A watershed is the area drained by a stream and its tributaries.
Watersheds range in size from under a square mile to hundreds of thousands
of square miles (the Chesapeake Bay watershed, for example, has an area
of 66,388 square miles). Any rain that falls within a watershed will eventually
drain from the bottom of the basin through the main stream channel. Because
watersheds may incorporate broad or diverse land areas, they are better
units for analyzing environmental problems than are political jurisdictions.
As a case in point, a study of water pollution in the Maryland portion
of the Chesapeake Bay watershed would have to take into account pollution
sources in the upper Chesapeake Bay watershed of New York and Pennsylvania.
But a watershed is more than a collection of streams and adjacent
land areas--it is a natural resource system in which humans and other organisms
interact with the land and its associated resources for sustenance, shelter,
and security. The physical condition of a watershed, therefore, directly affects
the health and well being of natural and social systems within its divides and
indirectly affects those systems beyond its divides.
What is a watershed?
A watershed is the area drained by a stream and all of its
tributaries. Any rain that falls within the watershed will pass
through the main stream channel.
A divide separates each basin from the surrounding drainage
basins. Divides follow ridges and hill tops. If a raindrop falls
on one side of a divide, it will flow down one side of the hill,
and into one drainage basin. If the raindrop falls on the other
side of a divide, it will flow into a different drainage.
Watersheds are composed of many smaller drainage basins. In
the diagram, a sub-basin has been drawn for every tributary.
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Stream Order
The streams occupying a drainage basin form a hierarchical
network of channels that, in humid areas, hold increasingly larger volumes
of water as you move toward the mouth of the basin. A stream's order
is its rank, or relative position, within the network. A first-order
stream is a channel with no tributaries--that is, a channel at its upper
reaches and near its source. A second-order stream is a channel fed
by at least two first-order tributaries. The joining of two-second order
streams forms a third-order stream. Stream ranking continues in this
manner until the highest-ordered channel is reached. First and second-order
streams are located in the headwater areas of watersheds and typically
convey small volumes of water. These lower-order streams are vulnerable
to pollution because they are unable to dilute contaminants, assimilate
much organic waste, or remove sediments deposited on channel bottoms.
Sub-watersheds and Watershed Order
A large watershed like the Chesapeake Bay Watershed is composed of
numerous sub-watersheds that are drained by tributary streams and
rivers. The Bay's sub-watersheds are shown on the following map. The
largest is the watershed of the Susquehanna River which drains parts
of New York, Pennsylvania, and Maryland before entering the bay at
Havre de Grace, Maryland. These sub-watersheds, in turn, are composed
of the yet smaller watersheds of streams draining into their main
channels.
Watersheds, like streams, are ranked according to order.
A first-order watershed is drained by a first-order stream, whereas
the main channel of a second-order watershed is a second-order stream,
and so on for each higher-ordered watershed. A large watershed, therefore,
is a nested hierarchy of numerous lower-ordered basins or sub-watersheds.
Nested Watersheds: Source Marsh 1998, 170.
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Basin versus Nonbasin Drainage
15% to 20% of runoff within a large watershed drains directly
into its highest-order stream or water body without first flowing through
lower-ordered basins. Areas that drain directly into higher-order water
bodies are called nonbasin drainage areas and include lands adjacent
to river, bay and ocean shorelines. In Maryland, nonbasin bay drainage
areas are subject to regulation under Maryland's Critical Areas Act.
This act restricts development within 1000 feet of waters under tidal
influence, and mandates 100-foot wide vegetated buffers between tidal
waters and adjacent land-uses to help control surface runoff and thus
control water pollution.
Nonbasin and basin drainage areas. Source: Marsh 1998, 170.
Stream Functions, Watersheds, and Urbanization
Stream systems have many functions. They provide habitats
for aquatic organisms, are important components of terrestrial ecosystems,
and function to convey runoff and sediment loads out of their watersheds.
A stream's load consists of three kinds of materials: dissolved materials
carried in solution, fine particles held in suspension, and heavier
or coarser materials pushed or bounced along the channel bottom. Over
time, a stream becomes graded. That is, a balance or equilibrium is
reached among channel slope (gradient), channel characteristics, available
discharge, and load. Stream banks and channels are relatively stable
under graded conditions. This balance is upset, however, by changes
to the land cover and surface characteristics of the watershed. The
urbanization of watersheds increases the imperviousness of land surfaces,
alters the density of channels, and diverts much of the surface drainage
to underground storm sewers. This, in turn, dramatically changes the
volume of water and the amount and type of material that streams in
urbanized watersheds convey. Urbanization also alters the physical configuration
and stability of stream channels, reducing their value as wildlife habitats.
The sections that follow will address the environmental impacts of increased
watershed imperviousness as a result of urban and suburban development.
