Hyporheic exchange, enhanced by complex stream channel morphology, can influence biogeochemical processing in the streambed. These processes chemically alter water passing temporarily through the streambed, which eventually returns to the stream channel and can potentially affect surface water quality. To assess the degree of biogeochemical cycling induced by complex streambed morphology, we instrumented two 20-m reaches of Red Canyon Creek, Wyoming, each containing a small log dam, with in-stream minipiezometers and temperature data loggers. We simultaneously observed pore water geochemistry and streambed temperature dynamics in several bedforms located upstream or downstream of the dams. We modeled seepage flux into the streambed using heat transport modeling. Upstream of the dams, low-permeability sediments have settled out in low-velocity pools, and enhanced anaerobic biogeochemical cycling occurred in the streambed. Rapid flux into the streambed occurred in glides immediately above the dams, where streambed temperature dynamics and geochemistry were nearly identical to the stream. In riffle sequences downstream of the dams, the streambed was oxygen rich, showed evidence of nitrification, and temperature dynamics indicated high connectivity between the streambed and the stream. Further downstream, streambed pore water geochemistry indicated ground water discharge occurring at the pool-riffle transition. Assessing streambed biogeochemical cycling may be facilitated by coupling streambed temperature measurements with pore water geochemistry and can aid in understanding how hyporheic exchange contributes to overall stream biogeochemistry.

Rapid exchange of stream water and groundwater in streambeds creates hotspots of biogeochemical cycling of redox-sensitive solutes. Although stream-groundwater interaction can be increased through stream restoration, there are few detailed studies of the increased heterogeneity of water and solute fluxes through the streambed and associated patterns of biogeochemical processes around stream restoration structures. In this study, we examined the seasonal patterns of water and solute fluxes through the streambed around a stream restoration structure to relate patterns of water flux through the streambed to morphology of the channel and biogeochemical processes occurring in the bed. We characterized different biogeochemical zones in the streambed using principal component analysis (PCA) and examined the change in spatial patterns of these zones during different seasons. The PCA results show that two principal components summarized 83% of the variance in the original data set. Streambed pore water was characterized as oxic (indicating production of nitrate), anoxic (indicating sulfate, iron and manganese reduction), or stream-like (indicating there was minimal change in the stream water chemistry in the bed). Regardless of season of the year, anoxic zones were predominantly located upstream of the structure, in a low-velocity pool, and oxic zones were predominantly located downstream of the structure, in a turbulent riffle. We expect structures that span the full channel, are impermeable, and permanent, such as those installed in natural channel design restoration will similarly impact biogeochemical processing in the streambed. The installation of these types of restoration structures may be a way to increase the degree of biogeochemical cycling in stream ecosystems.