This book aims to study the impact of water level control structures on the self-assimilative capacity and ecosystem of rivers. Constructing a water level control structure in a river reach will increase the water depths, decrease the velocities and increase the groundwater table. These changes in the river hydraulics will impact the water quality of the river, its thermal regime and the fish that dwell in it. A mathematical model was developed to simulate the hydraulics, water quality, pollution transport in the river, temperature regime and fish habitat. The study and results showed that water level control structures have negative impacts on the upstream self-assimilative capacity of the rivers, but they have a positive impact on the DO concentrations in the downstream side due to the reaeration that happens across them. The structures also have a positive impact on the fish. As for the temperature regime it was slightly affected by the existence of the control structure.The book should be especially helpful to professionals or anyone else interested in the field of hydraulics, water quality and water environment.