Runoff is a major water quality issue that is affected by topography, unlike many of the other aspects dealing with topographic features in the entire watershed. Rainwater drains into a body of water by first passing over, under, or through several landmarks. Many chemicals, pollutants, minerals, and sediments present where the rainwater falls can be carried to the body of water with precipitation. This can adversely affect water quality or bring essential nutrients to it.

In the event that the runoff flows over rural areas, many fertilizers and pesticides are moved along toward the body. In highly urban areas, chemicals from industrial factories, litter from just about any source, and even overflowing sewers may contribute to the runoff.

The geography also plays a crucial role here. If the water needs to seep through soil to reach its final destination, many of the chemicals may become filtered out. Furthermore, if the place where the rainwater initially lands is uphill from the water body, much more run-off will occur than if the opposite is true.

The concept of runoff, although not measured as a part of water quality, does contribute to the overall quality of water, which is why I felt the need to include it.