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.