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.
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