Does Hyporheic Intensity Explain Spawning Site Selection?

Abstract

Pool, riffle and runs are streambed features and important habitats for aquatic species. It is well-known that salmonid species utilize these streambed features as spawning site. Pools are characterized by deep water depth, low velocity and gentle water surface slope, whereas riffles are characterized by shallow depth, high velocity and steep slope. Past studies have used hydrodynamic model simulated hydraulic characteristics such as depth, velocity and shear stress to delineate these features. However, the magnitude of these hydraulic characteristics may vary significantly with the discharge and longitudinal slope of the stream. Additionally, these indexes still do not entirely explain the selection of spawning site. Here we hypothesize that hyporheic flow, which is an advective mechanism, and brings oxygen-rich surface water into the sediment, may help to understand the selection of Chinook salmon spawning sites. The curvature of the water surface has been suggested as a main driver for hyporehic exchange in gravel bed rivers with pool-riffle morphology. We studied the correlation between magnitude of water surface curvatures and locations of Chinook salmon redds in Bear Valley Creek, located in the central Idaho, USA. We used two-dimensional (2D) hydrodynamic model to simulate water surface elevations for low and bankfull discharges. Our results show that redd locations are highly correlated with areas of high potential hyporheic flow as indicated by the water surface curvature