What was a consequence of cities dumping sewage into natural waterways?

Dumping sewage into natural waterways leads to a significant increase in organic matter, primarily from human waste and other contaminants. When this organic matter breaks down, it is consumed by bacteria in a process that requires oxygen. As bacterial populations grow to decompose the sewage, they utilize a substantial amount of dissolved oxygen from the water. This decrease in oxygen levels, known as hypoxia, can result in dead zones where aquatic life cannot survive. Many species of fish and other organisms are highly sensitive to changes in oxygen levels and cannot tolerate the depleted conditions caused by sewage disposal. Consequently, this often leads to large-scale die-offs of aquatic life, severely disrupting the ecosystem and harming biodiversity.

While other options may mention beneficial aspects of organic matter or potential wildlife attraction, they overlook the critical impact of reduced oxygen levels, which is the most pressing consequence of sewage dumping in waterways.