Stable isotope composition of water (δ2H and δ18O) coupled with other measures of hydrochemistry have proved particularly useful for understanding key hydrologic and ecological processes in dryland catchments, including identification of zones where hyporheic exchange is particularly important. I will draw on our studies of the Fortescue River catchment in the semiarid Pilbara region of northwest Australia to demonstrate how water isotopes, characterisation of dissolved organic matter (DOM) and nutrient stoichiometry have provided new insight into the origin of surface water, identification of mean residence times of stream water, biological groundwater dependency and interactions between nutrient cycling processes and productivity of IRES at pool, reach and catchment scales. For example, the isotopic compositions of surface water in the Pilbara is strongly determined by the time since major rainfall and recharge events, fractionation due to evaporative pressure as well as the degree of connectivity to alluvial water. Surface water expression at individual stream reaches across the catchment may be on a spectrum from perennial (persisting for years) to ephemeral (persisting from weeks to months). As pools become progressively isolated across the catchment, longitudinal stream processes are overridden by hydrological connectivity and evaporation. In IRES, the evaporation:inflow ratio (E/I) derived from water stable isotopes thus better explains patterns in catchment biogeochemistry than geographic longitudinal gradients.