Aquatic organisms depend on oxygen for respiration. Oxygen is dissolved in water through aeration, via moving over rocks or waterfalls, and photosynthesis, by plants living in the creek. Cold water can hold more dissolved oxygen than warm water, because the molecules in the water move slower and thereby retain more oxygen.
In San Pablo Creek, dissolved oxygen levels range between 6-12mg/L; healthy streams should have more than 8mg/L. We see our lowest levels in the summer, when the water is warmer. Low flows and lack of shade may contribute to our low oxygen levels in the summer.