What is the exact meaning of the phrase "Cataract of rippling notes"?

Solution 1:

It's really helpful you mentioned that this quote came from Helen Keller's autobiography because the metaphor makes more sense in light of her blindness and deafness. Because of her disabilities, the majority of her sensory input comes from touch, so she "hears" sounds by feeling the vibrations of objects that caused the sounds. In this sentence, notes probably refers to the sound from the rippling brooks. For her, the sound of a running stream is felt as a waterfall and waves on her fingers.