Mountainous headwater stream networks are key drivers of freshwater and solute export to downstream waters and exert a strong influence on riverine chemistry due to their high surface-area-to-volume ratios and terrestrial-aquatic connectivity. Varied flow paths across heterogeneous terrestrial subcatchments result in distinct assemblages of geogenic and biogenic solutes delivered to streams, where they may serve as indicators of ecosystem function. Understanding how landscape characteristics influence freshwater and solute export is particularly important as high-elevation watersheds undergo climate-driven shifts in precipitation patterns and vegetation communities, affecting water resource resilience worldwide. Since 2023, we have conducted monthly catchment-wide synoptic sampling campaigns of streamflow and water chemistry across gradients of elevation, aridity, vegetation composition, topography, and hydrological connectivity within a major municipal watershed of the southwestern United States. We characterized the magnitude and variability of subcatchment contributions to total watershed export through a residual analysis and inferred stream network function via mass-balance-based ecohydrological metrics. Our residual analysis suggests that high-elevation subcatchments, which receive greater mean annual precipitation, export higher-than-expected and less variable amounts of freshwater, nitrate, phosphate, and dissolved organic carbon (DOC), relative to lower-elevation, drier subcatchments. These patterns reflect hydrological controls on solute export and likely arise from shallower, more dynamic flow paths associated with biogenic solute transport. However, several sites exhibit behavior opposite to that of adjacent subcatchments with similar terrestrial characteristics, suggesting heterogeneous terrestrial nitrate pools, contrasting source types, or localized land disturbance. Furthermore, ecohydrological metrics reveal seasonal DOC enrichment and nitrogen uptake at lower elevation stream reaches, highlighting differences across the catchment. Through these analyses, we identify subcatchments and stream reaches that disproportionately contribute to catchment-wide export and exert influence on stream network function. These findings provide insights to inform watershed management strategies aimed at maximizing the reliability of freshwater export, improving water quality, and supporting watershed resilience under changing climatic conditions.