Water quality is a key measure of ecosystem health. Catchments, or watersheds, are areas of land draining all inflows of water to a common outlet. Stream chemistry is typically measured at the outlet of a catchment, such that it reflects the many interconnected hydrologic and biogeochemical processes under the influences of climate, lithology, vegetation, land use, topography, and other factors. However, an important limitation of catchment-scale analyses is the general difficulty of applying site-specific findings to regional, continental, or global scales. In addition, stream chemistry data are often limited both spatially and temporally, while subsurface water chemistry data are very scarce in general. Thus, it is necessary to develop broad, conceptual frameworks and tools that can be applied to water chemistry at catchments across gradients of climate, lithology, vegetation, land use, and topography. Similarly, studies that couple catchment- and larger-scale analyses can be especially useful for bridging the gaps between our existing knowledge of each. In this work, we use a variety of data analysis and process-based reactive transport modeling approaches to study drivers and patterns in solute export and stream water quality at multiple catchments and spatial scales to broaden our understanding of major hydrological and biogeochemical processes. Chapter 1 provides an overview of key issues and knowledge gaps in catchment hydrology and biogeochemistry research that motivated each study in this dissertation. Chapter 2 provides direct data support for the shallow and deep hypothesis, which suggests that solute export behavior is driven by the distinct chemical signatures of shallow and deep source waters and shifting dominance of hydrologic flow paths. Chapter 3 identifies the differential controls of climate and hydrology on dissolved carbon production and export, where production is typically faster in the shallow subsurface under warm conditions, while export is determined by hydrologic conditions and the dominant subsurface flow paths that transport dissolved organic and inorganic carbon to the stream. Chapter 4 reveals a near-universal pattern of dilution behavior for dissolved inorganic carbon across the continental United States, primarily driven by the commonly observed profile of subsurface CO2 increasing with subsurface depth. This study also identifies climate as a key control on the long-term behavior of stream dissolved inorganic carbon, where arid sites typically have higher stream concentrations than humid sites. Chapter 5 summarizes the overall conclusions from each study, broader implications of the work presented in this dissertation, and future directions for research. Chapter 2 is currently published in Water Resources Research, and Chapter 4 is currently published in Global Biogeochemical Cycles. Chapter 3 is currently in preparation for publication.