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Oxygen is produced during the day through photosynthesis and consumed at night when plants respire and decompose. Therefore, streams with significant plant life can have dissolved oxygen levels that greatly fluctuate over a 24-hour period. Dissolved oxygen concentrations increase in streams wherever the water flow becomes turbulent, such as in a riffle area or waterfall. The turbulence allows more oxygen to dissolve into the water. This process is called reaeration.
Temperature, flow, lack of rapids and riffles, and pollution can all have an impact on the amount of dissolved oxygen in the water, and those impacts are most notable in the summer. Cold water holds more dissolved oxygen than warm water, so during the summer months, when the river and lake are warmer, they aren’t able to absorb as much. Low stream flows in late summer can compound the problem because the water heats up more rapidly. There is also less chance for reaeration as the water moves slowly through areas that were once riffles during higher flows, or when it stagnates in reservoirs.
Human impacts can also decrease the amount of dissolved oxygen in water. Wastewater discharges and stormwater runoff often carry oxygen-demanding substances (pollution) to streams and lakes. Most pollutants (nutrients, organic matter, and other chemicals) require oxygen for decomposition or other chemical reactions. The amount of oxygen required for some of these processes is called the biochemical oxygen demand, or BOD. As BOD concentrations increase, the river dissolved oxygen concentration decreases unless there is reaeration. Additionally, nutrients stimulate algae growth which uses dissolved oxygen during the night and adds to long-term depletion when it dies and decomposes in the bottom sediments.
The following graphic from a Spokane River computer model shows how dissolved oxygen changes in Lake Spokane throughout the year. Cold water fish, such as trout (salmonids), generally prefer the “green” range of the graphic. Click on the graphic to watch the change in levels from spring to fall 2001. Note the onset of “blue” or low dissolved oxygen in water in the summer months in the deeper parts of the lake, where salmonids will try to take refuge to avoid warm water. However, low dissolved oxygen levels in the deeper parts of the lake do not provide enough oxygen, so the habitat available to salmonids in summer is limited.
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