We are in Hot Water – Streams and Rivers are Heating Up
2025 was the hottest year for ocean temperatures in recorded history, and this was not an isolated occurrence. This continuing warming trend has been observed over more than two decades. Not surprisingly, similar warming trends have also affected river and stream temperatures globally and across the United States.
Waterways experience normal daily and seasonal temperature fluctuations, just as air temperatures change seasonally and throughout the day and night. Streams generally absorb heat during the day when exposed to solar radiation and cool down at night. Historically, streams and rivers were protected from high daytime temperatures by forested landscapes that provided shade and the existence of soil and vegetation that allowed water to flow slowly through the ground, cooling off as it moved through the soil. In addition, historical temperatures in the Northeastern United States experienced fewer extremely hot days and more of a temperature differential between day and night, preventing waterways from cooling normally.
Temperature affects biological, chemical, and physical stream health parameters that govern processes from the well-being and composition of aquatic species to nutrient cycling to pH buffering and much more. Increasing stream temperatures and larger, more frequent temperature fluctuations have a negative effect on stream health, especially when elevated temperatures persist and increase over time.
The chemistry of water molecules gives water the ability to absorb or lose a large amount of heat energy before water changes temperature, resulting in stream and river temperatures changing much more slowly than the surrounding land and air. This is a quality that provides a moderating effect for water-dwelling organisms, which require specific temperature ranges to live and thrive. Heat waves are typically defined as unusually hot weather lasting at least two days, with temperatures above the local historical averages. Consequently, when heat waves result in increasing air and land temperatures over time and when those temperatures last for longer periods of time, water temperatures rise above normal ranges and remain above normal temperatures for extended periods. This creates a cascading series of harmful effects on fish, macroinvertebrates, amphibians, aquatic vegetation, overall ecological function, and outdoor recreation for humans.
Why is temperature an important characteristic of waterways?
Because temperature controls reactions that make up biological and chemical processes, warmer water temperatures raise reaction rates for aquatic life. Many aquatic organisms are ectothermic, meaning that their body temperature is controlled by their surroundings. When waterways warm up, the organisms within the waterways also warm up, leading to an increase in body temperatures, respiration, and metabolism. At the same time, higher temperatures reduce the amount of oxygen dissolved in the water, so when the increased metabolism of aquatic organisms demands more oxygen, it may not be available. Additionally, these stressful conditions reduce the energy available for essential functions, such as growth, disease resistance, and reproduction, which can diminish the size, lifespan, and numbers of aquatic species over time. Different aquatic organisms have different suitable temperature ranges, but some waterways are now frequently reaching temperatures that are generally above a healthy range for most aquatic life. Unlike their terrestrial counterparts, animals that inhabit water cannot easily seek more favorable habitat.
What are the normal temperatures/ranges?
Waterways have varying temperatures, depending on size, volume, flow, time of day, season, location, surrounding landscape, stream morphology, stream order, and hydrogeology. Rivers tend to be warmer than streams, and forested headwater streams tend to be cooler. In Pennsylvania, streams may be at or near freezing for periods in the winter and may reach the mid to upper 70s during hotter parts of the summer. In general, cooler temperatures are better for a wide variety of aquatic species.
According to the Pennsylvania DCNR, these are general upper temperature ranges for some fish and aquatic macroinvertebrates:
Warmwater species (70 degrees F and up): largemouth bass, crappie, bluegill, carp, catfish, caddisfly
Coolwater species (65 to 70 degrees F:) perch, sauger, walleye, smallmouth bass, pike, muskellunge, pickerel, rock bass, stonefly, mayfly, caddisfly, water beetles
Coldwater species (70 degrees F and below:) trout, salmon, caddisfly, stonefly, mayfly
What temperature disruptions have been observed?
A 2025 Penn State study of 1,500 sites in the contiguous United States from 1980 to 2022 found that heat waves in water are increasing between two to four times as rapidly as heat waves in air. Heat waves in water also persist longer and have a greater intensity.
Analysis of historical stream temperature records in a 2010 publication found a significant increase in water temperatures in 20 of 40 streams and rivers. Areas downstream of urban communities had the most rapid rates of increase, with the greatest rate of increase found in the Delaware River outside of Chester, Pennsylvania.
A 2015 analysis of streams that feed into the Chesapeake Bay showed that between 1960 and 2014, there was a rise in water temperature in 79% of streams that were measured, with more than half showing a statistically significant temperature increase. For all sites, temperature increases averaged 1.2°F, while sites with statistically significant increases rose by an average of 2.2°F.
In June 2025, a heat wave occurred across much of Pennsylvania. In southeastern Pennsylvania, daily high temperatures ranged from 86 to 101 degrees for thirteen days between June 18 and June 26. Near the end of the heat wave, the water temperature of the Schuylkill River, measured at the USGS gauge at Norristown, reached 85°F on June 26, and for five days, water temperatures remained above 80 degrees, which is higher than the desirable threshold even for warm water fish. A typical June temperature at this gauge under normal summer conditions would be in the low to mid-70s.
