Acid precipitation can harm living organisms in rivers and streams, but some rivers are more vulnerable than others. Streams in the Adirondack Mountains, for example, were quickly acidified by acid precipitation in the 1970s and 80s, killing local trout populations. But rivers that ran through a bed of limestone (calcium carbonate), like the Clark Fork River in Montana, were less affected. What can explain the relative lack of effect of acid precipitation on the Clark Fork River