Biogeochemical processes that occur in hyporheic zones (HZ) of forested headwater streams appear to control stream export of carbon from these watersheds. The HZ is the zone where groundwater (GW) and surface water (SW) interact. Processes occurring there are difficult to study, however, because they are difficult to isolate from other watershed processes. The Stream Carbon Team, a collaboration between Oregon State University and the USFS Pacific Northwest Research Station, along with colleagues from Boise State University and University of Idaho, designed and built 5-m long hyporheic mesocosms to study biogeochemical processes that consume, produce, transport, and transform organic carbon along hyporheic flowpaths. The hyporheic mesocosms are made from pipes 20 cm in diameter and packed with streambed sediment. The mesocosms, which are located at the H. J. Andrews LTER (AND), provide unique opportunities to explore HZ biogeochemistry. We will examine how the quality and quantity of DOC interacts with hyporheic residence time to control carbon metabolism. Changing residence time will allow us to examine quantities of DOC enveloped in soil aggregates and incorporated into microbial biomass, to understand DOC quality along discrete HZ flow path, to examine change in DOC quality at the proximal end of HZ flow path compared to the distal end of HZ flow path, and to compare processing rates of fulvic acid and humic acid along defined HZ flow path. The mesocosms are instrumented with ProSens DO sensors spaced every 30 cm, with 3 sensors at each location spaced evenly across the width of the mesocosm to provide fine detail into the spatial variability of dissolved oxygen within the HZ. Concurrent measurements of concentrations of dissolved trace gases (CO2, N2O, and CH4) in interstitial water using headspace method and high spatial and temporal resolution of dissolved oxygen from ProSens DO sensors will help us to examine if carbon is being processed anaerobically within a globally oxic environment. Furthermore, greenhouse gas and DIC budgets can be calculated which will help us gain greater understanding of HZ either as a source or a sink of greenhouse gases and DIC. This poster summarizes conceptual design and instrumentation of the mesocosm, and proposed work for my PhD research.