Protecting Public Health and the Environment.

Water temperature has a profound effect on organisms that live or reproduce in the water, particularly Idaho's native coldwater fish such as salmon, Bull Trout, and steelhead and some amphibians (frogs and salamanders). When water temperature becomes too warm, salmon and trout suffer a variety of ill effects ranging from decreased spawning success to death. Most streams naturally warm as they flow from their headwaters to their mouth. Human-caused warming acts to shrink the available habitat that is suitable for coldwater-dependent species. For these reasons, it is important to protect the state's water from unnecessary warming.

Why Stream Temperature is Important

Stream temperature is an important part of water quality for the following reasons:

  • Coldwater fish such as salmon and trout need cold waters for optimum health during various stages of their lives. When temperatures are above optimum levels, fish are physically stressed and are more likely to get fungal infections and have difficulty getting enough oxygen. Thermal stress can also make fish more susceptible to toxic substances that may be present. If the temperatures stay above the lethal limit (77–78 °F) for very long, most salmonids will die.
  • Colder water holds more dissolved oxygen than warmer water, so as stream temperatures increase, the amount of dissolved oxygen available for fish and other aquatic organisms decreases. To make matters worse, warm water can also cause fish to need more dissolved oxygen.
  • Colder water slows the growth of bacteria and algae in water. When algae grow excessively, algal blooms can deplete the water's dissolved oxygen and cause changes in stream pH.
  • Stream temperature is the result of many different processes in the watershed. If stream temperatures are too warm, other water quality problems may be present as well, such as eroding streambanks and excessive sedimentation.

Causes of Elevated Stream Temperature

Elevated stream temperatures can result from both natural and human-caused events. Examples of natural influences on temperature include streams and rivers heating as they travel long distances over terrain that can't support streamside vegetation or when fires or floods remove significant portions of riparian vegetation. Climate and weather have an overriding effect on stream temperatures, so warmer temperatures are naturally seen in the day, during the summer, and at lower elevation and latitudes, which can make it quite difficult to detect unnatural warming. This leads us to look for a human cause of warming as well as at actual water temperature. Land management (human activity) can increase stream temperatures in the following ways:

  • Removing vegetation along streambanks reduces the amount of shade over the water thereby increasing the amount of solar radiation reaching the stream.
  • Withdrawing water for various purposes, including irrigation, reduces the amount of water in the stream during the summer when streams are already low. A shallow stream is heated more quickly by the sun than a deep stream. In addition, water in shallower streams moves more slowly than in deeper streams, which allows more time for heating.
  • Excessive sedimentation (i.e., build-up of boulders, rocks, gravel, sand, dirt, and silt) in a stream channel can result in a stream becoming wider and shallower, making it harder to shade and easier to heat. Sediment is a natural part of a stream system, but land management activities like road building, agriculture, forestry, and urban development can increase the amount of sediment entering a stream, delivering higher amounts of sediment than the stream can handle.
  • Changing the landscape can cause increased storm runoff. In some streams, peak streamflow can increase after changes to the landscape increase storm runoff. These high flows can scour the bottom of a stream, taking away gravel and rocks and leaving only bedrock. Bedrock absorbs the heat from the sun and later releases the stored energy and warms the water.

In addition to land management activities, discharge of wastewater that is warmer than the stream will also warm the stream. Such discharges can be the dominant factor in warming a small stream but in general are a less important factor than are land management activities. Impoundment and storage of water behind dams can either increase or decrease downstream water temperatures depending on the size and depth of the impoundment and how water is released. Impoundments, as well as natural lakes, create a lag in the annual cycle of warming and cooling in streams below their outlets.

While all streams warm, the best way to keep streams as cool as possible as long as possible is to maintain their natural shading provided by streamside vegetation.

Stream Temperature Standards

Federal regulations implementing the Clean Water Act call for states to adopt water quality standards that restore and maintain the chemical, physical, and biological integrity of the nation's waters. These standards include criteria limiting the temperature of the water to protect aquatic life, some of which are quite sensitive to changes in temperature. Concern over water temperature has come to the forefront because several stocks of salmon and Bull Trout, coldwater-dependent species, have been listed as threatened or endangered species.

DEQ and EPA do not agree on acceptable criteria for temperature for Idaho water bodies. At issue is a balance between temperature that is protective of coldwater-dependent species yet attainable in most water bodies. Numerous studies and investigations have been conducted by DEQ and others to determine the impact of temperature on aquatic life in various water bodies. In April 2003, EPA Region 10 issued guidance to states and tribes in the Pacific Northwest on temperature criteria to protect endangered salmonids. Idaho participated in developing this guidance but in the end dissented on most of the recommended criteria due to reservations as to their attainability. These reservations persist to this day.

Idaho's Current Water Temperature Criteria

DEQ's current stream temperature standards protect aquatic life uses, the only uses that have temperature requirements. The criteria vary by—and to a large degree define—the aquatic life uses: warm water, seasonal cold water, cold water, salmonid spawning, and bull Trout (see table below). The latter two are subcategories of the cold water aquatic life use. For all uses but bull Trout, DEQ uses a pair of criteria that limits the daily maximum and daily average temperatures.

Depending on the diurnal (day to night) temperature range in a given stream, one or the other of these paired criteria will be more limiting of the stream's warmth. Using a pair of criteria limits a broader range of stream temperatures than either alone could. For Bull Trout, the criterion is for a seven-day rolling average of daily maximums. This rolling average regulates high temperatures while allowing a few days to be slightly warmer.

All temperature criteria are limits on the maximum permissible value for a particular measure of temperature over a year or season. For assessment purposes only, the water quality standards allow DEQ to consider the health of the biological community as measured by its Beneficial Use Reconnaissance Program. If bioassessment shows the stream to support cold water aquatic life and the frequency of exceedance of temperature criteria is 10% or less, DEQ defers to the bioassessment.

Idaho’s rules also allow for exceedances of the criteria due to natural background conditions but not due to human activities (except for a small 0.3 °C human-caused increase in waters that are naturally warmer than criteria). Many waters in Idaho likely have temperatures naturally warmer than criteria; however, determining these natural temperatures in altered settings requires modeling of past conditions, which can be difficult.




33 °C
(91 °F)

26 °C
(79 °F)

22 °C
(72 °F)

13 °C
(55 °F)
MWMTb N/A N/A N/A N/A 13 °C
(55 °F)
MDATc 29 °C
(84 °F)

23 °C
(73 °F)

19 °C
(66 °F)
9 °C
(48 °F)

a MDMT = maximum daily maximum temperature
b MWMT = maximum weekly (7-day average) maximum temperature
c MDAT = maximum daily average temperature


DEQ undertook rulemaking in 2011 to revise its temperature requirements, in particular the treatment requirements at IDAPA and d and the salmonid spawning criteria at IDAPA The change to salmonid spawning criteria was aborted at EPA’s behest due to Idaho’s lack of data to describe everywhere in the state where the new criterion would apply. The revision to the thermal treatment requirement remains under consideration by EPA. Learn more.

DEQ Resources

EPA Region 10 Guidance

IDL Shade Rule

More Information

A Statistical Model for Estimating Stream Temperatures in the Salmon and Clearwater River Basins, Central Idaho (2002)

Aquatic Assemblages and Their Relation to Temperature Variables of Least-Disturbed Streams in the Salmon River Basin, Central Idaho (2001)