■ Stormwater BMPs for Trout Waters
Coldwoter Stream Design Guidance for Stormwater Wetlands,
Wet Ponds, and Bioretention
As our understanding of aquatic ecology has advanced, it has become clear that the
effects of stormwater runoff and urbanization are detrimental to organisms living in
streams and rivers. To reduce these negative impacts, a variety of stormwater best
management practices (BMPs) have been implemented to decrease stormwater flow
into streams and remove pollutants, such as metals, nutrients, and bacteria. While
these systems have become effective tools for treating many stormwater pollutants,
one contaminant has not been historically considered in BMP design: thermally pol¬
luted stormwater runoff.
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THERMAL POLLUTION
AND ITS IMPACT
Especially during the summer months,
pavement and rooftop materials cap¬
ture solar radiation, reaching tempera¬
tures much higher than those of natural
surfaces. During a storm event, heat is
transferred from pavement and roof¬
tops to stormwater runoff, with runoff
temperatures at times exceeding 1 10°F.
Because most of the heat is concentrated
near the pavement or rooftop surface,
runoff at the beginning of a storm often
exhibits a temperature spike with tem¬
peratures decreasing as rainfall contin¬
ues and surfaces cool. The effect of run¬
off on coldwater stream environments
depends upon both the temperature and
flow of water, making urban stonnwater
runoff especially dangerous with high
temperature and heavy flow.
Water temperature affects numerous
aspects of an aquatic ecosystem. Many
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aquatic organisms, such as fish and
insects, are ectotherms, meaning their
body temperatures are regulated by their
surroundings. Increased water tempera¬
tures can lead to behavioral changes,
such as increased feeding or aggressive¬
ness, as well as physiological changes,
such as increased metabolism or loss of
motor function. Fish, especially trout
and salmon, possess some of the most
stringent temperature requirements. Most
trout and salmon prefer water tempera¬
tures between 40 to 70°F, with increased
temperatures leading to injury or death.
In addition to the direct effects of
increased temperature, warm water has
a lower dissolved oxygen content, an¬
other vital habitat constraint for aquatic
organisms. The dynamics of an aquatic
ecosystem are impossible to predict.
Some life forms thrive under increased
temperatures, while others perish. This
complexity indicates the importance of
maintaining natural temperature ranges
