The red color comes from iron in the water. The source is old mining activity. It is called acid mine drainage.

"AMD results when mineral pyrite (FeS2) is exposed to air and water, resulting in sulfuric acid and iron hydroxide. Acid mine drainage is caused by water coming in contact with coal or mining activity. Pyrite is commonly present in coal seams and in the rock layers overlying coal seams."

https://www.geocaching.com/geocache/GC1V3Y8_the-red-waterfall?guid=d0c7f519-9c4b-46f0-9141-2b0c388df33e