Instream restoration: Its effects on lateral stream-subsurface water exchange in urban and agricultural streams in southern Ontario

Instream restoration strategies do not generally consider the subsurface environment. The study of recently restored stream reaches can provide an opportunity to assess the impacts of restoration on surface-subsurface exchange. In this study, lateral hyporheic zones occurring in a constructed gravel bar and re-meandered stream reach were examined, using hydrometric data in combination with differences in background conservative ion and tracer injection experiments. Both the constructed gravel bar and the meander bends induced lateral hyporheic exchange flow. In the gravel bar, lateral hyporheic exchange increased after a riffle-pool sequence was constructed in the channel adjacent to the bar. The substrate in the meander bends had low saturated hydraulic conductivity, and the stream-subsurface exchange was limited despite the large change in channel configuration. These results suggest that to enhance and maintain stream-subsurface water exchanges, restoration projects that modify horizontal geometry should involve construction of vertical morphologic features, and where floodplain sediments are fine-grained, the addition of coarse sediments should also be incorporated in the design.