Channel and Catchment Morphology, Spatial Intermittency, and Carbon Chemistry of a Headwater Stream

We investigated carbon dynamics in a steep, forested, headwater stream in the Cascade Mountains of western Oregon, USA. Measurements from a continuously recording pCO2 probe located near the mouth of the catchment showed that the stream was always super saturated with CO2 with respect to atmospheric concentrations, ranging from 500 ppm in mid-winter to as much as 3,500 ppm in late summer. Continuous measurements of pCO2 from a hyporheic well suggested that the hyporheic zone was a likely source of the super-saturated stream water because the hyporheic concentrations of CO2 ranged from a mid-winter low of 4,000 ppm to a late summer high of 16,000 ppm. Here, we investigate the causes for the large seasonal changes in pCO2 in the stream water. We conducted longitudinal synoptic surveys of flow and carbon chemistry over the period of baseflow recession during summer 2015. The channel is narrow and steep with occasional bedrock segments. However, debris flow deposits in the lower portions of the studied reach create wider valley floors where hyporheic exchange can capture 100% of the streamflow when discharge is very low. At the beginning of the summer when discharge was relatively high, flow was spatially continuous, but by mid-summer, stream flow became spatially discontinuous. Upwelling hyporheic water in these locations appears to be super saturated with CO2. In early summer, the amount of upwelling hyporheic water was small relative to stream discharge so that hyporheic exchange had only a modest influence on stream pCO2. Later in the summer, when discharge was much smaller relative to hyporheic exchange, we observed much greater spatial variability in CO2, which averaged 2720 ppm downstream of dry segments longer than 5 m but only averaged 980 ppm in wet segments and below shorter dry segments. Over the intervening wet segments, CO2 appears to be evaded from the stream as concentrations decreased rapidly. Also, upslope accumulated area appears to control lateral inputs of hillslope or groundwater which is low in CO2, leading to local minimums in stream CO2 concentrations. Overall, channel and watershed morphology exert strong controls on both spatial patterns and temporal dynamics of CO2 in the stream.