Relationships Between Stream and Intragravel Temperatures in Coastal Drainages, and Some Implications for Fisheries Workers
By a depth of 10 cm into the streambed, water temperatures are likely to be different from those in the open water of the stream. Combined results from three independent studies on disparate streams on the Pacific Northwest coast indicated that there are widespread similarities in the thermal behavior of intragravel water. In general, the thermal mass of the substrate causes parallel but lagged and buffered heating and cooling trends in infiltration-source intragravel water compared with surface water. Intragravel mean daily temperatures were generally 0.5–1.0 °C warmer in winter and 0.5–1.5 °C cooler in summer, with cross-overs around March and October; intragravel daily maximum temperatures could be up to 6 °C different in summer (a difference of 4 °C was common). The degree of difference showed considerable site-specific variation, and potentially can be affected by several factors. Such intragravel temperature differences have implications for those involved in salmonid egg incubation and fry emergence studies, enhancement projects, benthic invertebrate research, and environmental impact assessments.