A simple matrix developed by a small group of ecologists, geomorphologists and an hydraulic engineer at a workshop in February 1995 was proposed as a standardised classification tool for the recognition of hydraulic biotopes, instream flow environments recognised on the basis of surface flow type and substrate class. Research was carried out in the Buffalo River, South Africa, to test the validity of this classification with respect to measurable hydraulic attributes. Hydraulic biotopes were classified at a number of sites representing a range of stream morphologies, channel scale and discharge. The hydraulic attributes at each sampling point were measured. Using simple measures of flow depth, velocity and bed roughness, hydraulic indices were calculated to describe both the near bed conditions (shear velocity and ‘roughness’ Reynolds number) and the mean flow in the water column (Froude number and Reynolds number). Results indicate that hydraulic biotope classes can be considered as significantly different in terms of flow hydraulics. Hydraulic characteristics appear to be consistent within classes across different spatial scales. An assessment of the influence of discharge showed that differences are recognised in the classes of the lower energy environments (pool and run) but despite considerable within class variability, no significant differences were observed across discharges in the higher energy environments. The hydraulic biotope classification and associated hydraulic biotope matrix shows good potential as a standardised method for classifying instream environments over a range of channel scales and discharges.

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