Flow, in cubic-feet/second, is measured in Chester Creek using a Marsh-McBirney velocity meter.
What exactly is a cubic foot per second? |
Stream Flow
Stream flow, also called discharge
and indicated by the symbol Q, is determined with rating curves
developed for each stream and subsequently used to calibrate
flow. Stream elevation (called "stage height") is
measured remotely using a pressure transducer and discharge
is determined using rating curves based upon a set of cross-sectional
and discrete depth in-stream velocity measurements made with
a Marsh-McBirney velocity meter (other manufacturers will work
also) over a range of discharge conditions. Flow, in cubic-meters/second
is estimated as the average velocity of the water, in meters/sec,
multiplied by the cross-sectional area of stream at that point,
in square meters.
We actually measure velocity at various
depths and positions across the stream to estimate the true
"average flow". If we do this over a range of flow
conditions, from baseflow to high flow while simultaneously
measuring the height of the stream, we can generate a graph
relating flow to stage height. This curve is then used to
convert the remotely measured stage height into flow values.
Sampling intervals have been set at 15 minutes that allows
more than enough time for the sensors to equilibrate between
readings. Although more frequent readings are possible, the
extra data is not useful for our purposes and simply use up
available computer memory, add to processing time and slow
the use of the data visualization tools.
Why is it important?
Flow
is a fundamental property of streams that affects everything
from temperature of the water and concentration of various
substances in the water to the distribution of habitats and
organisms throughout the stream. Low flow periods in summer
allow the stream to heat up rapidly in warm weather while
in the fall and winter temperatures may plummet rapidly when
flow is low. Flow directly affects the amount of oxygen dissolved
in the water. Higher volumes of faster moving water, especially
if it creates "white water," increases the turbulent
diffusion of atmospheric oxygen into the water. Low flow conditions
are much less conducive to oxygenation and when water temperature
is high, DO levels can become critically low. The amount of
sediment and debris a stream can carry also depends on its
flow since higher velocity increases stream bank and stream
channel scouring and erosion, and also keeps particulate materials
suspended in the water. The precipitation inputs that cause
higher flows may also wash higher amounts of particulate and
dissolved materials from the watershed directly into the stream.
Stream flow, acting together with the downward slope (gradient),
and the geology of the channel (its bottom substrate), determines
the types of habitats present (pools, riffles, cascades, etc),
the shape of the channel, and the composition of the stream
bottom.
Raindrop impact. One of many types of erosion.
Fig 2.13 in Stream Corridor Restoration. Principles Processes and Practices (10/98).
Interagency Stream Restoration Working Group (15 federal agencies)(FISRWG)
Flow paths of water over a surface.
The portion of precipitation that runs off or infilitrates
to the ground water table depend s on the soil's permeability
rate; surface roughness, and intensity of precipitation.
Fig 2.10 in Stream Corridor Restoration. Principles Processes
and Practices (10/98).
Interagency Stream Restoration Working Group (15 federal agencies)(FISRWG)
Reasons for Natural Variation
The volume of stream flow is determined by many factors.
Precipitation is of course the primary factor- the more rain
or snowmelt, the higher the flow. However, there is usually
a lag period between the time a storm reaches it highest intensity
and the time the stream reaches it peak flow. This lag time
is affected by land use practices in the watershed. Vegetation
increases the time it takes the water to reach the stream
by allowing it to slowly infiltrate into the soil before it
reaches the stream. Wetlands and ponds in the watershed also
add to this temporary storage. If it rains hard enough and
long enough, the ground may saturate with water and then the
precipitation will run off directly into the stream. In winter
and spring, the potential of the natural soil and vegetation
to absorb water is also affected by the depth to which it
is frozen. This is why even moderate spring rainstorms may
bring severe flash flooding. The precipitation also melts
snow and ice that further adds to the problem. In Duluth,
stream flow regimes throughout the year are typically characterized
by low, or base-flow conditions that most commonly occur in
summer and winter, the spring snowmelt runoff (high flow)
period, and sporadic periods of storm runoff (high flow).
Duluthians know better than most people about how variable
these periods are and how different years can be. Because
flow is such an important factor in determining the overall
ecology of a stream, we have to be particularly careful about
how we modify it. Because of its natural variability, we also
must be careful to interpret water quality data in light of
how the streams are flowing.
Expected Impact of Pollution
The increased, and more variable flows associated with
stormwater runoff pose a direct threat to the aquatic organisms
in Duluth's streams by modifying their physical habitat. Organisms
are adapted to certain ranges and intensities of water velocity.
Urbanization increases impervious surfaces such as roofs,
roads and parking lots that speed the delivery of water into
streams. They become "flashier." Higher velocities
alter habitats by moving cobbles and boulders and flushing
large woody debris (snags and shoreline brush). Increased
flows create secondary impacts by increasing erosion, modifying
the channel and riparian zone in addition to delivering added
"natural" pollutants (leaves, soil, animal droppings),
road surface chemicals (metals, hydrocarbons, salts), lawn
materials (grass and garden clippings, fertilizer nutrients,
pesticides), and just plain litter - cigarette butts, cans,
paper, and plastic bags. Increased erosion severely affects
habitats by producing increased sedimentation of fine silt
that fills the spaces between gravel and cobbles where aquatic
invertebrates live, scours organisms and clogs their gills.
Although perhaps less important in Duluth's
urban streams, many streams and rivers in the U.S. have been
severely impacted by flow modifications due to impoundment
(creating dams) and channelization. The management of dams
on the St. Louis River and some of its feeder lakes was the
focus of intensive studies in the 1990's regarding effects
on both habitat and water quality.
References:
Water on the Web Module 4/5 - Stream Ecology Lectures 2 and 3
http://waterontheweb.org/curricula/ws/unit_01/U1mod4_5.html
(download slides or view from browser)
Gordon, N.D., T.A. McMahon, and B.L. Finlayson. 1992.
Stream Hydrology: An Introduction for Ecologists. Wiley.
New York, N.Y.
FISRWG (10/1998). Stream Corridor Restoration: Principles,
Processes, and Practices. By the Federal Interagency Stream
Restoration Working Group (FISRWG)(15 Federal agencies of
the US gov't). GPO Item No. 0120-A; SuDocs No. A 57.6/2:EN
3/PT.653. ISBN-0-934213-59-3.
http://www.nrcs.usda.gov/technical/stream_restoration/
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