CHANNEL RESTORATION PROJECTS AND STREAM-HYPORHEIC INTERACTIONS

The effects of stream restoration on hyporheic exchange have been neglected although channel restoration projects often involve the modification of channel bedform and alignment, and have a potential to alter stream-groundwater interactions. We studied stream-subsurface water interaction induced by recently constructed riffles and meander bends in agricultural and urban streams in Southern Ontario, using networks of piezometers and solute injections. The riffles and bends studied increased the vertical and lateral extent of the hyporheic zone. However, hyporheic exchange was limited in a re-meandered stream reach by low hydraulic conductive floodplain sediments that are characteristic of many lowland streams. We expected that increased water exchange would enhance the transfer of nitrate and dissolved oxygen (DO) between stream and streambed sediments in artificial riffles constructed with cobbles and boulders. Deeper nitrate penetration was limited to the zone just downstream from the riffle crest where the stream water fraction was high. DO concentrations were rapidly depleted with depth, and the hyporheic zone remained anoxic in the constructed riffle, despite the very coarse substrate. Although not directly measured, the effects of surface clogging may have reduced the downwelling of oxygen and nitrate-rich stream water. If the function of the hyporheic zone as a habitat is of concern, restoration projects need to involve efforts at the watershed scale to minimize sediment and nutrient runoff to streams.