Quality status of fresh waterways in Egypt, especially those that receive agricultural drainage water such as Rosetta Branch (RB), is critical for most of its water uses. However, the country depends on this marginal quality water to fill the gap between demand and supply. Therefore, the need for effective/economic water management tools turn into an obligation. Mathematical models can be considered as effective and practical tools for the quality assessment of water bodies. This paper carries out a statistical comparison between simulated and observed data, error quantification and simulation efficiency in order to assess the functionality of water quality (WQ) models for simulating the WQ of RB. This approach was set up to evaluate the cost-effective RB simulation adequacy using different WQ models and assess the gap between simulation simplicity and results accuracy. The simulation case of RB was compared using advanced MIKE-11 and simple QUAL2K WQ models. Despite the simplicity of the QUAL2K model, it showed a good adequacy compared to MIKE-11. Both mathematical models outputs showed a good agreement against field observations. However, MIKE-11 gives results that are more precise in general and for nutrients specifically.