Graph showing the water temperature increase in June 2025 in the Schuylkill River at Norristown, Pennsylvania
What factors affect the increasing temperature of streams and rivers?
Air temperature affects the temperature of water, so rising air temperatures result in rising water temperatures. Dams and physical disturbance of streams and rivers can create shallow areas or reservoirs that heat up more readily and transfer heat if the water is discharged. Direct sunlight raises the temperature of unshaded waterways, and the large-scale loss of forested riparian buffers has left streams and rivers more likely to heat up. Groundwater that enters streams helps regulate stream temperature because it flows underground and tends to be cooler in the summer and warmer in the winter. During heat waves, however, groundwater temperatures also increase.
Development and urbanization have increased the amount of impervious surface, such as parking lots, roads, sidewalks, and buildings. In areas with a high percentage of impervious cover, rather than infiltrating into the soil and slowly cooling before being returned to streams, rain runs rapidly across hot surfaces, absorbing heat from concrete and asphalt, and flows directly into streams and rivers, elevating the temperature of the waterway.
Pennsylvania and the surrounding region have already begun to experience the effects of changing climate, such as lengthier and more frequent heat waves, longer periods of drought, and more unpredictable precipitation. Drought reduces water levels, causing temperatures to rise, and if this happens in conjunction with extended periods of hot weather, overheating can reach harmful levels. Flooding can also elevate water temperatures because floodwaters typically absorb heat as they travel across heated surfaces, and this heated floodwater enters waterways rapidly and in large amounts. Climate change projections indicate that these conditions are expected to worsen, which will intensify and lengthen the periods of temperature extremes in waterways. At the same time, increasing development increases the volume of routine heated runoff.
What are the problems that arise from higher water temperatures?
As temperatures increase, the amount of dissolved oxygen available in the water decreases. Oxygen is essential to the survival of most organisms that live in streams and rivers. When oxygen levels drop, this leads to fish kills, a reduction in food sources at all trophic levels, and changes to the overall stream or river ecology.
High temperatures, especially when repeated and long-lasting, degrade the habitat quality of streams. Over time, less heat-tolerant species are less likely to survive, and the biodiversity of the waterway throughout the food web is reduced, from plant life to macroinvertebrates to fish. Eventually, the only remaining species are those that are highly tolerant of extended periods of warmer temperatures.
Invasive aquatic species, such as Northern Snakehead fish, are often more tolerant of a wider range of temperatures and dissolved oxygen levels than our native fish species, such as Brook Trout, that require clean, cold water. Many invasive aquatic plants also benefit from elevated water temperatures, giving them a competitive advantage over native plants that are better adapted to cooler water that was historically found in Pennsylvania. The number of invasive species can increase dramatically when water temperatures rise, ultimately shifting populations toward invasive fish, plants, and other aquatic life, while limiting the survival of native species.
Higher temperatures also increase the growth of algae and other aquatic plant life. As the algae rapidly grow, die, and decompose, available oxygen is used up. In addition to algal blooms being harmful to other organisms living in waterways, they can also create an unpleasant odor and appearance and unhealthy conditions that prevent people from enjoying recreational activities, such as swimming, fishing, or boating.
Waterborne pathogen numbers increase with higher temperatures. Coliform bacteria and other pathogens can grow and multiply rapidly when temperatures are elevated, leading to an increased likelihood of disease concerns.
What can we do to protect waterways from temperature extremes?
Riparian Buffers. Trees and shrubs along the banks of waterways provide shade and protection from direct sunlight, helping to moderate water temperatures and prevent overheating. Planting, restoring, extending, and maintaining riparian buffers, ideally a minimum of 35 feet wide on each side of the waterway, is one of the most high-impact interventions to protect streams and rivers from extreme temperatures, in addition to providing habitat and reducing pollutants.
Community science monitoring. One useful action that individuals can take is to participate in water quality monitoring programs that measure and record temperature along with visual, biological and chemical parameters, and assess overall stream health and biodiversity by conducting macroinvertebrate sampling. Measuring and tracking the temperature of a water body over time provides valuable data that can contribute to evaluation, stewardship, and public education activities that help communities maintain or improve stream health. Local watershed groups, fishing organizations, river keeper networks, and environmental organizations often have existing volunteer programs that provide training.
Impervious surface reduction. Promoting vegetated or porous surfaces that allow infiltration wherever possible and encouraging residential, community, and municipal planning that limits or reduces impervious surfaces can reduce the amount of heated stormwater runoff that enters local waterways.
Stream and wetland restoration. Participating in watershed groups that advocate for and protect local waterways can improve local conditions. When communities implement stream restorations to achieve more natural conditions, this can reduce high-temperature water inputs and help stabilize stream temperatures. Wetland restoration and protection can act as a buffer to hold and cool water before it enters streams, reducing flooding and temperature impacts.
Communication. Educating fellow community members about the risks of high stream temperatures and the interventions that can improve conditions helps increase public awareness and spark solutions. Using any data and observations you have collected strengthens the case and tells the story of local waterways in a compelling and specific way.
来源：Penn State Extension 科普简报
We are in Hot Water – Streams and Rivers are Heating Up
