Influences of Forest Harvest on Water Quality in Goat Creek, Swan River Basin, Montana

The purpose of this study was to examine the effects of timber harvest on attributes of water quality in Goat Creek of the Flathead River Basin, Montana, in comparison to the adjacent Upper Lion Creek watershed, which was unroaded and unharvested.

We concluded that water quality, measured in terms of nutrient and suspended solids concentrations and dynamics, was poorer in Goat Creek than in Lion Creek. During runoff, soluble phosphorus, temperature and conductivity were higher in Goat than Lion Creek. During base flow, nitrate plus nitrite nitrogen, soluble phosphorus, total phosphorus and conductivity were higher in Goat Creek than Lion Creek. Although particulate carbon and total suspended solids (during runoff) and total nitrogen (during base flow) were not significantly higher in Goat than Lion Creek at the 90% confidence level, the p values were only slightly over 0.10, suggesting possible differences in these variables as well.

Studies elsewhere have shown that undisturbed watersheds tend to retain nitrogen and phosphorus and export dissolved organic carbon. Many studies have shown elevated nitrogen, phosphorus and total suspended solids associated with harvest activities in watersheds. Hence, our findings were consistent with results found elsewhere.

The only biophysical factors that we measured that could explain the differences observed in the water quality attributes were the harvest legacy in Goat Creek. Establishing a conclusive linkage of incremental effects of harvest on water quality was beyond the scope of this study. Nor can we say that the harvest activities were necessarily harmful to Goat Creek because no specific impairment thresholds or criteria were established a priori. We can only say that the two watersheds are very similar and that water quality, measured in terms of nutrient and suspended sediment concentrations and dynamics, was poorer in the harvested watershed in comparison to the unharvested watershed. Longer period of record, coupled with more frequent sampling would allow a much more accurate determination. Additional paired basin studies also are needed to see if the patterns and correlations observed in Goat and Lion are consistent across a range of paired basin studies to allow a true analysis of variation design. The best scenario would be to initiate the comparison before harvest in either catchment.