National Park designation preserves the natural environment for the benefit of future generations and protects landscapes from most human development. Yellowstone National Park (USA) is the oldest national park in the world and protects most of the catchment of the Yellowstone River. The Greater Yellowstone Inventory and Monitoring Network has been monitoring water quality in the Yellowstone River for over a decade. However, until now, this data had not been assessed for changes in water quality parameters over time. Using a weighted regression on time, discharge, and season (WRTDS) model, we analyzed long-term trends in water quality data from the Yellowstone River at Corwin Springs (YRCS) gage site. Salinization parameters (e.g. Na+ concentration, specific conductance) showed virtually no trend over time, indicating that salinity levels are stable in the Yellowstone River. This contrasts with many other large rivers in the United States, which have been steadily increasing in salinity in recent decades. The YRCS monitoring site is unique in that it is home to one of the oldest U.S. Geological Survey gages in the United States, with discharge data dating back to the 1890’s. After the record-breaking flood in June of 2022, we analyzed the historical discharge dataset to contextualize the flood and to assess changes in the flow of the Yellowstone River over the past century. Using quantile regression, we found that the median values of discharge have not been changing over time. However, the 5th percentile of flows has been significantly decreasing and the 95th percentile of flows has been significantly increasing over time, raising economic and environmental concerns. Our analyses show that National Park designation can protect rivers from water quality degradation, but does not necessarily protect them from water quantity issues.