Water temperature affects how much oxygen the water can hold. The warmer the water the lower the DO, and vice versa. It is no accident that fish requiring a lot of oxygen are found in cold water. If DO levels are too low, fish and other aquatic animals will move away or die.

When water holds all the dissolved oxygen it can hold at a given temperature, it is said to be 100% saturated. If water holds half-as-much oxygen as it can hold at a given temperature, it is 50% saturated.

Sometimes water can be supersaturated- that is, water can hold more oxygen than it is supposed to be able to hold at a given temperature. Supersaturation may result when water is tumbled over rapids and falls. Conditions of supersaturation usually don't last long. Because tiny air bubbles rise to the surface of the water and pop, the percent of saturation soon drops to 100% percent or less. Supersaturation is a common problem along rivers controlled by dams. Supersaturation, even for short periods of time, can be very harmful to fish. Capillaries in the fishes gills can rupture or tear from too much oxygen. Fish with this condition usually die. It is referred to as gas bubble disease.

Streams that have a dissolved oxygen saturation percent value of 80- 120% are considered to be excellent. If the DO saturation percent is 60-79%, it is okay, but not great. If the DO percent saturation value is over 125% or below 60%, the water quality is poor.

You can determine the percent saturation using a Celsius thermometer, DO test kit, and the percent saturation chart below. Instructions for determining the percent of saturation follow the chart.

1. Use a thermometer to determine the water sample's temperature in degrees Celsius. Use your pencil to put a dot on the top line of the scale that corresponds to the water's temperature.

2. Use a test kit to determine the amount of dissolved oxygen (DO) in the sample. The DO amount should be in measured in mg/l. Use your pencil to put a dot on the bottom line of the scale that corresponds to the amount of DO in the water sample.

3. Use a straight edge to draw a line between the two dots.

4. Read your water sample's percent saturation off the middle diagonal line labeled percent saturation.

EXAMPLE: A water sample with a temperature of 15°C containing 11 mg/l of DO would be about 110 percent saturated. That value would be considered excellent because it lies between 80 and 120 percent.