What is a Watershed?
A watershed is an area of land that is drained
by a distinct stream or river system and is separated from other similar systems
by ridge-top boundaries.
Often called a drainage basin or hydrologic unit, a watershed can cover a
large multi-state area like the Columbia River watershed or a relatively small
area, like the watershed of a small pond. In turn, each watershed is made up
of numerous smaller watersheds or sub-basins.
Most watersheds are composed of a mixture of landscape types including uplands,
wetlands, riparian areas, streams and lakes. The most common component of almost
all watersheds is the upland area, covering in many cases over 99% of the total
watershed area. The rain and snow that falls into a watershed, and that doesn't
evaporate, is stored in the soil, and over a period of time is released down
slope through ground water, wetlands and streams. This water continues to flow
down slope and eventually reaches the mouth of the river that drains the watershed.
In a healthy watershed, vegetation and wetlands are present to intercept and
slow the flow of water as it travels through the watershed, removing sediment
and allowing large quantities of water to enter the soil and percolate into
the ground water or aquifer. This water stored below the surface is then available
to contribute to late season stream flow, to the benefit of fisheries and water
quality.
Most human activities and development have the potential to adversely affect
the overall health and quality of a watershed. Timber harvest on unstable slopes
can cause erosion. Agricultural activities can increase levels of harmful bacteria
and overload runoff with nutrients. Also, poorly planned urban and industrial
growth can cause many of the same problems as farming and timber harvest in
addition to contamination from toxic chemicals. Even seemingly harmless activities
such as rural development and recreational activities along rivers and creeks
can be harmful, destroying and replacing the watershed's sensitive riparian
vegetation which is important for water quality protection and wildlife habitat.
When viewed individually, most human activities have little effect on the
general health of the watershed. However, the effects of numerous activities
within a watershed are cumulative and when combined can greatly diminish the
watershed's overall health. Every activity has the potential to impact the
area of the watershed downstream.
As people place more demands on a watershed, greater efforts must be made
to reduce these cumulative effects. Managing a land area according to watershed
boundaries rather than the arbitrary boundaries of a county or state line or
a city limit makes sense because the natural boundary defined by a watershed
is self-contained.
Cumulative effects, therefore, can better be addressed.
Wetland: A low lying
area of land that is wet during extended periods of the year and that
has been significantly shaped by the presence of water over time. Wetlands
have the ability to remove pollutants from runoff and provide important
habitat for a multitude of plant and animal species. Bogs, swamps,
ponds and marshes are types of wetlands. |
Riparian: A zone transition
between an upland area and a river, lake or stream. This area often
contains a great diversity of plant and wildlife species and can serve
an important water quality function.
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Upland: An area, which
due to its slope or permeability of its soils, is fairly well drained
and where standing water is not found over extended periods of time.
The majority of land area in most watersheds is upland. |
Aquifer: Water below the
land's surface in large quantities. An aquifer is fed by water percolating
from the surface through porous soils and rock.
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