Some organisms require high levels of dissolved oxygen, but other animals and plants thrive in waters with low dissolved oxygen content. Rivers with high dissolved oxygen content are usually stable ecosystems capable of supporting many different kinds of plants and animals. Oxygen in the air gets mixed with water in the river when river water tumbles over rocks or down waterfalls. Green plants conduct photosynthesis to make their food, and oxygen is a by-product of this process. Dissolved oxygen levels may rise from morning through the afternoon as a result of photosynthesis, reaching a peak in late afternoon.
Gases dissolve better in cold water.
The warmer the water temperature, the lower the dissolved oxygen content. Some animals such as mayflies, stone flies, caddis fly, and aquatic beetles, require high dissolved oxygen content to survive. Worms and fly larvae, which can survive in low dissolved oxygen rivers, are indicators of an unhealthy river. We measured dissolved oxygen content with a kit. We are not confident of the results of the 37.8 Mile test.