Forest restoration has been widely promoted as a nature-based solution to protect water quality by reducing nutrient and sediment inputs to streams. Nevertheless, the magnitude over which restored riparian forest functions as an effective buffer, resulting in lower in-stream concentrations and reduced pollutant exports, remain poorly constrained, particularly across landscapes with contrasting disturbance histories. Here, we assessed how different stages of riparian vegetation influence stream water quality and in-stream pollutant exports across headwater catchments spanning a gradient of ecological conditions in two contrasting environments: one with low ecological resilience shaped by intensive agricultural practices, and another with high resilience, characterized by lower historical disturbance and greater landscape connectivity. Across these environments, we sampled a total of fifty-seven catchments (approximately thirty per environment) during the dry season of 2025 and the rainy season of 2026. Catchments were classified into four forest-cover classes according to the type and extent of vegetation cover: (i) catchments dominated by native forest remnants; (ii) pasture-dominated catchments with well-developed riparian buffers; (iii) pasture-dominated catchments with narrow riparian thicket buffers; and (iv) catchments largely covered by pastures. Stream water was sampled to determine concentrations of total dissolved carbon, dissolved organic carbon, dissolved inorganic carbon, total suspended solids, fixed and volatile suspended solids, total nitrogen, nitrite, nitrate, total phosphorus, and orthophosphate. Landscape metrics, including vegetation indices and land-use/land-cover composition, were derived using GIS at reach, riparian, and catchment scales. Stream discharge was measured using the pulsed release of a conservative solute, enabling the calculation of mass loads and the estimation of pollutant exports across multiple temporal scales. Our results provide novel, process-based insights into how riparian forest restoration stage and landscape context jointly regulate in-stream water quality and pollutant export in headwater systems. These findings offer actionable guidance for the design and prioritization of riparian restoration strategies aimed at improving water quality and reducing downstream pollutant delivery.