Quantification of the habitat available for fish species named Rainbow Trout (Oncorhynchus mykiss) were evaluated in Lees Ferry, Colorado River using water depth, water temperature, sediment transport, flow velocity in 2004 as environmental index. For the flow velocity and temperature distribution calculations in the river, the Navier-Stokes equation and energy conservation equations with finite volume approach has been employed. Sediment transport and river bed deformation in Lees Ferry were also calculated based on Engelund-Hansen equations. The suitability index (SI) curves based on these four biological, ecological and hydraulic factors were obtained and fish habitat suitability function was established. The HSI (habitat suitability index), WUA (weighted useable area), OSI (overall suitability index) of the fish species were quantitatively calculated using SI curves coupled with habitat suitability function. The effects of these variables on the Lees Ferry river areas were analyzed. The results showed that the model system can correctly represent the Rainbow Trout living situation. The WUA and OSI are generally low in Less Ferry and the proportions of high quality of OSI (HSI³0.7) are even less which is in accordance with the real situation. The results also revealed that there is a nonlinear relationship between flow discharge and suitable habitat areas.