By Watershed Conservation Research and Data Specialist Anna Willig
One of the most basic, yet most critical, aspects of stream health is temperature. Temperature sits behind the scenes, governing nearly everything that happens in a stream. Make a stream too warm or too cold, and the wildlife will disappear.
While measuring the temperature of a stream seems straightforward — just stick a thermometer in the water — the reality is much more complicated. In small streams, such as the headwaters of Ridley, Crum, and Darby Creeks, temperature is incredibly variable. At the same spot in a stream, temperature can change by several degrees depending on the time of day, current weather, and time of year. Each day, water temperature rises and falls with the sun. Weather matters — a hot, sunny day will warm a stream more than a cool, cloudy day. Temperature also changes with the seasons, rising throughout the spring and summer and dropping in the fall and winter.
Location in a stream is important as well. In shady areas, streams will be cooler than in sunny areas. Slow-moving stretches tend to be warmer than fast-moving areas. Muddy water gets warmer than clear water. Shallow areas can warm more than deep sections. The variation of water temperature over time and across space in streams makes it crucial to measure temperature many times at many places in a stream in order to understand temperature trends.
Changes in land use further alter water temperature. When forests are leveled and replaced with strip malls and subdivisions, streams heat up. In undeveloped areas, rainfall soaks into the ground and cools as it flows underground towards streams. In developed areas, rainfall instead runs off of (often hot) parking lots, roads, houses, and sidewalks right into streams. This water cannot cool off and can make streams hotter. Removing streamside trees, building dams, and creating ponds all expose water to more sunlight, ultimately heating the stream.
Add in climate change, which will warm our region drastically in the next few decades, and it is clear that our streams are heating up. But why is this a problem? What does temperature have to do with the animals that live in our streams?
The danger of warm water temperature starts at the molecular level. Temperature governs the rate of chemical reactions and, in water, the amount of oxygen available. As a stream warms, chemical reactions occur faster and the water holds less oxygen. Most animals that live in streams — insects, mussels, fish, frogs, turtles — are cold-blooded, meaning their body temperatures change with the stream temperature. If a stream warms up, so will body temperature, speeding up respiration, metabolic reactions, and all other internal chemical processes that keep the animal alive. However, since warm water holds less oxygen, there will be less oxygen to support these processes, causing stress.
This thermal stress affects how fast organisms grow, how big they get, and how well they reproduce. Researchers from Stroud Water Research Center tested the impacts of water temperature on aquatic insects, which form the base of the food chain in streams. While there was a threshold above which all insects died, there were concerning effects even before temperature reached lethal levels. As temperatures warmed, insects developed and matured faster, but they did not become as large and consequently struggled to reproduce. If streams become too hot for aquatic insects to survive and reproduce, they will disappear, with catastrophic consequences for the entire ecosystem.
Fish are also sensitive to changes in stream temperature. Brook trout, Pennsylvania’s only native trout, currently cannot reproduce in local streams because temperatures are too warm. Even though trout are stocked by anglers every spring in local streams, the thermal stress forces these fish to devote so much energy to survival that they have no energy left for reproduction. Even less sensitive types of fish, like bass and sunfish, will show signs of stress when temperatures get too high.
If we want to keep our streams healthy, if we want to protect all the animals that depend upon them, we need to keep them cool. Limiting deforestation, especially along stream banks, will protect shady areas that cool streams. Establishing and expanding forested streamside buffers will create new shade and cool streams. Limiting new development will prevent the expansion of hot pavement. Shifting from stormwater management strategies that hold water, such as retention basins, to strategies that encourage infiltration, such as rain gardens, will force more water into the soil, where it can cool before reaching a stream.
You can make a difference at home by replacing lawns with gardens or meadows full of native plants, which will allow more water to enter the soil. Adding rain barrels will reduce the amount of hot runoff flowing off rooftops into streams. Planting trees, even if you are far from a stream, will also cool water down. Climate change means that rising temperatures will remain a constant threat, but making stream-conscious decisions can help ensure our streams remain healthy and full of life.
References and Resources
Check out this lecture on temperature from Dr. John Jackson of Stroud Water Research Center (lecture starts at 18:20): https://youtu.be/8Y1ey45053Q?t=1100.
Check out this research article about the impacts of temperature on mayflies from Stroud Water Research Center.
To see a streamside buffer restoration in progress, visit the Watershed Team’s Tree Planting at Ashbridge Preserve! Since 2019, we have put in nearly 1,500 trees with the goal of lowering water temperature in Ridley Creek.