What is a negative impact of reservoirs or dams?

The choice regarding decreased dissolved oxygen is correct as a negative impact of reservoirs or dams. When a dam is constructed, it often leads to the creation of a reservoir, which can disrupt the natural flow of a river. This alteration of water flow can result in water becoming stagnant in certain areas, which decreases the oxygen levels in the water. Stagnant water does not mix as effectively with the atmosphere, leading to lower oxygen saturation levels. Dissolved oxygen is crucial for aquatic life, and a decrease in these levels can harm fish and other organisms that rely on oxygen for survival.

In contrast, the other options present benefits rather than negative impacts. Increased biodiversity is generally associated with healthy ecosystems; however, while a reservoir can create new habitats, it can also alter existing ecosystems negatively. Enhanced recreational activities are often seen as a positive outcome of dams and reservoirs, providing opportunities for boating, fishing, and other leisure activities. Improved irrigation methods can also be a benefit, as reservoirs can provide a stable water supply for agricultural needs. Thus, the negative impact of decreased dissolved oxygen stands out in this context.