Groundwater-surface water interactions in a constructed side channel complex

Constructed groundwater-fed side channels are a common restoration tool used in the Pacific Northwest to mitigate loss of floodplain features important for salmon spawning and rearing. The former floodplain of the Mamquam River, in Squamish, British Columbia, has a series of groundwater-fed side channels that consistently lose surface flow during the dry season, stranding juvenile coho salmon (Oncorhynchus kisutch). This study characterizes temporal and spatial patterns of streamflow in a side channel relative to mainstem streamflow, shallow groundwater levels, and local water use through synoptic flow measurements, water quality tracers, and a relative elevation transect. The construction and subsequent removal of a beaver dam enables a natural experiment, establishing a hydraulic connection between the side channels and a nearby water rights holder. Results suggest that the side channels become a local surface water sink and are susceptible to water withdraw under certain conditions during the dry season.