Publication Summary

The Forest Practices Rules and Regulations contain Best Management Practices (BMP) which include requirements for Riparian Management Zones (RMZ) on certain water types affected by timber harvest activities. The purpose of this study was to test the effectiveness of the BMPs (i.e., RMZs) at achieving water quality standards for temperature.

The Forest Practices Rules and Regulations contain Best Management Practices (BMP) which include requirements for Riparian Management Zones (RMZ) on certain water types affected by timber harvest activities. The purpose of this study was to test the effectiveness of the BMPs (i.e., RMZs) at achieving water quality standards for temperature.

Recording thermographs were deployed upstream and downstream of 13 RMZs statewide during the summer of 1990 to monitor stream temperature response to timber harvests. Streams and riparian zones were characterized to evaluate factors influencing the observed temperature conditions.

Maximum observed water temperatures ranged from 12.8°C to 19.9°C. Maximum water temperature change between upstream and downstream monitoring sites ranged from 0.3°C to 5.2°C. Definitive determinations of whether applicable water quality criteria were met or exceeded were not possible for many of the study sites due to uncertainties related to thermograph accuracy and/or representativeness of the monitoring period. Water temperature criteria were met or judged likely to be met at three of the thirteen study sites. These RMZs were considered effective. Temperature conditions at five of the 13 study sites exceeded applicable criteria for maximum allowable temperature, with conditions at an additional three sites possibly exceeding criteria. The BMP was considered ineffective at six of the sites where maximum temperature criteria were exceeded or possibly exceeded. At two of the five sites where maximum allowable temperature criteria were exceeded, the exceedances were attributed to factors other than timber harvesting at the study site, and the BMP was considered effective. Possible exceedance of the criteria for allowable temperature change due to timber harvesting was indicated by the monitoring results at two of 13 sites. At one additional site, exceedance of the temperature change criteria was suspected based on spot field checks.

The primary factors influencing BMP effectiveness appear to be site elevation, post-harvest shade levels, groundwater flux within the reach, and stream morphology. Stream modification by beavers was a significant factor influencing the effectiveness of RMZs at some sites. The proposed new TFW method for identifying temperature-sensitive streams takes the most important factors into account, and is expected to correctly identify streams where enhanced RMZs are needed in a majority of cases. To optimize the effectiveness of RMZs, procedures to identify and address site-specific anomalies which result in temperature sensitivities that would not be identified by the new TFW stream temperature screen and/or model should be incorporated into the BMPs.

This document is Ecology Publication No. 92-64, and TFW Publication No. WQ6-92-001.