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.

DEQ's stream temperature standards are designed to protect aquatic life uses, which are the only uses that have temperature requirements. The criteria vary by aquatic use-warm water, seasonal cold water, cold water, salmonid spawning, and bull trout (see table below). The latter two uses are subcategories of the cold water use. For all but bull trout, DEQ uses a pair of criteria, targeting 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 limit the stream's warmth. Using a pair of criteria provides regulation over a broader range of streams than either alone could. For bull trout the criterion is for a seven-day rolling average of daily maximums. This rolling average regulates maximums while allowing a few individual days to be slightly warmer.

^aMDMT = Maximum Daily Maximum Temperature

^bMWMT = Maximum Weekly (7-day average) Maximum Temperature

^cMDAT = Maximum Daily Average Temperature

After three years of development, EPA Region 10 issued guidance to states and tribes in the Pacific Northwest on temperature criteria to protect endangered salmonids in April 2003. DEQ is evaluating how to best apply this guidance to Idaho waters.

All the criteria limit the maximum permissible value for the particular metric over the course of a year or season. However, the rule language for cold and seasonal cold water states that all the associated criteria are values not to be exceed due to human activities. Thus, if the criteria are exceeded due to natural background conditions, that is acceptable. When natural background conditions do exceed numeric criteria, the pollutant levels shall not exceed the natural background condition. There is an exception for temperature that allows a small 0.3 °C human caused increase in waters that are naturally warmer than criteria. There are many waters in Idaho where this is likely to apply.