pH is measured on a scale from 1 to 14 that tells us how acidic or basic water is.  pH depends on the amount of hydrogen (H+) and hydroxide (OH-) in the water.  More acidic water is indicated by a pH value below 7, while more basic water is indicated by a pH value above 7.  An equal concentration of hydrogen and hydroxide ions results in neutral (pH = 7) water. 

pH is important to measure because most plants and animals can only survive in a pH range from approximately 6.5 to 8.  pH can be altered by acid rain, rocks in the stream, and wastewater discharges.