pH is a value on the scale that measures the hydrogen ion concentration of a solution. The pH level in rivers can be altered by several factors including runoff, leakages from human activity (such as septic systems), acid rain and interactions with surrounding rock (particularly in carbonate form). Negatively charged ions, such as phosphates, can remove hydrogen ions from the water solution thus making the system basic. The additions of positive ions, such as calcium and iron, can cause hydrogen ions to be disassociated and enter the water, thus making the system acidic. ​