[1] Natural channel design restoration projects in streams often include the construction of cross-vanes, which are stone, dam-like structures that span the active channel. Vertical hyporheic exchange flux (HEF) and redox-sensitive solutes were measured in the streambed around four cross-vanes with different morphologies. Observed patterns of HEF and redox conditions are not dominated by a single, downstream-directed hyporheic flow cell beneath cross-vanes. Instead, spatial patterns of moderate (<0.4 m d À1 ) upwelling and downwelling are distributed in smaller cells around pool and riffle bed forms upstream and downstream of structures. Patterns of biogeochemical cycling are controlled by dissolved oxygen concentrations and resulting redox conditions, and are also oriented around secondary bed forms. Strong downwelling into the hyporheic zone (0.5-3.5 m d À1 ) was observed immediately upstream of structures, but was limited to an area 1-2 m from the cross-vane; these hyporheic flow paths likely rejoin the stream at the base of cross-vanes after residence times too short to alter nitrate concentrations or accumulate reaction products. Total hyporheic exchange volumes are $0.4% of stream discharge in restored reaches of 45-55 m. Results show that shallow hyporheic flow and associated biogeochemical cycling near cross-vanes is primarily controlled by secondary bed forms created or augmented by the cross-vane, rather than by the cross-vane itself. This study suggests that cross-vane restoration structures benefit the stream ecosystem by creating heterogeneous patches of varying HEF and redox conditions in the hyporheic zone, rather than by processing large amounts of nutrients to alter in-stream water chemistry.Citation: Gordon, R. P., L. K. Lautz, and T. L. Daniluk (2013), Spatial patterns of hyporheic exchange and biogeochemical cycling around cross-vane restoration structures: Implications for stream restoration design, Water Resour.

Stream restoration goals include improving habitat and water quality through reconstruction of morphological features found at analogous, pristine stream reaches. Enhancing hyporheic exchange may facilitate achieving these goals. Although hyporheic exchange at restoration sites has been explored in a few previous studies, comparative studies of restored versus reference or control streams are largely absent. We hypothesized that restoration cross-vanes enhance hyporheic exchange, resulting in biogeochemical alteration of stream water chemistry in the streambed. Two streams restored using cross-vanes to control erosion and improve habitat were compared with their associated reference reaches, which provided the basis for the restoration design. Thirteen temperature profile rods with vertically stacked sensors were installed at each site for 2 weeks. Heat tracing was used to quantify vertical flux in the streambed from the diurnal temperature fluctuations in the subsurface. Stream water and bed pore waters from mini-piezometers were analysed for ion and nutrient chemistry. In general, mean vertical flux rates through the streambed were small throughout reference sites (À0.3 to 0.3 m/day) and at most locations at restored sites. Immediately adjacent to cross-vanes, vertical flux rates were larger (up to 3.5 m/day). Geochemistry of pore waters shows distinct differences in the sources for the reference and restored sites. Strong downwelling zones adjacent to cross-vanes showed high dissolved oxygen (10.75 mg/l) and geochemistry in the streambed similar to surface water. Reference sites had lower dissolved oxygen in the streambed (0.66-5.14 mg/l), and geochemical patterns suggest a mixture of discharging groundwater and surface water in the hyporheic zone. Restored sites also clearly show sulfate and nitrate reduction occurring in the streambed, which is not observed at the reference sites. The stream restoration sites studied here enhance rapid hyporheic exchange, but upwelling of groundwater has a stronger influence on streambed geochemistry at reference sites. Figure 4. Mapped bed temperature ( C) at a depth of 7 cm at each field site, derived from field measurements with a hand-held temperature probe at the points indicated 3736 T. L. DANILUK ET AL