What is it?
Hardness is a measure of dissolved minerals in water – primarily calcium and magnesium. The types of rocks (and the minerals they contain) in an area will influence the hardness of the water. For example, limestone is rich in calcium and is easily eroded by water.
Other types of rocks do not contain as much magnesium or calcium, and do not erode as easily. Lake and river water in these areas would most likely have low hardness and would be considered “soft”.