It is relative to the amount in the air and the temperature of the water. This influences what can live in the river. This is the balance of carbon dioxide in the river. It is a measure of the buffering capacity of the river. These are the most common ions in fresh water.
Hard water is more product than soft.
We test hardness because it helps metal toxicity by out competing on the gill uptake sites on fish. SO the harder the water, the more metals that can be in the water. Trout need a range of 6 to 8 to not be stressed. We test this using a Beckman pH meter with an automatic temperature compensator. The CDOW runs the test at their lab in Fort Collins.