The accidental release of sewage into a river would likely lead to a decrease in oxygen levels in the water due to microbial decomposition of organic matter.
When sewage is accidentally released into a river, it introduces a significant amount of organic matter into the water. This organic matter serves as a nutrient source for bacteria and other microorganisms. As these microorganisms break down the sewage through the process of decomposition, they consume dissolved oxygen in the water. The increased microbial activity and oxygen consumption can lead to a significant decrease in the levels of dissolved oxygen available for aquatic organisms.
Furthermore, the decomposition of organic matter by bacteria is an aerobic process, meaning it requires oxygen. As the microbial population increases to process the influx of organic material from the sewage, the demand for oxygen also rises. This can result in what is known as biochemical oxygen demand (BOD), a measure of the amount of dissolved oxygen consumed by microorganisms during the breakdown of organic matter.