This chapter discusses how humans affect stream temperatures both directly and indirectly. Thermal pollution directly arises from heat contained within wastewater, precipitation warmed through the urban heat island phenomenon, and runoff from hot pavements. Indirect routes include riparian deforestation, warmed groundwater, stream base flow reductions that limit thermal dilution, and climate change. This chapter also demonstrates that runoff temperatures and subsequent stream temperature pulses depend on features as varied as dewpoint temperatures, watershed imperviousness, reservoir discharges and distance from a stream's source. Warm stormwater runoff also affects groundwater through water quality measures such as conductivity and dissolved oxygen. This chapter considers the size of receiving streams and the potential for buffering thermal impacts. Small, headwater streams are generally close to groundwater temperatures, except for sudden heat pulses from urban storms, whereas large streams have higher temperatures with little daily variation. Temperature affects ecological competition between various phytoplankton species, with high temperatures leading to the dominance of green and blue-green algae associated with algal blooms. Typically, urbanization results in heat tolerant invasive trout species displacing native trout with lower temperature requirements.
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