Streams in the City

Streams in the City
It’s a hard (surface) life
Close your eyes and imagine a
cool, running stream. What
comes to mind? Green
forests? Leaping trout? Warm
breezes? If you’re like most people,
you probably don’t envision skyscrap-ers, business suits, or city buses.
Walden comes to mind, not Wal-Mart.
Streams are an important part of
every landscape, no matter whether
that landscape is a forest, a city, or a
suburb. Often they lie low when
people, buildings, and streets are
around. But rest assured, streams are
there, flowing under sidewalks,
meandering past ball fields, and
rippling by shopping centers.
Streams, stream banks, and the
low lands around them provide
important habitat for animals and
plants that share the urban/suburban
landscape with us. They are also part
of the network of channels that drains
rain and melting snow off our streets,
parks, and yards.
The Difference Between
Streams in Forests and
Streams in Urban Areas
Streams in a town or suburb are
usually very different from streams in
a forest or other natural area. Urban
streams tend to carry more water at a
faster speed after a storm than their
country cousins. The reason for this
has to do with what happens to rain
after it hits the earth’s surface. In
forests, meadows, and other natural
areas, about half the water that falls to
earth soaks into or infiltrates the soil.
Most of what remains on the ground
and in the grass, tree leaves, and other
plants gets returned to the atmosphere
by evapotranspiration, a combination
of evaporation and transpiration (loss
of water vapor by plants). Only a
small portion of rainfall (about 10
percent) travels across the land as
runoff and drains into a stream, lake,
or pond.
When people move into an area,
they build houses, buildings, streets,
and parking lots. When rain falls in
urban areas it doesn’t land on nice
soft ground and plants. Instead it hits
impervious surfaces like hard pave-ment and rooftops and has no chance
to infiltrate the soil. Storm water has
no choice but to flow downhill into
street drains and ditches and then into
streams. The panels below illustrates
how the fate of rainwater changes as
cities grow.
Scientist use percent impervious-ness to describe how much of a given
area is covered by hard surfaces.
Many cities have areas that are 75 to
95 percent impervious. This means
that most of the rain that falls will not
infiltrate into the soil and instead will
flow off streets and parking lots. If all
this extra water is diverted directly
into a stream channel, several impor-tant changes will occur.
• More Flooding During Rainstorms
Streams that used to carry only a
small portion of rain that falls are
now asked to carry most of it.
Natural channels easily become
overwhelmed when a big storm
hits. The extra water overflows the
banks and floods the surrounding
land.
• Less Flow During Dry Times
Where do you think the water that
infiltrates into the ground goes?
Some of it moves slowly under-In a natural landscape, about
half the precipitation that
falls soaks into the soil.
The land is more impervious
in cities. Instead of soaking
into the soil, most of the
precipitation runs off hard
surfaces into storm sewers,
which empty into streams,
lakes, and ponds.
ground and empties into stream
channels days and even months
after soaking into the soil. This
groundwater provides flowing
water in the channel even during
dry times. When impervious
surfaces prevent infiltration of
rainwater, there is less groundwater
to move into the channel.
• Wider Channels with More Sedi-ment in Them
Imagine rushing water moving
down a channel after a rainstorm.
Everything in its path that is not
tied down or too heavy to move
gets carried away. This includes all
the loose sand, sediment, and dirt
on the banks of the stream. Over
time, pulses of rushing water erode
the banks away and the channel
gets wider and wider. Wider
channels also allow large volumes
of water to heat up from the sun,
raising temperatures higher than
what fish can tolerate.
• More Sand and Sediment on the
Channel Bottom
City slicker streams have more sand
and sediment at the bottom of their
channels than their country cousins.
Part of this material comes from
eroding stream banks. A large
Learn about your
urban streams at...
wwwga.usgs.gov/edu/
urbaneffects.html
The USGS has developed a
web site that shows how
urbanization affects the local
water system.
* Do you see any of the land
use changes listed at the
web site occurring in your
neighborhood?
Shower sleuthing
It’s raining!—a perfect time to
observe impervious surfaces
and runoff in action.
* Walk around your yard and
draw a map showing the
location of impervious
surfaces.
* Draw arrows showing the
path water takes off the
property.
* Follow the runoff and see
how it leaves your neigh-borhood. (Does it run into a
stream? A storm sewer?)
Storm water in a city is usually collected in storm sewers and
transported to a stream in large pipes. This photo shows
water exiting a pipe into a stream.
A planner develops
long- and short-term land
use plans for local governments to
provide for growth and revitaliza-tion of urban, suburban, and rural
communities.  Planners promote
the best use of a community’s land
and resources for residential,
commercial, institutional, and
recreational purposes.
A landscape architect designs
residential areas, public parks,
college campuses, shopping
centers, golf courses, and other
areas so that they are functional,
beautiful, and compatible with the
natural environment.
A civil engineer designs and
supervises the construction of
roads, buildings, airports, tunnels,
dams, bridges, and water supply
and sewage systems.
   

   




fraction, however, is carried by
runoff water washing the pavement
and land clean. When runoff
reaches the stream it drops its load
of sediment on the bottom of the
channel.