Unlike lakes and ponds, rivers are open systems, where frequent water exchange occurs. Despite this fact, the organisms that depend on rivers require some equilibrium. Various indicators give a measure of the quality of a river. These measurement include dissolved oxygen, temperature, and pH, which is a measure of hydrogen ion concentration.

The scale for measuring pH goes from 0 to 14 with 7 or under representing an acidic environment, and a reading over 7 indicates an alkaline environment.

A study published in the journal Nature directly linked acidity in Norwegian rivers and lakes to acid rain, which decimated local salmon and trout populations.

Low pH levels cause fish kill by stressing animal systems and causing physical damage, which in turn makes them more vulnerable to disease.

The effects of low pH levels can be lessened by the presence of limestone along river banks and in soil.

Other external factors that can cause fluctuations in the river pH include agricultural runoff, acidic mine drainage (AWD), and fossil fuel emissions such as carbon dioxide, which creates a weak acid when dissolved in river water.

Testing pH levels indicates the acidity or alkalinity of a sample. Rivers have some capacity to prevent changes in pH by the structure and composition of the river bed. However, drastic changes in pH can have detrimental effects on river health.