Declining Snowpack in the Presence of Stable Precipitation May Not Negatively Impact Baseflow or Floodplain Vegetation in the Middle Fork Rock Creek Watershed, Montana, USA

Emily Iskin, Anna Bergstrom, Jodi Brandt

In the age of snow droughts and megafires, water availability and changes in precipitation, snowpack, and baseflows are active areas of research. Headwater streams are where all large rivers begin, but their seasonal water availability is difficult to measure because they are so abundant and remote. Remote sensing can help monitor small streams semi-arid areas if there is an appropriate proxy for water availability given the coarse spatial resolution of satellite data. In this study, 40 water years (1984-2024) of climate, streamflow, and floodplain vegetation data are compiled in the relatively undisturbed and gaged Middle Fork Rock Creek watershed in Montana to investigate if and how climate, streamflow, and vegetation are changing through time and/or are correlated with each other. We find that temperatures are warming, snowpacks are shrinking, and total flow volume is growing, but total precipitation, late season baseflow volume, and floodplain vegetation remain stable. In a given water year, greater coverage of floodplain vegetation in September is correlated with more precipitation and higher late season baseflow volume, indicating that the stream and the vegetation might not be in direct competition for available water. Additionally, floodplain vegetation measured with remote sensing is likely an appropriate proxy for late season baseflow in this watershed and could potentially be used to monitor ungaged watersheds. More study is needed to determine if floodplain vegetation is an appropriate proxy for water availability generally across the semi-arid U.S.,; and how these relationships between climate, streamflow, and vegetation might vary across watersheds of varying size, topography, and ecoregion.