Water measurements: Temperature 
pH Specific conductance Turbidity
Dissolved oxygen Hardness Suspended sediment All water measurements
The U.S. Geological Survey (USGS) has been measuring water for decades; millions of measurements and analyses have been made. Some measurements, such as specific conductance and pH are taken almost every time water is sampled and investigated, no matter where in the United States water is being studied. Another common water-quality measurement often taken water temperature.
Temperature exerts a major influence on biological activity and growth. To a point, the higher the water temperature, the greater the biological activity. Temperature also governs the kinds of organisms that can live in a lake. Fish, insects, zooplankton, phytoplankton, and other aquatic species all have a preferred temperature range. As temperatures get too far above or below this preferred range, the number of individuals of the species decreases until finally there are none.
Temperature is also important because of its influence on water chemistry. The rate of chemical reactions generally increases at higher temperature, which in turn affects biological activity. An important example of the effects of temperature on water chemistry is its impact on dissolved oxygen. Warm water holds less oxygen than cool water, so it may be saturated with oxygen but still not contain enough for survival of aquatic life. Some compounds are also more toxic to aquatic life at higher temperatures.(Source: A Citizen's Guide to Understanding and Monitoring Lakes and Streams)
You might not think that water temperature is considered an important water-quality measurement. After all, temperature is not a chemical and it doesn't have physical properties. But, if you ask a fish if the temperature of the water it is living in is important, it would yell yes! (if it could talk). In natural environments, temperature is not too much of a concern for aquatic life, since the animals and plants in the water have evolved to best survive in that environment. It is when the temperature of a water body changes, either by a natural event or by a human-induced event, that the fish start to worry. Thus, this picture of a parking lot. Parking lots and roads, which are examples of impervious surfaces, where water runs off into local streams instead of soaking into the ground, as in natural environments, act as "fast lanes" for rainfall to make its way into streams. If the rainfall occurs duing summer, then the runoff water can be much hotter than the stream water. This heated water can be a shock to the aquatic life in the stream and can, thus, harm the water quality of the stream.
Temperature is also important in lakes and reservoirs. It is also closely related to the dissolved-oxygen concentration in water, which is very important to all aquatic life. Many lakes experience a "turning" of its water layers when the seasons change. In summer, the top of the lake becomes warmer than the lower layers. You've probably noticed this when swimming in a lake in summer - your shoulders feel like they're in a warm bath while your feet are chilled. Since warm water is less dense that colder water, it stays on top of the lake surface. But, in winter, some lake surfaces can get very cold. When this happens, the surface water becomes more dense than the lower water (which is now warmer than the surface), and the lake "turns", with the surface water sinking to the lake bottom.
The way that temperatures vary in lakes over seasons depends on where they are located. In warm climates the surface may never get so cold as to cause the lake "to turn." But, in climates that have a cold winter, then temperature stratifications and turning do occur. This chart is an illustration of termperatures profiles for a lake in Minnesota, USA (where it gets really cold during winter). You can see that in May the surface starts to warm (green color), but the warming only goes down to about 5 meters in depth. Even though the surface continues to warm all summer, the warmer water still stays on top of the lake. Even in summer the bottom half of the lake still stays almost as cold as it was in winter. During summer, the warmer water "floats" on top of the colder water; no mixing of water occurs. Notice in October, probably when the first freeze occurs, that the surface water cools, becomes the same temperature and density as the water in the bottom of the lake, and, thus, mixing occurs. After October, the temperature throughout the vertical column of water is about the same, cold temperature. The lake " has turned."
Certain industries have to be very concerned with water temperature. The best example of this is the thermoelectric-power industry that produces most of the electricity that the Nation uses. One of the main uses of water in the power industry is to cool the power-producing equipment. Water used for this purpose does cool the equipment, but at the same time, the hot equipment heats up the cooling water! Overly hot water cannot be released back into the environment -- fish downstream from a power plant releasing the hot water would protest. So, the used water must first be cooled. One way to do this is to build very large cooling towers and to spray the water inside the towers. Evaporation occurs and water is cooled. That is why large power-production facilities are often located near rivers.
If you look at National water-use trend data, you can see that thermoelectric-power water use kept rising until about 1980.
Thermoelectric-power water withdrawals primarily were affected by the Federal legislation that required stricter water-quality standards for return flow and by limited water supplies in some areas of the United States (U.S. Congress, Amendments to the Federal Pollution Control Act of 1972 and 1977; Micheletti and Burns, 2002). Consequently, since the 1970s, power plants increasingly were built with or converted to closed-loop cooling systems or air-cooled systems instead of using once-through cooling systems, which used more water.
Sources and more information
Water test kits are available from World Water Monitoring Day (WWMD). Teachers and water-science enthusiasts: Do you want to be able to perform basic water-quality tests on local waters? WWMD offers inexpensive test kits so you can perform you own tests for temperature, pH, turbidity, and dissolved oxygen.
World Water Monitoring Day is an international education and outreach program that builds public awareness and involvement in protecting water resources around the world.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu//temperature.html
Page Contact Information: Howard Perlman
Page Last Modified: Tuesday, 06-Oct-2009 07:59:47 EDT
