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

The consequence of cities dumping sewage into natural waterways primarily leads to lowered oxygen levels, which is detrimental to aquatic life. When sewage is introduced into these ecosystems, the organic matter present in the waste provides a food source for bacteria. As these bacteria break down the organic matter, they consume oxygen in the process, leading to a significant decrease in the available dissolved oxygen in the water.

Aquatic organisms, such as fish and other marine animals, rely on sufficient oxygen levels to survive. When these levels drop dramatically—often referred to as hypoxia—many species cannot survive, leading to die-offs and a significant decline in biodiversity in the affected areas. This phenomenon can cause "dead zones" in waterways where most aquatic life cannot exist due to insufficient oxygen, disrupting the ecosystem and causing long-term ecological damage.

While options discussing organic matter promoting plant growth or attracting wildlife might seem plausible at first glance, they overlook the immediate and severe impact of oxygen depletion on aquatic ecosystems, making them less relevant in the context of sewage dumping. The focus on the consequences of oxygen depletion underscores the critical nature of maintaining healthy water systems for supporting life.