pH
pH describes the acidity or alkalinity of water and represents the balance between hydrogen ions (H⁺) and hydroxide ions (OH^!) in water.

The value for pH is expressed on a scale ranging from 0 to 14. Solutions with more H⁺ than OH^! ions have a pH value lower than 7 and are said to be acidic. Solutions with pH values higher than 7 have more OH^- than H⁺ ions and are said to be basic, or alkaline. If the pH value is 7, the solution is said to be neutral (an equal number of H⁺ and OH^! ions) and is neither acidic nor alkaline.

It is important to note that the pH scale is logarithmic. This means that each step on the scale represents a tenfold change in the H⁺ concentration. For example, water with a pH of 5 has ten times the number of H⁺ ions than water with a pH of 6 and is ten times more acidic.

pH and Water
Water with a pH of less than 4.8 or greater than 9.2 can be harmful to aquatic life. Most freshwater fish prefer water with a pH range between 6.5 and 8.4 (colored band on the graph). The pH is also a useful indicator of the chemical balance in water. A high or low pH will adversely affect the availability of certain chemicals or nutrients in the water for use by plants.

pH in the Red River
As seen in the graph below, the pH in the Red River at Fargo-Moorhead (FM) is relatively constant year-round. The average pH of the Red River is around 8, or slightly basic. The slightly basic pH and the stability of the pH result from the alkalinity of the regional environment.

Graph of pH (relative acidity/alkalinity) for the Red River in the FM metro area for the
period January 2001 to April 2003 in relation to neutral pH (red line) and the optimal pH conditions for fish (green band).