Department of
Environmental Quality

Water temperature has a profound effect on organisms that live or reproduce in the water. This is particularly true of Idaho's native coldwater fish such as salmon, bull trout, and steelhead, and some amphibians (frogs and salamanders). When water temperature becomes too high, salmon and trout suffer a variety of ill effects ranging from decreased spawning success to death. 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 because:

• 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 oxygen, and, if the temperatures stay very long above the lethal limit (77-78 °F), most salmonids will die.
• Colder water holds more dissolved oxygen than warmer water, so as stream temperatures go up, the amount of dissolved oxygen available for fish and other aquatic organisms goes down. To make matters worse, warm water can also cause the fish's need for dissolved oxygen to increase.
• Colder water slows the growth of bacteria and algae in water. When algae grow excessively, algal blooms can use up the water's dissolved oxygen and cause changes in stream pH levels.
• Stream temperature is the result of many different processes in the watershed. If stream temperatures are too high, other water quality problems may be present as well, such as eroding stream banks and excessive sedimentation.
• Warm water can make other water quality problems worse. For instance, warm water can lead to increased bacteria and nuisance aquatic plant growth and intensify water chemistry problems involving dissolved oxygen and pH.

Causes of Elevated Stream Temperature

Elevated stream temperatures can result from both natural and human-caused events. Examples of natural influences on temperature include creeks and rivers heating if they travel long distances over terrain that can't support streamside vegetation or when fires or floods remove significant portions of riparian vegetation.

Land management (human activity) can increase stream temperatures through:

• Removing vegetation along the banks of streams, which reduces the amount of shade over the water, which increases the amount of solar radiation reaching the stream.
• Withdrawing water for various purposes, including irrigation, which 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, the water in shallower streams moves more slowly than in deeper streams, which allows more time for heating.
• Contributing excessive sediment (boulders, rocks, gravel, sand, dirt, silt) to a stream channel, which 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 have the potential to greatly increase the amount of sediment entering a stream, delivering higher amounts of sediment than the stream can handle.
• Changing the landscape, which can cause increased storm runoff. In some streams peak stream flow can increase after changes to the landscape increase storm runoff. These high flows can scour out the bottom of a stream, taking away gravel and rocks, leaving only bedrock. Bedrock absorbs the heat from the sun and later releases the stored energy and warms the water.

While all streams warm, the best way to keep streams as cool as possible as long as possible is to maintain their natural shading from 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. This includes criteria limiting the temperature of the water so as to protect aquatic life, some species of which are quite sensitive to changes in temperature. Concern over water temperature has come to the fore because of the listing of several stocks of salmon and bull trout as threatened or endangered species.

DEQ disagrees with EPA over acceptable criteria for temperature for Idaho water bodies. At issue is a balance between temperature that is protective of cold-water dependent species yet attainable in most waterbodies. 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 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 use. For all but bull trout, DEQ uses a pair of criteria - limiting 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 temperturess 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 individual days to be slightly warmer.

All temperature criteria limit the maximum permissible value for the particular measure of temperature over a year or season. Idaho’s rules allow for the 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). There are many waters in Idaho where temperatures naturally warmer than criteria are likely to occur; however determining these natural temperatures takes modeling of past conditions which can be difficult.

^aMDMT = Maximum Daily Maximum Temperature
^bMWMT = Maximum Weekly (7-day average) Maximum Temperature
^cMDAT = Maximum Daily Average Temperature

DEQ is undertaking rulemaking in 2011 to revise its temperature requirements, in particular the treatment requirements at IDAPA 58.01.02.401.01.c&d and the salmonid spawning criteria at IDAPAS 58.01.02.250.f.ii