In the river there is not enough sulphur for the acid to react in great quantities. However in the soil there is a great collection of sulphur to aid the reaction. When it joins the water the pH becomes much lower. The water itself does not contain enough sulphur to kill off it's population of fish and plants. But with the sulphur in the soil it does.
Acid rain is a big problem.
It causes the death of our lakes, our rivers, our wild life and most importantly us. As well it causes other problems that are very serious as well such as the release of aluminium and lead into our water supplies. We are suffering because of it. In Scotland there are many birth defects being attributed to it. We must cut down the releases of chemicals that cause it.