Which part of the river system is most affected by acid rain?

Acid rain greatly affects river water by changing its chemical composition. The chemical elements found in river freshwater are organic, and all play a role in a river's ecosystem. One of the most important aspects of a river's health is its pH levels. Acid rain can substantially lower a river system's pH level, causing acid imbalance in the water. Acid rain itself is composed of synthetic chemicals, usually originating from factories and general pollution.
River banks are also affected by acid rain. They contain various plant life, including trees, that contribute to the ecosystem of the river. Those plants and trees also provide a light flood barrier when the river's height increases during rainy seasons. If acid rain kills these plants, flash flooding along the river can increase. Animals and aquatic life that live in river systems and are crucial to their ecosystems are also affected by the chemicals of acid rain.
http://www.apis.ac.uk/acid-deposition-rivers-and-streams

https://www.britannica.com/science/acid-rain/Effects-on-lakes-and-rivers