Acid rain contains unusual and increased levels of hydrogen ions causing it to become, as the name suggests, acidic. Emissions of nitrogen oxide and sulphur dioxide react with water molecules in the atmosphere to become acidic. Sulphur dioxide and nitrogen oxide can be produced from man-made sources as well as natural sources such as lightning strikes and volcanic eruptions.
Whilst a lot of river systems can be badly affected by acid rain, some have a natural barrier in the form of a buffer solution. An aqueous solution of weak acid or base and its conjugate counterpart, the pH of buffer solutions changes very little when exposed to moderate amounts of acid or base.