Stream Restoration Projects: Effects on Stream-Subsurface Water Interactions and Nitrate Dynamics

Stream channel restoration has become a common practice in management of degraded lowland streams. This study examined the effects of various scales of restored channel features, such as steps, riffles, gravel bars and channel meandering on stream-subsurface water interactions (hyporheic exchange) and nitrate dynamics in streams near Toronto, Canada. The hyporheic zone was delineated using background stream and groundwater chloride concentrations and injected bromide tracer. The extent of hyporheic zone was dependent on the size of channel features, and the amount of hyporheic exchange was strongly influenced by saturated hydraulic conductivity. We observed penetration of stream water 0.1- 0.15 m into the streambed under a small constructed riffle, whereas stream water advection to a depth of 0.8 m was observed under a large constructed riffle. Stream water was a source of nitrate for the hyporheic zone at all study sites. As stream water enters the streambed, a steep decline in nitrate concentration occurred along the initial portion of the hyporheic flow paths. These patterns indicate that examined restored channel features increases stream-subsurface water interactions, and the hyporheic zone functions as a nitrate sink in these streams.