Modeling of biological parameters in every case calls for systematic function analysis of the river. The IFIM model (Instream Flow Incremental Methodology) and simulation system PHABSIM (Physical Habitat Simulation System) were used for this modeling. PHABSIM is used to analyze relations between the discharge, morphology of the riverbed and the environmental biological units that are represented by criteria curves. Big demands on the evaluation of criteria curves prevent the methodology IFIM from wide using. This article gives information from the first phase of the research, which is aimed to generalize criteria curves. First results from alluvial rivers of Slovakia confirm precondition that it is possible to generalize criteria curves – this helps to simplify the application of simulation system PHABSIM.