Human effects on water quality in the northern Rocky Mountains: relationships among solute sources, transformations, and flow

Abstract

Humans have profoundly altered water quality within streams and rivers. Degradation of water quality has manifested through direct effects on stream water chemistry via changes to physical and chemical mechanisms of solute delivery across the landscape. Indirect effects on water quality have also resulted from the influence of human-induced climate change on the hydrologic regimes that determine solute transport. This dissertation contributes research on the controls of water quality in streams and rivers of the northern Rocky Mountains (NRM), which drain a snowmelt dominated region consisting of parts of Montana and Idaho in the United States (US) and southern British Columbia, Canada. This area includes the largest contiguous wilderness area in the continental US and contains watersheds that have undergone land use change from large-scale mining and urban development. The NRM consists of headwater systems draining high elevation regions subject to disproportionate effects of climate change. We consider pathways of human influence on water quality in the NRM via examination of spatiotemporal relationships among solute concentrations, solute loads, and the flow of water. First, an investigation of the influence of large-scale coal mining operations on solute delivery provides evidence of mechanisms underlying increases in selenium and nitrate contributions to a river. These results suggest hydrologic processes responsible for delivering solutes to the river have shifted over time with changing mining operations. Next, we describe how chronic nitrate loading from distributed releases of treated wastewater have fueled autotrophic growth that influences both seasonal and diel concentration dynamics. These results highlight the importance of understanding biologic feedbacks on nutrient concentration dynamics for biogeochemically reactive solutes. Finally, application of a novel hydrograph differentiation technique reveals evidence that climate change is shifting ecologically relevant transitions in snowmelt hydrologic regimes to earlier in the year for over half the NRM watersheds assessed. These results reveal an objective approach to identifying inflections of hydrographs that are related to key transitions in catchment storage behaviors underlying snowmelt hydrologic regimes. Results across these studies suggest that effective decisions surrounding mitigating water quality problems sit at the nexus of understanding sources of contaminants, their potential to be transformed by watershed systems, and the hydrologic regimes that control their transport.