Sewage treatment plants remove anaerobic conditions during treatment by aerating the wastewater. Extra fertilizers promote excessive plant growth, which at first adds plenty of oxygen to the water. This process is known as eutrophication. These organic wastes also need to be broken down by bacteria, using up oxygen. High levels of BOD are bad for a stream.
Humans can also add too much oxygen to the water, which can be toxic to aquatic organisms.
One common example is turbulent water released from a dam. Water Quality Criteria For aesthetic purposes, the EPA requires only enough oxygen needed to maintain aerobic conditions. A summary of these data is below to provide an understanding of past values of dissolved oxygen and demonstrate relationships of past dissolved oxygen levels. The highest DO concentration is in the winter months while the lowest concentrations are in the late summer months of August and September. This change in dissolved oxygen concentration shows an inverse relationship to changes in temperature.