Fresh water sources such as rivers, lakes, streams and brooks constitute less than one percent of all available water on earth. However, pollution activities have been impacting negatively on these sources of water supply thereby compromising quality and creating environmental problems. Studies have shown that these water bodies auto-purify themselves from pollutants discharged into them under certain conditions. This work investigated the auto-recovery processes of River Illo in Ota, Nigeria from abattoir wastes which is one of the most common pollution activities in developing countries. The analysis was done using the modified Streeter-Phelps equations. This work should help shed light on how the natural purification processes of rivers can be further enhanced. This book will be useful to professionals who engage in research, public water supply, standards enforcement and environmental policy making.