As water becomes warmer (from 0 degrees C to 100 degrees C), water becomes less dense. So the water of the warmer summer months has a higher dissolved oxygen content than the water in the cooler months. According to the data collected, the water of the Great Dismal Swamp usually has a annual dissolved oxygen level ranging from 1 to 3 mg/L, but the extremely hot and dry summer of 1997 caused the oxygen level to rise, almost to 6 mg/L at one sampling date. Also, periods of rainfall raise the oxygen content of the water, due to the fact that rain dissolves oxygen into itself while falling through the air (Berner 1987).