pH is a measure of how acidic or basic the water is. pH in streams can be affected by many things, including urban runoff, mining activities, and even pine needles! Typically, the presence of organic matter and decomposition lower pH and urban runoff tends to increase pH. A pH range of 6 to 9 is optimal for the health of freshwater fish and bottom-dwelling macroinvertebrates; in San Pablo Creek, pH has mostly stayed between 6 and 8 at all sites.