HYPORHEIC EXCHANGE AND BIOGEOCHEMICAL CYCLING AROUND STREAM RESTORATION STRUCTURES: IMPLICATIONS FOR THE HEALTH OF RESTORED STREAMS

Natural channel design restoration projects in streams often include the construction of cross-vanes, which are static, stone restoration structures that span the active channel. Channel-spanning structures are recommended in the stream restoration literature as tools to improve connectivity between the stream and hyporheic zone, and thereby increase biogeochemical processing of in-stream pollutants.

Vertical hyporheic exchange flux and concentrations of redox-sensitive solutes were measured in the streambed around four cross-vanes in Central New York State. Observed patterns of hyporheic exchange and redox conditions are not dominated by a single, downstream-directed hyporheic flow cell beneath cross-vanes. Instead, spatial patterns of moderate (less than 0.4 md^-1) upwelling and downwelling are distributed in smaller cells around pool and riffle bed forms upstream and downstream of structures. Strong downwelling into the hyporheic zone (0.5-3.5 md^-1) was observed immediately upstream of structures, but was limited to an area 1-2 m from the cross-vane. Total hyporheic exchange volumes are approximately 0.4% of stream discharge in restored reaches of 45-55 m.

Results show that shallow hyporheic exchange and associated biogeochemical cycling near the studied 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-vanes in the studied restored streams benefit the stream ecosystem by creating heterogeneous patches of varying hyporheic exchange and redox conditions in the hyporheic zone, rather than by processing large amounts of nutrients to alter in-stream water chemistry.