Do stream restoration projects enhance hyporheic functioning?

摘要

Hyporheic exchange is a hydrologic link between stream and the sediments below and adjacent to the channel, and the hyporheic zone is important for stream ecosystem functioning. Although stream restoration projects, which have increased in numbers in recent years, aim to rehabilitate stream ecosystem health, they seldom incorporate the subsurface environment. However, channel restoration projects modify stream bedform and have a potential to enhance hyporheic exchange flow. This is the first study to address the effects of channel restoration projects on the hyporheic zone.; Four different types of channel bedforms (riffles, steps, gavel bars and meander bends) that were recently constructed in streams in Southern Ontario were examined. A combination of field observations of hydraulic head and water chemistry, tracer injection experiments and numerical groundwater flow modelling were used to study hyporheic exchange induced by the constructed bedforms and nitrate and oxygen dynamics in the hyporheic zone.; All the restoration projects studied enhanced the size of the hyporheic zone in the study reaches; however, the amount of hyporheic exchange was largely limited by saturated hydraulic conductivity. For example, the substrate at the riffle construction sites experienced clogging by fines and had saturated hydraulic conductivity similar to the natural riffles despite the use of cobble and boulders. A substrate with very low saturated hydraulic conductivity at the channel re-meandering site limited the amount of hyporheic exchange, despite the drastic changes in channel morphology.; Oxygen and nitrate concentrations rapidly decreased as stream water entered the streambed at all sites. The anoxic conditions of the hyporheic zone, despite the use of cobbles and boulders in construction, indicated that the hyporheic zone had a low quality as a habitat for stream invertebrates. The hyporheic zone at all study sites functioned as a nitrate sink of stream water that had a relatively high nitrate concentration, although its impact may be small due to a small amount of hyporheic exchange induced. To successfully restore hydrologic connection between a stream and the subsurface and enhance hyporheic functioning, restoration projects should also involve watershed scale planning to reduce suspended sediment load that may clog the stream.