Granitic headwater streams are not only interfaces between groundwater and surface water but also represent poorly understood but highly reactive components of the terrestrial carbon cycle. Especially their low buffering capacity makes these environments particularly sensitive to short-term hydrological and biogeochemical fluctuations. This study examined how groundwater discharge and episodic high-flow events affect the hydrochemistry and CO₂ dynamics in a forested, high-gradient headwater stream (White Main, Fichtelgebirge Mountain, Bavaria). Over a sampling period of 15 months, baseflow conditions showed a downstream (spring to pond) decline in partial pressure (pCO₂), CO₂ fluxes (F_CO2), accompanied by increasing δ¹³C_DIC, reflecting rapid degassing from CO₂-rich groundwater. However, during episodic high-flow events, pCO₂ and F_CO2 rose sharply at downstream sites, coinciding with brown water coloration and distinct charge balance anomalies. These event signatures indicate an increased coupling between stream water and organic-rich soil layers. This leads to the mobilization of Fe-Al-bearing colloids, clay minerals, and natural organic matter (NOM), which temporarily modify proton balances, alkalinity, and carbonate equilibria. The resulting short-term CO₂ supersaturation demonstrates that hydrological pulses can bias apparent carbon fluxes in low-buffered headwaters. Recognizing this reactive soil-stream coupling is essential for correctly scaling CO₂ emissions from inland catchments.