Headwater streams comprise nearly 90% of the total length of perennial channels in global catchments. They mineralize organic carbon entering from terrestrial systems, evade terrestrial carbon dioxide (CO2), and generate and remove carbon through in‐stream primary production and respiration. Despite their importance, headwater streams are often neglected in global carbon budgets primarily because of a lack of available data. We measured these processes, in detail, over a 10 year period in a stream draining a 96 ha forested watershed in western Oregon, USA. This stream, which represents only 0.4% of the watershed area, exported 159 kg C ha−1 yr−1, similar to the global exports for large rivers. Stream export was dominated by downstream transport of dissolved inorganic carbon (63 kg C ha−1 yr−1) and by evasion of CO2 to the atmosphere (42 kg C ha−1 yr−1), leaving the remainder of 51 kg C ha−1 yr−1 for downstream transport of organic carbon (17 kg C ha−1 yr−1 and 34 kg C ha−1 yr−1 in dissolved and particulate form, respectively).

We investigated carbon dynamics in the hyporheic zone of a steep, forested, headwater catchment western Oregon, USA. Water samples were collected monthly from the stream and a well network during base flow periods. We examined the potential for mixing of different source waters to explain concentrations of DOC and DIC. We did not find convincing evidence that either inputs of deep groundwater or lateral inputs of shallow soil water influenced carbon dynamics. Rather, carbon dynamics appeared to be controlled by local processes in the hyporheic zone and overlying riparian soils. DOC concentrations were low in stream water (0.04–0.09 mM), and decreased with nominal travel time through the hyporheic zone (0.02–0.04 mM lost over 100 h). Conversely, stream water DIC concentrations were much greater than DOC (0.35–0.7 mM) and increased with nominal travel time through the hyporheic zone (0.2–0.4 mM gained over 100 h). DOC in stream water could only account for 10% of the observed increase in DIC. In situ metabolic processing of buried particulate organic matter as well as advection of CO2 from the vadose zone likely accounted for the remaining 90% of the increase in DIC. Overall, the hyporheic zone was a source of DIC to the stream. We suggest that, in mountain stream networks, hyporheic exchange facilitates the transformation of particulate organic carbon buried in floodplains and transports the DIC that is produced back to the stream where it can be evaded to the atmosphere.

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It is safe to say that no portion of this process would have been completed without the help of my thesis mentor Tim Ku. From the start he has worked tirelessly to help me acquire this research opportunity, develop the study itself, conduct intensive laboratory analyses, modify unworkable methods, fix malfunctioning machines, interpret data, and edit my written analysis and discussion. Though the process was at times overwhelming, in retrospect the amount I have learned from it is staggering. In addition, I want to thank Tim for his guidance over the past three years, as it was his enthusiastically taught geochemistry classes that ultimately fostered my interest in geochemistry and led me to choose Earth & Environmental Sciences as a major. The research portion itself would not have been possible without the Keck Geology Consortium, my project directors David Dethier and Will Ouimet, and my fellow Keck Colorado 2010 student researchers, in particular the rest of the soils crew. I also thank Joel Labella, who willingly tackled every piece of equipment that succumbed in the face of concentrated acid solutions, and Ginny Harris, who provided encouragement each step of the way. My gratitude also extends to the rest of the E&ES department, in particular my readers Peter Patton and Dana Royer, who, when presented with drafts only days before the deadline provided insightful edits in a very short time. I also want to thank Peter for convincing me to keep going with this project back in November and for motivating me and cheering me up throughout. Thanks also go to my lab-mate and friend Hannah Bourne, who provided an immense amount of advice, and who was with me in the lab until the very end. Considerable support was provided by my friends Alisa Fersch, Mary Weir, Eleni Healey, Rosemary Ostfeld, and Lisa Gränitz, who listened to my trials every step of the way, as well as by my housemates and the members of my ultimate frisbee team. I also thank my parents and siblings, who always encouraged me and gave me their love even when I was uncertain of this project's success, and my grandfather Dale Corson, who never failed to ask me about my work when I called. Lastly I want to thank my niece Wilma, to whom the word phosphorus means nothing, but whose smile, when I am lucky enough to see it, never fails to make my day.