-fish bearing reaches required an equipment exclusion zone within 30 feet of the ordinary high water mark or definable bank. Any fish bearing stream which bordered the harvesting was protected with an SPZ of at least 75 feet, with a requirement to leave at least 75% of existing shade. Timber harvesting occurred in the summer and fall of 2001. Temperature data were collected in the non-fish bearing sites for the summers of 2001 and 2002. Water temperature at the downstream fish-bearing sites has been collected continuously since 1991. Water temperature maxima in the non-fish bearing reaches increased by 1.5 to 2.0° C in the partially clearcut watershed, probably due to increased solar radiation from decreased canopy cover. There was also an apparent slight 0.2 to 0.5° C upward trend at the selective cut sites when compared to the control tributaries. Despite the temperature increases in the 1st-order reaches, there was no apparent increase in water temperature maxima in the fish bearing reaches downstream. Continued monitoring at these sites is planned to evaluate stream temperature trends over subsequent years.