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.
Stream temperature is an important part of water quality for the following reasons:
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:
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.
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.
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)
23 °C(73 °F)
a MDMT = maximum daily maximum temperatureb MWMT = maximum weekly (7-day average) maximum temperaturec MDAT = maximum daily average temperature
DEQ undertook rulemaking in 2011 to revise its temperature requirements, in particular the treatment requirements at IDAPA 58.01.02.401.01.c and d and the salmonid spawning criteria at IDAPA 58.01.02.250.02.f.ii. 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.
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)