Water temperature is an important driver of fish persistence and survival, and spatial patterns in water temperature can define species distributions and affect fish life histories. To understand longitudinal stream temperature patterns in rivers, USGS scientists used thermal infrared remote sensing of 53 rivers—16,866 km total—in the Pacific Northwest and found unexpected patterns of downstream warming. Water temperature did not always get warmer as water moved downstream, in contrast to prevailing theory. In fact, more than half the rivers showed unexpected patterns due to local climatic conditions, such as coastal fog and cold-water inputs from springs or tributaries. The variability within and among rivers studied suggests fish that depend on cold water may be able to use thermal diversity in rivers to survive in a warming climate. Findings could have implications for models that predict the response of water temperature to climate change.