The USGS Water Science School
Water properties: Temperature
The U.S. Geological Survey (USGS) has been measuring how much water is flowing in rivers, determining the water levels of groundwater, and collecting water samples to describe what the quality of those waters are for over a century. Millions of measurements and analyses have been made. Water temperature is taken almost every time water is sampled and investigated, no matter where water is being studied.
Significance of water temperature
Temperature exerts a major influence on biological activity and growth. Temperature governs the kinds of organisms that can live in rivers and lakes. 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. Water, particularly groundwater, with higher temperatures can dissolve more minerals from the rocks it is in and will therefore have a higher electrical conductivity. It is the opposite when considering a gas, such as oxygen, dissolved in the water. Think about how much bubblier a cold soda is compared to a warm one. The cold soda can keep more of the carbon dioxide bubbles dissolved in the liquid than the warm one can, which makes it seem fizzier when you drink it. How warm stream water is can affect the aquatic life in the stream. Warm water holds less dissolved oxygen than cool water, and may not contain enough dissolved oxygen for the survival of different species 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)
Impervious surfaces contribute hot water to 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.
The picture to the left shows a typical parking lot after a strong summer rainstorm. 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. Rain that falls on a parking lot that has been baking in the sun all day during summer gets super heated and then runs off into streams. This heated water can be a shock to the aquatic life in the stream and can, thus, harm the water quality of the stream.
Along with the heat, runoff from parking lots can contain pollutants, such as leaking motor oil, hydrocarbons from exhaust, leftover fertilizer, and normal trash. Some communities are experimenting with using permeable pavement in the parking lot and water gardens and absorbent plants alongside the lot to see if this cuts down on harmful runoff from the lots into streams. In the right side picture the parking surfaces are tilted so that they drain into a natural area that allows runoff to soak into the ground. Water-loving plants are also being grown in the area. A significant amount of the runoff should be captured by these areas, and by the time a portion of the runoff reaches a stream, the water temperatures should be closer to normal stream temperatures.
Seasonal changes in lakes and reservoirs
Temperature is also important in lakes and reservoirs. It is 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 deeper water with a more constant year-round temperature (which is now warmer than the surface), and the lake "turns", when the colder surface water sinks 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, temperature stratifications and turning do occur. This chart is an illustration of temperatures 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 less dense 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 less dense warmer water stays on top of the colder water; no mixing of water occurs. Notice in October, as the temperature starts to consistently get down near freezing at night, the surface water cools, becomes a little colder in temperature and a little more dense than the water in the bottom of the lake, and, thus, sinks, causing mixing. The lake " has turned." After October, the temperature throughout the vertical column of water is about the same, cold temperature, until the ice is melted and the sun can warm the top of the lake again.
Temperature Effects of Dam Operations
Aerial view of Cougar Dam and Reservoir, Oregon, looking south. (Bob Heims, U.S. Army Corps of Engineers)
I'm sure fish have been living in the McKenzie River in Oregon for many thousands of years—way before many people lived there and definitely before the Cougar Dam was built. For eons fish were adapted to live and reproduce in a river having certain environmental characteristics that would not change quickly. But, after the construction of Cougar Dam, one thing that did change for the fish was the water-temperature patterns below the dam at certain times of year. The McKenzie River supports the largest remaining wild population of Chinook salmon in the upper Willamette River basin, and the South Fork McKenzie River provides good spawning habitat. It was found that the altered temperature pattern downstream of Cougar Dam created problems with regard to the timing of migration, spawning, and egg hatching for the fish (Cassie, 2006).
This detrimental environmental consequence was realized in the mid 2000s and to restore the suitability of this reach for salmon spawning, the U.S. Army Corps of Engineers (USACE) added a sliding gate assembly to the intake structure at Cougar Dam. Water temperature patterns below the dam have become more like natural patterns recently, with the result being a lot of smiling salmon. The chart below shows the differences in temperature patterns for sites above and below the dam before any adjustments were made to fix the situation.
1 Caissie, D., 2006, The thermal regime of rivers—A review: Freshwater Biology, v. 51, p. 1389-1406.
Impoundements can alter natural temperature patters of a river
This chart compares a year's temperature pattern for monitoring sites on the South Fork McKenzie River upstream and downstream of Cougar Dam. The intent is to show how, due to certain construction aspects of the dam, that seasonal temperature patterns below the data were severely altered after the dam became operational. The altered temperature patterns had adverse effects on fish populations below the dam.
The light grey line shows, for the upstream site, a pattern as you might expect—temperatures heating up in late spring and rising during the summer with the fall bringing lower temperatures. It shows a normal bell-curve type of pattern that closely follows seasonal air temperature patterns. Fish living in this reach of the river would be adapted for these normal temperature patterns.
Cougar Dam controls the flow and greatly influences the temperature in the South Fork McKenzie River downstream of the dam. Cougar Reservoir becomes thermally stratified in summer, with warmer, less-dense water near the surface and colder, denser water at the bottom. Western Oregon's warm and sunny summer weather adds additional heat to the reservoir's surface, stabilizing its stratification throughout the summer. Because the dam was built with its major release point at a relatively low elevation, the dam historically released relatively cold water from near the bottom of the reservoir in mid-summer. As the reservoir was drawn down in autumn to make room for flood-control storage, the heat that was captured in the reservoir's upper layer during the summer was released downstream. As a result, the seasonal temperature pattern (darker line on chart) downstream of Cougar Dam through 2001 was quite different from the pattern upstream of Cougar Reservoir.
Power plants must cool their used water
Aerial photo of Beaver Valley Power Station in Pennsylvania, showing evaporation from the large cooling towers.
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.
Do you want to test your local water quality?
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 your 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.