What factor does NOT affect the quality of surface water?

The geographic elevation of a reservoir does not directly affect the quality of surface water in the same way that the other factors do. While elevation can influence aspects such as temperature and pressure, which may indirectly impact water quality, it is not a primary driver of water quality changes.

In contrast, the watershed and area drained play a crucial role in determining surface water quality, as they can influence the types and amounts of pollutants that enter the water source. The location and source of pollution are significant because they dictate the kinds of contaminants present and their concentrations, which directly affect water safety and usability. Natural agencies of purification, like wetlands and certain biological processes, actively work to remove impurities from water, thereby improving its quality.

Overall, geographic elevation is less directly linked to the quality of surface water compared to the other factors listed, which have a more pronounced and immediate effect on water quality.