also called the drainage basin,
is all of the land and water areas that drain toward a particular river or
lake. Thus, a watershed is defined in terms of the selected lake (or river).
There can be subwatersheds within watersheds. For example, a tributary
to a lake has its own watershed, which is part of the larger total drainage
area to the lake.
A lake is a reflection
of its watershed. More specifically, a lake reflects the watershed's
soil fertility and erodibility, and vegetation. The impact of the watershed
is evident in the relation of nutrient
loading to the watershed:lake surface area ratio (Figure 7).
water quality decreases with an increasing ratio of watershed
area to lake area. This is obvious when one considers that as
the watershed to lake area increases there are additional sources
of runoff to the lake. In larger watersheds, there is also a greater
opportunity for water from precipitation to contact the soil and leach
minerals before discharging into the lake. Lakes with very small watersheds
that are maintained primarily by groundwater flow are known as seepage
lakes. In contrast, lakes fed primarily by inflowing streams
or rivers are known as drainage
lakes. In keeping with the watershed:lake area relationship, seepage
lakes tend to have good water quality compared with drainage lakes.
However, seepage lakes are often more susceptible to acidification
rain because of their low buffering