Managing Semi-Arid Watersheds: Watershed Basics - What is a Watershed?
"Watershed" is a new term to
many people. Its definition is almost as simple as the well-known phrase
"water runs downhill". The drain board that carries rinse water
into your kitchen sink can be compared to a watershed. On the land, water
that does not evaporate or soak into the soil usually drains into ditches,
streams, marshes, or lakes. The land area from which the water drains to
a given point is a watershed.
When you were a small child, you probably had a favorite mud puddle in
which you liked to play. The part of the yard from which the water drained
into the puddle was its watershed. Possibly a small stream ran by your
house. It may have been dry most of the year or it may have flowed continuously.
Water from a few acres drained into that little stream. Those few acres
were its watershed. This small stream and others like it ran into a larger
one. The land areas drained by the small steams made up the watershed
of the larger stream into which they flood. Small watersheds make up the
larger ones. The Mississippi River, for example, drains a watershed of
about 1,243,000 square miles. This large watershed is made up of thousands
of smaller ones. So wherever you live you are in a watershed. It can be
just your own backyard or the area drained by a small creek or by a large
You and the other people who live in the watershed are part of the watershed
community. So are the animals, the birds, and the fish. All depend on
the watershed, and they, in turn, influence what happens there. What happens
in your small watershed also affects the larger watershed downstream.
If water runs off the land too fast, it cuts gullies and carries off
topsoil. This soil along with other debris the water carries into streams
and lakes may spoil fishing. As soil fills the lakes or reservoirs, the
amount of water they can hold is reduced. Therefore the water supply for
your town and your home may be reduced. Although erosion is a natural
process, accelerated erosion degrades productivity of the land. Such sediment
carried downstream by runaway water may greatly increase the cost of filtering
the water you get from the faucet. It can interfere with the hydroelectric
plant that produces your electricity. This may make your electric bills
If too much water runs away too rapidly, it causes a flood that damages
farms, ranches, crops, property, homes, highways, and utilities. It may
take lives. Stream channels may be choked with sediment. Then the flood
is more serious because the choked-up channels carry less water. However,
much of our productive farmland was created by flood waters that deposited
soil. This soil was eroded from the uplands of the watershed.
Water can be slowed down and used to advantage when soil and water conservation
practices and other flood-prevention measures are put in over all the
watershed. Terraces, strip cropping, more grass and legumes in crop rotations,
and improved pastures are practices that make more water soak into the
soil. Small dams can hold back runoff water that would otherwise cause
flood damage. Conservation irrigation systems waste less water and thus
leave more for other irrigators to use. Later some water will go into
streams, lakes, or underground storage to be used in other ways. It doesn't
carry sediment to clog streams and water supplies. Thus, more water is
available for the many uses people make of it.