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.