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These brief rainy periods in August 2003 warmed Tischer Creek by
4 - 7° F, near the upper limit for Brook trout. Temperature is
shown by the black line and the background color; flow is shown in
blue and the green bars show rainfall at the airport. It looks like
in rained harder at Tischer in East Duluth than at the airport on
the first day and vice versa on the second. You can see how precipitation
varies across the City by using the Data
Viewer for WLSSD's rain gauges.
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Stream Temperature
Duluth Critters Can't Take the Heat
Temperature is one of the most important factors determining what lives
in a stream. Warm streams support sunfish, bluegill, bass, and bullheads,
and cold streams support trout species including our native Brook trout.
Brook trout need very cool water, cooler than most other trout species.
They prefer temperatures between about 52° F (11° C) and 61° F
(16° C), and can’t live for long periods in water temperatures
above about 75° F (24° C). The cooler water is needed for spawning
and embryo survival. Many of the streams in the Duluth area are cold
water streams that support Brook trout; further south in Minnesota, streams
are generally too warm for trout unless spring fed.
Warm water can also lead to oxygen problems, since warm water holds less
gas than cold water when saturated (warm root beer goes flat). When flows
are low and the water is warm in summer, relatively small amounts of
extra decomposing organic matter can lead to low oxygen levels that stress
fish. Sources include soil and vegetation from erosion, oils and greases,
and excess algae caused by fertilizer nutrient enrichment.
Stream temperature is determined by many factors, including:
- Air temperature - the only parameter in this list
that does not directly change with watershed development
(although big cities are, in fact, warmer than the
suburbs)
- The amount of light hitting the water - clearing
stream side (riparian) vegetation allows more sunlight
to reach the stream, warming the water
- Water depth - more volume leads to cooler habitats
at depth
- How dirty the water is - dirty water absorbs more
heat from the sun. Erosion causes the water to become
turbid with suspended sediment
- How much groundwater is coming into the stream
and the presence/absence of cold springs
- The types of land surfaces in the watershed, such
as impervious surfaces, that get hot from sunlight
Impervious surfaces result in water running off warmer surfaces (such
as parking lots and rooftops) and carrying this water quickly to the
stream. They cause more water to run into the streams with less infiltration
into the ground to become ground water. This leads to higher flows
during storms causing more erosion and dirtier water, but lower flows
between rain events due to less groundwater. The reduced water depths
cause the streams to warm up more quickly.
As water runs across a surface, it tends to warm up to (or cool down
to) the temperature of the surface. On a hot summer day, rain falling
on hot asphalt or other warm surfaces will warm up dramatically (on
a typical 80° F summer day, pavement temperatures can be well above
100° F!), and as this water drains into our streams, the streams
warm up as well. You can see this effect in the data from Tishcer Creek
in the graph above.
This graph shows how often the temperature of Kingsbury Creek in the
Duluth Zoo reaches unhealthy levels during the summer. For 15 straight
days the temperature exceeded the optimal range for brookies, and on
10 days it exceeded their upper limit. We don't know how this really
affects the fish. Perhaps they find cooler pools to get past those
warm spells. Regardless, it's one additional chronic stress on top
of sediment effects and other "insults" that result from
how we treat the watershed.
What you can do:
- If you live on a creek, keep or plant trees and
shrubs along the stream to provide shade. The South
St. Louis Soil and Water Conservation District can
help you determine what the best types of trees for
your location, and has an annual tree sale every
spring.
- Prevent water from running off your driveway,
sidewalk, or rooftop from going directly into the
gutter or into the street. Collect rainwater from
your roof in rain barrels, plant a rain garden in
your yard, and make sure your downspouts are draining
onto pervious areas, such as your lawn or a wooded
area, rather than onto your driveway. You want it
to soak into the ground.
- Keep it Clean: don't sweep or dump dirt (or anything
else) into the gutter or catch basins on your street
and sweep dirt and lawn clippings off your driveway
before it gets washed away. Everything we put in
the gutter ends up in a creek, and eventually into
Lake Superior! Click
here to learn more about this topic.
(Sources: EPA 1986, Newbury et al. 1993, Raleigh 1982,
Raleigh et al. 1984a, Raleigh et al. 1984b
http://www.water.ncsu.edu/watershedss/dss/estuary/trout/temperature.html and
http://wlapwww.gov.bc.ca/wat/wq/BCguidelines/temptech/temperaturetech-05.htm.
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