Dynamics of dissolved oxygen (DO) and greenhouse gases (GHG; CO2, CH4) in flowing waters are strongly controlled by both physical and biological processes, primarily metabolism and gas exchange. Although strengths of biological and physical processes can vary within reaches due to flow variability and geomorphology, gas dynamics have conventionally been studied by one- or two-station measurements in streams, assuming a uniform upstream gas concentration. We questioned the validity of this assumption, asking how DO and GHG dynamics vary at the reach scale in a stream with both lentic behaving (pools) and lotic-behaving (riffle) habitats. We measured DO (continuous) and GHG (synoptic) concentrations at 9 sites in a ~ 5 km forested reach of New Hope Creek, North Carolina, where physical conditions (e.g., gas exchange rate, residence time) vary longitudinally, from June 2023 to February 2024. Under baseflow, both DO and GHG concentrations were longitudinally heterogeneous due to reach-scale variations in gas exchange rate and residence time. In contrast, gas concentrations converged to the saturation level (100% saturated in O2 and GHG) during high flow events. Analysis on daily DO saturation patterns showed that the amplitude of daily DO variation was longitudinally heterogeneous under baseflow but became uniformly negligible under high flow. Longitudinal variation in DO and GHG concentrations was comparable to the variation found at the US continental scale, implying that reach-scale variation can impact our understanding of broad scale variability in both metabolism and greenhouse gas flux. These results suggest that longitudinal discontinuity should be considered when investigating true reach-scale gas dynamics with proper understanding of reach-scale heterogeneity in physical controls and temporal flow variability.