Methodology

Stream temperature is a critical element of stream health, affecting aquatic life functions, habitat suitability, and water quality. Many fish and insects have a narrow temperature range to support optimal growth and reproduction. Elevated temperatures can increase metabolic stress, reduce dissolved oxygen availability, and alter how pollutants behave in the water. The water temperature assessment includes two metrics: chronic and acute temperatures.

TEMPERATURE METRIC WEIGHTING

Lower of the two grades, Chronic or Acute. Temperature standards are designed to protect fish and other aquatic life from water that is too warm for too long or that rises too quickly. These standards vary based on stream classification, which reflects the type of aquatic life the stream is expected to support.

CHRONIC TEMPERATURE

Chronic temperature standards look at longer-term exposure to elevated temperatures. They are intended to protect fish from conditions that may affect growth, reproduction, or overall health over time. Chronic conditions are assessed using a Maximum Weekly Average Temperature (MWAT), calculated from a seven-day rolling average and compared to the applicable standard.

ACUTE TEMPERATURE

Acute temperature standards address short-term temperature spikes. These standards are intended to protect fish from sudden high temperatures that could cause immediate stress or mortality. Acute conditions are assessed using a daily maximum two-hour rolling average over a 24-hour period and compared to the applicable standard.

DEGREE DAYS

Degree days are used to describe how much and how long stream temperatures exceed a standard. This measure combines the amount of warming above the standard with the number of days it occurs, providing an indication of cumulative thermal stress on aquatic life.