Thermal Impacts

The Thermal Impact of Untreated Storm Water

The thermal impact of untreated storm water to segments of the Kinnickinnic River within the City of River Falls, especially in the downtown and Glen Park areas, have been clearly documented by temperature monitoring research conducted by the local Kiap-TU-Wish Chapter of Trout Unlimited. These thermal impacts are also evident in the South Fork of the Kinnickinnic River. When summer rainfall flows over warm impervious surfaces, it absorbs heat (thermal pollution) that is discharged to the Kinnickinnic River through storm sewers, causing a rapid and marked temperature increase at the beginning (first flush) of a storm water runoff event. These rapid temperature increases (thermal spikes) in the Kinnickinnic River may be particularly harmful to macroinvertebrates (aquatic insects), which serve as a critical food source for trout. Scientific research (Galli, 1990) shows that macroinvertebrates have little ability to compensate for rapid temperature changes. Hence temperature changes of 1-2 degrees centigrade can reduce insect size and reproduction, while temperature changes of 2-3 degrees centigrade could eliminate sensitive insect species.


Storm Water Ordinance

The City’s Storm Water Ordinance targets new development, however, large quantities of runoff come from the old parts of town also. Due to this, the City of River Falls has been evaluating several storm water management options for 24 direct storm sewer discharges to the Kinnickinnic River between County Road MM and Lake George. In June 2005, the city, with consulting assistance from Bonestroo Rosene Anderlik and Associates, completed the Lake George Area Storm Water Treatment Concept Plan, which presents several alternatives for managing untreated stormwater from these 24 discharges.

Thermal Preferences of Brown Trout for Survival & Growth

(Armour, 1994) 

Physiological Optimum: 17.0 C (12.0 - 20.0 C)
Growth Optimum: 16.4 C (7.0 - 19.0 C)
Zero Net Gain Range: 19.5 - 21.2 C
(No Growth; Survival = Mortality)
Visible Thermal Stress: 19 - 30 C


Lower Value: Avoidance, restlessness, disturbing temperature
Upper Value: Maximum survival temperature for brief periods
Upper Ultimate Incipient Lethal Temperature: 25 - 30 C
Critical Thermal Maximum: 29.8

Summary: Brown trout will begin showing stress at 19 C (66F) and will die above temperatures of 29 C (84F)

Thermal Preferences of Macroinvertebrates

(Galli, 1990)

Many aquatic insects (esp. mayflies) require a fluctuating diurnal temperature regime. Temperatures > 17 C (63F) exceed the optimum for many stoneflies, mayflies, and caddisflies.

For more information on the impacts of storm water on the Kinnickinnic, the local Trout Unlimited Chapter (Kiap-TU-Wish) has some good papers available: