This booklet is so beneficial for environmental engineers, ecologists, hydrologists, and environmental science students. This book introduce point and non-point source pollution in one of the most important river that is located in Iran and in Isfahan city. Also this book presents L-THIA (long term hydrological impact assessment) model for modeling of non-point source pollution. this model can show critical area of the basin, so we can manage the condition of water in these areas.