Form-based river restoration decreases wetland hyporheic exchange: Lessons learned from the Upper Colorado River

Restoration of river-wetland systems to recover lost ecosystem services and restore consistent flood regimes is commonly directed at modifying in-channel storage and hyporheic exchange. Here, we monitored the hydrologic response to channel realignment in a montane river-wetland system by comparing pre- and post-restoration measurements. In 2015, an earthen berm and 190 m segment of the Upper Colorado River were constructed to consolidate flow from multiple channels into the historic thalweg. We injected a sodium chloride tracer during baseflow and used mass-balance calculations and electrical resistivity imaging to assess changes in near-channel hyporheic exchange. Results indicate a decrease in hyporheic exchange within the wetland due to lost complexity along the consolidated flow path. Subsurface complexity appears to control hyporheic exchange more than surface complexity. Flow consolidation increased the area-adjusted wetland water yield by 231 mm, indicating a loss of wetland water storage capacity. One year of post-restoration monitoring suggests that the form-based channel restoration directed at consolidating flow into a single thread adversely affected the hyporheic exchange functioning in the pre-restoration system. Results from this case study are applicable to restoration planners as they consider the effects of form-based projects on water storage capacity in similar systems. (c) 2018 John Wiley & Sons, Ltd.