When teaching about rain aboard Clearwater, we explain that acid rain is usually a result of sulfur emissions from powerplants to the west of us, carried aloft and turned into Sulfuric Acid [but very dilute] and rained down.

Various minterals in exposed rock, especially limestone, interact with the acid rain and neutralize before it even gets to the river.

There are bogs and other sites which do not have limestone/marble in structure accessible to the acid rain. In those sites, you often see a lot of dead plants and trees standing forlorn.