Temperature Information Sheet

Background

The temperature characteristics of stream water directly and indirectly control aquatic ecosystems and water quality. Thermal pollution refers to the addition of warmer or colder water that causes an unstable jump in the temperature of a waterway.

The sun's energy affects water temperature, and every waterway's temperature will naturally fluctuate from season to season. The more sunlight that hits the water's surface, the warmer the water will get. Narrow, well-shaded headwater streams are often cooler than wider, larger streams that are not fully shaded by streamside (riparian) forests.

In addition to shading, the physical dimensions of the waterway will also affect the temperature. Shallow water will fluctuate in temperature faster than deeper water. Running water tends to be cooler than stagnant, still water. In a stream, the shallow riffles or rapids are often cooler than the slow moving, deep pools. The most downstream stretches of creeks and rivers are often warmer than the upstream sections, and may even have a slight thermal stratification (temperatures differ at various depths) in these deep, slow sections.

Temperature affects some of the chemical parameters of water. Probably the most important is dissolved oxygen, which can be found in the air spaces between water molecules (H2O). At lower temperatures, more oxygen can be dissolved in the water because the gas molecules are moving slower and are more compact. At higher temperatures, dissolved oxygen and other gases in water move faster and spread farther apart, including out of the water. Also at higher temperatures, the water molecules may move faster and bump out oxygen. Think of how the gases (carbon dioxide or the fizz) in soda pop eventually escape as it warms up.

As water temperature rises, it increases the metabolic rates of fish and aquatic insects. These rates require more oxygen; therefore, dissolved oxygen levels are reduced even further in the water. Temperature also influences another natural stream process. Under warm conditions, the rate of decomposition (break down of plants and animals) speeds up. Decomposition uses oxygen and produces carbon dioxide instead.

A rise in temperature can increase the concentration of total dissolved solids (the ions and particles in the water that you cannot see, such as salt). At higher temperatures, evaporation rates increase and the water vapor leaves behind the total dissolved solids. Concentrations of the dissolved solids will increase, as the body of water becomes shallower. High total dissolved solid concentrations can harm aquatic life. Refer to the Total Dissolved Solids Information Sheet for more information. Increased turbidity (cloudiness of water) can also increase the water temperature because the suspended visible particles can absorb the sun's rays.

There is a natural fluctuation of waterway temperature from season to season, even day and night, and aquatic life can cope with these natural changes. When humans alter the temperature of waterways, it may harm aquatic life; a thermal change of 2°C or more is harmful to stream organisms. All species have a specific range of temperature in which they are adapted. Fishermen know that trout like cold water streams, while other fish like carp and bluegills can tolerate warmer waters. If a stream changes temperature, organisms that cannot tolerate the change are stressed and must either reduce activity, move somewhere else, or in extreme cases, perish. Many life cycles of fish and aquatic insects are tied to water temperature. Temperature cues are used by these creatures to determine when to spawn, lay eggs, when the eggs will hatch, and when insect larvae will emerge from a stream to fly away. Thermal pollution can disrupt the timing of the life cycles, possibly causing eggs to hatch before sufficient food resources are available or larva to emerge when it is too cold atmospherically.

Human Impact

Humans can alter natural temperature characteristics of a stream by direct actions to the waterway or indirectly through alterations to the watershed. Industries and power plants discharge warm water that was used in the manufacturing process (boilers) or to cool machinery and turbines. When industries and community water authorities withdraw water from a stream, it may decrease the water depth. Since shallower water heats up more readily than deeper water, water withdrawal may increase stream temperatures. Water released from dammed lakes can also alter temperatures because it is often withdrawn from near the lake bottom and is often cooler than the stream temperature in the summer, warmer in the winter. The shock of these rapid temperature changes can be too much for aquatic life to handle.

Humans can alter the land use around a waterway and affect the water temperature. Removing the streamside forests for farming or during timbering eliminates the shade provided to the stream. Sunlight is no longer blocked. The removal of streamside forests and any other actions that promote soil erosion also increase the water temperature as more heat soaking sediment enters the stream. Urbanization is another heat causing pollution source. Rainwater that flows over hot paved surfaces warms up and eventually enters the stream.

Water Quality Standard

The Environmental Protection Agency (EPA) has established a formula for two important temperature extremes for streams: upper temperature limit and a weekly average. The upper temperature limit, or short term maximum, is set at 30.6°C for the area from the southern shore of Long Island, New York to Cape Hatteras, North Carolina. The weekly maximum is set at 27.8°C for this same area. No regulations are established for the zone containing Western Pennsylvania.

Example Temperature Date - French Creek

The Department of Environmental Protection (DEP) has systematically collected water quality data from French Creek, at Meadville, since 1973. A summary of these data is below to provide an understanding of past values of temperature and demonstrate trends or relationships of past temperatures.

Average 11.5 C

Minimum 0.0 C

Maximum 33.0 C

Graph 1 shows a trend of increasing temperature from January to August with a seasonal peak in August. The temperature of French Creek decreases from August to January. Graph 2 displays all temperature data from 1975-1994 for French Creek at Meadville. Note that the maximum and minimum temperatures have changed. Changes in temperature are mainly from the natural differences between seasonal weather. Soil erosion, storm drainage, and riparian zone removal may cause some artificial temperature influences. Withdrawal of water for agricultural irrigation and municipal water supplies drop water levels, allowing the shallower stream to heat quicker. Water released from flood controlled reservoirs (Woodcock Lake, Union City) tends to be cooler than natural water, especially during summer months, but these temperature changes would have rebounded before reaching Meadville.