Temporal and spatial hydrodynamic variability in a gravel-bed river:measurement, characterisation, and significance for spawning salmonids
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The spatial and temporal flow field characteristics of a gravel-bed river supporting a robust rainbow trout (Onchorhynchus mykiss) and brown trout (Salmo trutta) fishery were studied in high resolution in attempts to hydraulically quantify the selection preferences and incubation characteristics of redds. Measurements were obtained at both the riffle- and redd-scale post-spawning, in addition to pre- and post-bed mobilization flow events. A Pulse Coherent Acoustic Doppler Profiler (PCADP) was adapted for use in the lotic environment where between 4000 to 5000 three-dimensional velocity measurements could be obtained on a daily basis at the riffle scale. Bed material samples were obtained and scour chains installed to characterise the bed mobility, critical shear, and macro-erosion of each riffle studied. At the riffle-scale, seasonal variability in the mean and variance of commonly used hydrodynamic parameters (velocity, flow depth, Froude number, bed roughness, turbulent kinetic energy per unit area) was often explicitly related to changes in sampling discharge and channel structure. Riffle-scale directional semivariograms isolated varied scales of spatial dependency, and the occurrence of bed mobilizing and channel-forming discharge created a less smooth and poorly defined spatial structure. Redd-scale omnidirectional semivariograms presented the same response to channel-forming discharge, however overall did not reveal a coherent spatial structure. Although the commonly used Froude number did not present patterns or characteristics which could distinguish the redd properties from the riffle, the results for local velocity and turbulence at the redd-scale revealed evidence of optimal physical conditions for salmonid incubation, particularly convective flow acceleration which promotes downwelling at the egg pockets and unidirectional flow with low turbulence, which allows this behaviour to persist through a range in environmental flows.