Hydro-Morphologically Related Variance in Benthic Drift and Its Importance for Numerical Habitat Modelling

Numerical fish-habitat modelling on various scales has to be seen as state of the art in river management issues. However, most of the applied concepts use steady state hydraulic parameters like e.g. flow velocity and water depth. The presented paper analyses and discusses the possibility of including a drift feeding parameter (SI_F) into habitat evaluation based on multiplying suitability indices. “Sources” and “Sinks” of benthic drift were identified according to the zero-crossing and hydraulically-threshold method in an alpine gravel bed river. Minor differences could be determined between the two methods in a well developed riffle-pool section. Macroinvertebrates, used for simulating benthic drift, were collected by multi-habitat-sampling and appraised according to their critical threshold (t_cr.) for motion on the bed surface and sinking velocity (v_s). The findings of the calculation of drift rates due to 1D and 2D hydrodynamic-numerical models highlight a specification of best feeding position for drift feeding fish (i.e. brown trout, grayling) in considering the SI_F parameter. Moreover, the results indicate, that (artificial) lateral obstruction (e.g. dams) may lead to a reduced transport rate of benthic organisms due to low bottom shear stress (< 0.25 Nm^-2). Thus, consequently a deposition of drifting macroinvertebrates occurs inside of backwaters with downstream impacts on benthic and fish community.