Cold groundwater gushes from a cliff face along the Yellowstone River at the town of Big Timber, Montana. Groundwater is usually much colder than the summer temperatures of temperate zone rivers, and hence cools the river water. On the August day of this image, the afternoon river temperature was 72F (22 C), whereas the spring water was 52F (11 C). In contrast, in winter, the groundwater is warmer than stream and river water, and helps keep it warm.
Warm temperatures stress trout, because they cannot get sufficient oxygen, and consequentely they reduce their feeding activities because digestion of the food requires additional oxygen (i.e. - gut cramps if you run after eating a large meal).