River-groundwater interactions in salmon spawning habitat: riverbed flow dynamics and non-stationarity in an end member mixing model

Salmonids commonly spawn in locations of upwelling vertical flow, but it is unclear whether this upwelling water is groundwater, river water which has spent some time flowing through the riverbed before re‐emerging downstream or some mixture of the two. Vertical specific discharge and riverbed water chemistry were monitored in spawning habitat of Sockeye (Oncorhynchus nerka) and Chinook Salmon (Oncorhynchus tshawytscha) in contiguous natural and modified sections of Okanagan River in southern British Columbia, Canada. A quasi‐stationary end member mixing model was developed to describe mixing between end members as well as hydrochemical modification along a subsurface flowpath. For this work, a hydrochemical category called ‘hyporheic water’ was defined as river water which spent some time flowing within the riverbed and developed a distinct chemistry. The resulting mixing triangle was used to categorise intragravel water chemistry as (1) river‐like, (2) mixed river and regional groundwater, (3) hyporheic water or (4) mixed hyporheic and regional groundwater. Sockeye Salmon spawned in locations with all combinations of vertical specific discharge and intragravel water chemistry, which was interpreted as a high degree of habitat use plasticity. Chinook Salmon redds were more common where regional groundwater mixed with river or hyporheic water in locations with upwelling or near‐neutral vertical specific discharge. Current efforts at restoring or enhancing salmonid spawning habitat can be improved by considering the subsurface flow dynamic needs of different target species. Copyright © 2016 John Wiley & Sons, Ltd.