Cold groundwater entering the warm Yellowstone River
Groundwater falls from a cliff and enters the Yellowstone River below the town of Big Timber, Montana.
Groundwater typically is much colder than the summer temperatures of rivers, and thus helps moderate temperatures. In this case, the groundwater was 11C (52F) and the Yellowstone River afternoon temperature was 22 C (72F). The high temperatures decrease the oxygen carrying capacity of the water, but increase the metabolic oxygen demand of the trout (and other fish) in the river. An additional metabolic demand for oxygen comes when fish digest food. At the high temperatures, there isn't enough oxygen to go around, so fish decrease their feeding, and fishing becomes poor.