Freshwater salinization impacts the availability of water for human use and ecosystem needs across the globe. In the southwestern United States, it has been estimated that total dissolved solids (TDS; a proxy for salinity) from the Upper Colorado River Basin cause upwards of $300 million per year in economic damages. Substantial resources are devoted to reducing TDS loading to streams and rivers in the basin, with targeted mitigation efforts that have been informed by long-term average models that have identified ubiquitous dissolution of minerals in soils and rocks and relatively small areas of irrigated lands as dominant sources of TDS. Recent work has demonstrated that between 65% and 80% of TDS loading to streams in the basin originates from baseflow, a proxy for groundwater discharge to streams, highlighting the importance of subsurface processes in TDS delivery to streams. This study describes the development and application of two coupled, temporally dynamic Spatially Referenced Regressions on Watershed attributes (SPARROW) models that estimate sources and processes influencing the transport of dissolved solids to streams in the basin at a seasonal time-step over a 35-year period. The key advance is using seasonal estimates of baseflow TDS loads from a baseflow-specific model as explicit time-varying inputs to a total in-stream TDS model, which allows for source tracking through the subsurface and across the landscape over a range of timescales. Results suggest that baseflow is a dominant source of TDS loading throughout the basin, and its relative source proportion varies spatially and across timescales as a function of climate, geology, and land use. Given the lagged delivery associated with baseflow contributions of constituents to streams, this approach of first quantifying baseflow and its drivers relative to all sources has implications for how, when, and where mitigation efforts aimed at reducing TDS loading to streams may be effective.