The dominance of 'old' pre-event water in headwater storm runoff has been recorded in numerous upland catchment studies; however, the mechanisms by which this pre-event water enters the stream channel are poorly understood. Understanding these processes is fundamental to determining the controls on surface water quality and associated impacts on stream ecology. Previous studies in the upland forested catchment of the Afon Hafren (River Severn) at Plynlimon, mid-Wales, identified an active bedrock groundwater system that was discharging into the stream channel during storm response. Detailed analysis showed that these discharges were small and could not account for the majority of pre-event storm water response identified at this site; pre-event storm runoff had to be sourced predominantly from further upstream. An intensive stream survey was used to determine the spatial nature of groundwater-surface water (GW-SW) interactions in the Hafren Catchment. Detailed physico-chemical in-stream profiling identified a marked change in water quality indicating a significant discrete point of bedrock groundwater discharge upstream of the Hafren Transect study site. The in-stream profiling showed the importance of high spatial resolution sampling as a key to understanding processes of GW-SW interaction and how quick and cost-effective measurements of specific electrical conductance of stream waters could be used to highlight in-stream heterogeneity. This approach is recommended for use in headwater catchments for initial characterisation of the stream channel in order to better locate instrumentation and to determine more effective targeted sampling protocols in upland catchment research.