Effects of road construction on water quality parameters and bull trout (Salvelinus confluentus) populations in three Montana water bodies

Abstract

Road construction adjacent to rivers and ensuing clearing of timber and shrubs can lead to alterations in the dynamics and morphology of channel features and water chemistry that provide habitats for aquatic biota. We examined associations between bull trout (Salvelinus confluentus) populations and water quality parameters related to road construction in three western Montana rivers over a 15-year study period. Bull trout have specific habitat requirements that can influence their overall abundance and distribution within a watershed, making them an important indicator species of general ecosystem health. We used average annual bull trout population data from Montana Fish, Wildlife, and Parks and daily water quality observations from the United States Geological Survey between 2000 and 2014 to analyze the correlation between the data sets. We used a nonparametric statistical test to determine whether any significant change was observed between the medians of pre- and post-construction water quality parameters and bull trout population numbers. The association between the water quality parameters and bull trout numbers was visually examined with scatter plots created in R where the Y-axis was population numbers and the X was the water quality parameter. The plots were then fit with a linear regression line and from this a visual interpretation of the association and strength of said associated was determined. The analysis yielded unexpected results with only some of the water quality parameters exhibiting a negative relationship with road construction disturbance. Two water bodies (Flathead River and Warm Springs Creek) showed no significant changes in bull trout population numbers, and the control water body with no road construction (the Blackfoot River) displayed significant differences between the pre- and post-construction water quality parameter medians. The parameters that did display expected outcomes (cadmium concentration in the Flathead River; water temperature, pH, and instantaneous discharge in Warm Springs Creek; and instantaneous discharge, pH, and cadmium concentration in the Blackfoot River) were not all strongly correlated with decreased bull trout population numbers. The water quality parameters common to all the study rivers that presented a negative association with bull trout numbers was pH. High levels of recreation and management activities on the Blackfoot River (the control) could explain these unexpected findings. The results do, however, partially corroborate previous studies on the negative association between bull trout population numbers and road construction-related disturbance. A longer study period combined with finer-grained data would be beneficial for future studies.