Turbulence Signatures of Natural River Morphology in Four Dimensions

Turbulent flow in natural river channels drives geophysical processes and exerts afundamental influence on aquatic biota. An extensive range of turbulence propertieshave previously been synthesized into four categories or“dimensions”with ecologi-cal relevance: intensity, periodicity, orientation and scale (IPOS). We apply this frame-work across three rivers with differing morphologies in order to assess the statisticalcoherence of the four IPOS categories within turbulence field data and their utility indiscriminating between fundamental units of river habitat. Intensity, periodicity-scaleand orientation were identified as the key gradients in the turbulence data set usingmultivariate analysis. These gradients all revealed statistically significant differencesbetween rivers and/or geomorphic units. The intensity gradient accounted for thehighest variance and most pronounced inter-reach differences, but the periodicity-scale and orientation gradients were also useful in distinguishing between certaincombinations of rivers and/or geomorphic units. Different turbulence gradients, orcombinations of gradients, were important in characterizing differences between riv-ers and geomorphic units (riffes, pools, steps). The gradients provided improved pre-diction of geomorphic units compared to standard hydraulic variables (mean velocity,depth), although the extent of improvement in prediction varied between river mor-phologies. The analysis reveals the statistical coherence of the four categories or“dimensions”of turbulence in multivariate space, connects the ecologically definedIPOS categories of turbulence properties with river types and fundamental units ofriver habitat (geomorphic units). Turbulence signatures of natural channel morphol-ogy are expressed across all four dimensions of turbulence, providing clear evidencethat these four dimensions should be routinely considered in ecohydraulics andhydromorphology research to facilitate a full understanding hydraulic habitat.